The Unsaturated Hydrocarbons: Alkenes, Alkynes, and Aromatics

325

OR

GA

NIC

CH

EM

IST

RYEach year millions of dollars are lost because of crop damage by pests.

OutlineCHEMISTRY CONNECTION:

A Cautionary Tale: DDT andBiological Magnification

12.1 Alkenes and Alkynes:Structure and PhysicalProperties

12.2 Alkenes and Alkynes:Nomenclature

A MEDICAL PERSPECTIVE:Killer Alkynes in Nature

12.3 Geometric Isomers: AConsequence ofUnsaturation

12.4 Alkenes in Nature12.5 Reactions Involving

AlkenesHydrogenation: Addition

of H2 to an AlkeneHalogenation: Addition of

X2 to an AlkeneA HUMAN PERSPECTIVE:

Folklore, Science, andTechnology

Hydration: Addition ofH2O to an Alkene

Hydrohalogenation:Addition of HX to anAlkene

Addition Polymers ofAlkenes

A HUMAN PERSPECTIVE:Life without Polymers?

AN ENVIRONMENTAL PERSPECTIVE:Plastic Recycling

12.6 Aromatic HydrocarbonsStructure and PropertiesNomenclatureReactions Involving

BenzeneA HUMAN PERSPECTIVE:

Aromatic Compounds andCarcinogenesis

12.7 Heterocyclic AromaticCompounds

Summary of ReactionsSummaryKey TermsQuestions and ProblemsCritical Thinking Problems

12The UnsaturatedHydrocarbons:Alkenes, Alkynes, andAromatics

Learning Goals

1 Describe the physical properties of alkenes andalkynes.

2 Draw the structures and write the I.U.P.A.C.names for simple alkenes and alkynes.

3 Write the names and draw the structures ofsimple geometric isomers of alkenes.

4 Write equations predicting the products ofaddition reactions of alkenes: hydrogenation,halogenation, hydration, andhydrohalogenation.

5 Apply Markovnikov’s rule to predict the majorand minor products of the hydration andhydrohalogenation reactions of unsymmetricalalkenes.

6 Write equations representing the formation ofaddition polymers of alkenes.

7 Draw the structures and write the names ofcommon aromatic hydrocarbons.

8 Write equations for substitution reactionsinvolving benzene.

9 Describe heterocyclic aromatic compoundsand list several biological molecules in whichthey are found.

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Unsaturated hydrocarbons are those that contain at least one carbon-carbondouble or triple bond. They include the alkenes, alkynes, and aromatic com-pounds. All alkenes have at least one carbon-carbon double bond; all alkynes have

326 Chapter 12 The Unsaturated Hydrocarbons: Alkenes, Alkynes, and Aromatics

12-2

We have heard the warnings for years: Stop using non-biodegradable insecticides because they are killing many ani-mals other than their intended victims! Are these chemicals notspecifically targeted to poison insects? How then can they beconsidered a threat to humans and other animals?

DDT, a polyhalogenated hydrocarbon, was discovered inthe early 1940s by Paul Müller, a Swiss chemist. Müller showedthat DDT is a nerve poison that causes convulsions, paralysis,and eventually death in insects. From the 1940s until 1972,when it was banned in the United States, DDT was sprayed oncrops to kill insect pests, sprayed on people as a delousingagent, and sprayed in and on homes to destroy mosquitoes car-rying malaria. At first, DDT appeared to be a miraculous chem-ical, saving literally millions of lives and millions of dollars incrops. However, as time went by, more and more evidence of adark side of DDT use accumulated. Over time, the chemicalhad to be sprayed in greater and greater doses as the insectpopulations evolved to become more and more resistant to it.In 1962, Rachel Carson published her classic work, SilentSpring, which revealed that DDT was accumulating in the envi-ronment. In particular, high levels of DDT in birds interferedwith their calcium metabolism. As a result, the egg shells pro-duced by the birds were too thin to support development of thechick within. It was feared that in spring, when the air shouldhave been filled with bird song, there would be silence. This isthe “silent spring” referred to in the title of Carson’s book.

DDT is not biodegradable; furthermore, it is not water-soluble, but it is soluble in nonpolar solvents. Thus if DDT isingested by an animal, it will dissolve in fat tissue and accu-mulate there, rather than being excreted in the urine. WhenDDT is introduced into the food chain, which is inevitablewhen it is sprayed over vast areas of the country, the result is bi-ological magnification. This stepwise process begins when DDTapplied to crops is ingested by insects. The insects, in turn, areeaten by birds, and the birds are eaten by a hawk. We can imag-ine another food chain: Perhaps the insects are eaten by mice,which are in turn eaten by a fox, which is then eaten by an owl.Or to make it more personal, perhaps the grass is eaten by asteer, which then becomes your dinner. With each step up oneof these food chains, the concentration of DDT in the tissues be-comes higher and higher because it is not degraded, it is simplystored. Eventually, the concentration may reach toxic levels insome of the animals in the food chain.

Consider for a moment the series of events that occurred inBorneo in 1955. The World Health Organization elected to

spray DDT in Borneo because 90% of the inhabitants were in-fected with malaria. As a result of massive spraying, the mos-quitoes bearing the malaria parasite were killed. If this soundslike the proverbial happy ending, read on. This is just the be-ginning of the story. In addition to the mosquitoes, millions ofother household insects were killed. In tropical areas it is com-mon for small lizards to live in homes, eating insects foundthere. The lizards ate the dead and dying DDT-contaminatedinsects and were killed by the neurotoxic effects of DDT. Thehouse cats ate the lizards, and they, too, died. The number ofrats increased dramatically because there were no cats to con-trol the population. The rats and their fleas carried sylvaticplague, a form of bubonic plague. With more rats in contactwith humans came the threat of a bubonic plague epidemic.Happily, cats were parachuted into the affected areas of Borneo,and the epidemic was avoided.

The story has one further twist. Many of the islanders livedin homes with thatched roofs. The vegetation used to makethese roofs was the preferred food source for a caterpillar thatwas not affected by DDT. Normally, the wasp populationpreyed on these caterpillars and kept the population undercontrol. Unfortunately, the wasps were killed by the DDT. Thecaterpillars prospered, devouring the thatched roofs, which col-lapsed on the inhabitants.

Every good story has a moral, and this one is not difficult todecipher. The introduction of large amounts of any chemicalinto the environment, even to eradicate disease, has the poten-tial for long-term and far-reaching effects that may be very dif-ficult to predict. We must be cautious with our fragileenvironment. Our well-intentioned intervention all too oftenupsets the critical balance of nature, and in the end we inad-vertently do more harm than good.

A Cautionary Tale: DDT and Biological Magnification

Introduction

OCOOCOCl

ClA

ClOA

ClA

HA

ClA

DDT: Dichlorodiphenyltrichloroethane

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at least one carbon-carbon triple bond. Aromatic compounds are particularly sta-ble cyclic compounds and sometimes are depicted as having alternating single anddouble carbon-carbon bonds. This arrangement of alternating single and doublebonds is called a conjugated system of double bonds.

Many important biological molecules are characterized by the presence ofdouble bonds or a linear or cyclic conjugated system of double bonds (Figure 12.1).For instance, we classify fatty acids as either monounsaturated (having one doublebond), polyunsaturated (having two or more double bonds), or saturated (havingsingle bonds only). Vitamin A (retinol), a vitamin required for vision, contains aten-carbon conjugated hydrocarbon chain. Vitamin K, a vitamin required for bloodclotting, contains an aromatic ring.

12.1 Alkenes and Alkynes: Structure andPhysical Properties

Alkenes and alkynes are unsaturated hydrocarbons. The characteristic functionalgroup of an alkene is the carbon-carbon double bond. The functional group that

12.1 Alkenes and Alkynes: Structure and Physical Properties 327

12-3

Fatty acids are long hydrocarbon chainshaving a carboxyl group at the end. Thusby definition they are carboxylic acids. SeeChapters 15 and 18.

These vitamins are discussed in detail inAppendix E, Lipid-Soluble Vitamins.

Figure 12.1(a) Structural formula of the sixteen-carbon monounsaturated fatty acidpalmitoleic acid. (b) Condensed formulaof vitamin A, which is required for vision.Notice that the carbon chain of vitamin Ais a conjugated system of double bonds.(c) Line formula of vitamin A. In the lineformula, each line represents a carbon-carbon bond, each double line representsa carbon-carbon double bond. A carbonatom and the appropriate number ofhydrogen atoms are assumed to be atthe point where two lines meet. Thevertical lines are assumed to terminate ina methyl group. (d) Line formula ofvitamin K, a lipid-soluble vitamin requiredfor blood clotting. The six-member ringwith the circle represents a benzene ring.See Figure 12.6 for other representationsof the benzene ring.

HOCAH

AH

OCAH

AH

OCAH

AH

OCAH

AH

OCAH

AH

OCAH

AH C

DH

PCG

H

CAH

AH

OCAH

AH

OCAH

AH

OCAH

AH

OCAH

AH

OCAH

AH

OCA

OAH

OC

CH3

Palmitoleic acid

Vitamin K

OCH PACH3

P

O

CC

C

CH2

CH2

OHCO

O

H3C

CH3

O

CH3 CH3 CH3

CH3

H2C

H2C CHP

OH

OCACH3

PCHOCH

CH3

OCH

Vitamin A

H3C CH3

CH3

CH3

CH

Vitamin A

(a)

(b)

(c)

(d)

BO

O

H

H

O

Learning Goal

1

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characterizes the alkynes is the carbon-carbon triple bond. The following generalformulas compare the structures of alkanes, alkenes, and alkynes.

General formulas: Alkane Alkene AlkyneCnH2n�2 CnH2n CnH2n�2

Structural formulas:

Molecular formulas: C2H6 C2H4 C2H2

Condensed formulas: CH3CH3 H2CPCH2 HCqCH

These compounds have the same number of carbon atoms but differ in thenumber of hydrogen atoms, a feature of all alkanes, alkenes, and alkynes that con-tain the same number of carbon atoms. Alkenes contain two fewer hydrogens thanthe corresponding alkanes, and alkynes contain two fewer hydrogens than the cor-responding alkenes.

In alkanes the four bonds to the central carbon have tetrahedral geometry.When carbon is bonded by one double bond and two single bonds, as in ethene(an alkene), the molecule is planar, because all atoms lie in a single plane. Eachbond angle is approximately 120�. When two carbon atoms are bonded by a triplebond, as in ethyne (an alkyne), each bond angle is 180�. Thus, the molecule is lin-ear, and all atoms are positioned in a straight line (Figure 12.2).

The physical properties of alkenes, alkynes, and aromatic compounds are verysimilar to those of alkanes. They are nonpolar. As a result of the “like dissolves like”rule, they are not soluble in water but are very soluble in nonpolar solvents such asother hydrocarbons. They also have relatively low boiling points and melting points.

12.2 Alkenes and Alkynes: NomenclatureTo determine the name of an alkene or alkyne using the I.U.P.A.C. NomenclatureSystem, use the following simple rules:

• Name the parent compound using the longest continuous carbon chaincontaining the double bond (alkenes) or triple bond (alkynes).

• Replace the -ane ending of the alkane with the -ene ending for an alkene orthe -yne ending for an alkyne. For example:

CH3—CH3 CH2PCH2 CHqCH

Ethane Ethene Ethyne

CH3—CH2—CH3 CH2PCH—CH3 CHqC—CH3

Propane Propene Propyne

• Number the chain to give the lowest number for the first of the two carbonscontaining the double bond or triple bond. For example:

CH3CH2CHPCH2

4 3 2 1

1-Butene(not 3-butene)

CHqCOCH2CH2CH3

1 2 3 4 5

1-Pentyne(not 4-pentyne)

HOCOCOH

HA

Ethane(ethane)

HA

HA

HA

CPCH

HH

HG

G D

D

Ethene(ethylene)

HOCqCOH

Ethyne(acetylene)


12-4

Learning Goal

2

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• Name and number all groups bonded to the parent alkene or alkyne, andplace the name and number in front of the name of the parent compound.Remember that with alkenes and alkynes the double or triple bond takesprecedence over a halogen or alkyl group, as shown in the followingexamples:

CH3OCHPCOCH3

4 3 2 1

2-Chloro-2-butene

CH3CHOCqCOCH2CH3

1 2 3 4 5 6

ClA

2-Bromo-3-hexyne

BrA

12.2 Alkenes and Alkynes: Nomenclature 329

12-5

i f

f iH

H

C

Ethene Ethyne

C

Planar

H

Allbondanglesapproximately120�

Hi&@

iH

H

$CH H

HCC

Ethane

Tetrahedral

Allbondanglesapproximately109.5�

CC

Linear

Bondangles180�

HOOH

(a)

H

C&@

HH

GCC

C%∆

(¨f

{&

HH

HH

HH

H H

C%∆

H H

A long-chain alkane(pentane)

D

H

i C

DC

H C

H

CC

A long-chain alkene(1-pentene)

DHG

H H%C

( ¨H H( ¨

H H

C

H H$C

( ¨H HC

COH

A long-chain alkyne(1-pentyne)

C C

H H$C

Hi

(b)

(b)

Figure 12.2(a)Three-dimensional drawings and ball-and-stick models of ethane, ethene, andethyne. (b) Examples of typical long-chain hydrocarbons.

Remember, it is the position of thedouble bond, not the substituent, thatdetermines the numbering of thecarbon chain.

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12-6

Killer Alkynes in Nature

There are many examples of alkynes that are beneficial to hu-mans. Among these are parasalamide, a pain reliever, pargyline,

an antihypertensive, and 17-ethynylestradiol, a synthetic estro-gen that is used as an oral contraceptive.

17-Ethynylestradiol

O

CHC CH

3

OH

HO

ParagylineParasalamide

NH

C N CH H

2

2CH2CH3

O CH2C CH

O

C NCH2 CCH2

CH3

CH

Alkynes used for medicinal purposes.

But in addition to these medically useful alkynes, there arein nature a number that are toxic. Some are extremely toxic tomammals, including humans; others are toxic to fungi, fish, orinsects. All of these compounds are plant products that mayhelp protect the plant from destruction by predators.

Capillin is produced by the oriental wormwood plant. Re-search has shown that a dilute solution of capillin inhibits thegrowth of certain fungi. Since fungal growth can damage ordestroy a plant, the ability to make capillin may provide asurvival advantage to the plants. Perhaps it may one day bedeveloped to combat fungal infections in humans.

Ichthyotherol is a fast-acting poison commonly found inplants referred to as fish-poison plants. Ichthyotherol is a very

toxic polyacetylenic alcohol that inhibits energy production inthe mitochondria. Latin American native tribes use these plantsto coat the tips of the arrows used to catch fish. Althoughichthyotherol is poisonous to the fish, fish caught by thismethod are quite safe for human consumption.

An extract of the leaves of English ivy has been reported tohave antibacterial, analgesic, and sedative effects. The com-pound thought to be responsible for these characteristics, aswell as antifungal activity, is falcarinol. Falcarinol, isolated froma tree in Panama, also has been reported by the Molecular Tar-gets Drug Discovery Program, to have antitumor activity. Per-haps one day this compound, or a derivative of it, will be usefulin treating cancer in humans.

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12.2 Alkenes and Alkynes: Nomenclature 331

12-7

Cicutoxin has been described as the most lethal toxin nativeto North America. It is a neurotoxin that is produced by the wa-ter hemlock (Cicuta maculata), which is in the same family ofplants as parsley, celery, and carrots. Cicutoxin is present in allparts of the plants, but is most concentrated in the root. Eatinga portion as small as 2–3 cm2 can be fatal to adults. Cicutoxinacts directly on the nervous system. Signs and symptoms of ci-cutoxin poisoning include dilation of pupils, muscle twitching,rapid pulse and breathing, violent convulsions, coma, anddeath. Onset of symptoms is rapid and death may occur withintwo to three hours. No antidote exists for cicutoxin poisoning.The only treatment involves controlling convulsions andseizures in order to preserve normal heart and lung function.Fortunately, cicutoxin poisoning is a very rare occurrence. Oc-casionally animals may graze on the plants in the spring, re-sulting in death within fifteen minutes. Humans seldom comeinto contact with the water hemlock. The most recent cases

have involved individuals foraging for wild ginseng, or otherwild roots, and mistaking the water hemlock root for an edibleplant.

O

IchthyotherolCapillin

C C CH3C C C C C C CCH3 C C CH CH

OH

H CCH2 C C C CCH C

HO

HH2 ( )CH2 CH37CH C

Falcarinol

CH2 CH OH2 CH3 CH2

HCH C HCH C HCHCH C

HO

Cicutoxin

CH2

C C C CCH2

CH2

Alkynes that exhibit toxic activity.

Cicuta maculata, or water hemlock, produces the most deadly toxinindigenous to North America.

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EXAMPLE 12.2

EXAMPLE 12.1

• Alkenes having more than one double bond are called alkadienes (twodouble bonds) or alkatrienes (three double bonds), as seen in these examples:

CH3CHPCH—CHPCHCH3 CH2PCHCH2CHPCH2

2,4-Hexadiene 1,4-Pentadiene 3-Methyl-1,4-cyclohexadiene

Naming Alkenes and Alkynes Using the I.U.P.A.C. Nomenclature System

Name the following alkene and alkyne using I.U.P.A.C. nomenclature.

Solution

Longest chain containing the doublebond: octene

Position of double bond: 3-octene(not 5-octene)

Substituents: 3-methyl and 4-propyl

Name: 3-Methyl-4-propyl-3-octene

Longest chain containing the triplebond: hexyne

Position of triple bond: 3-hexyne(must be!)

Substituents: 2,2-dimethyl

Name: 2,2-Dimethyl-3-hexyne

Naming Cycloalkenes Using I.U.P.A.C. Nomenclature

Name the following cycloalkenes using I.U.P.A.C. nomenclature.

Solution

Parent chain: cyclohexene

Position of double bond: carbon-1 (carbons of thedouble bond are numbered 1 and 2)

Substituents: 4-chloro

Name: 4-Chlorocyclohexene

Parent chain: cyclopentene

Position of double bond: carbon-1

Substituent: 3-methyl

Name: 3-Methylcyclopentene

1 2

34

5

P

C

HA

HA

C

HA

HA

C

HA

CH3A

CCD

H HG

C

HA

HA

C

ClA

HA

C

HA

HA

C

HA

HA

P CCD

HHG1 2

3

45

6

ACH3


12-8

Alkenes with many double bonds areoften referred to as polyenes (poly—many enes—double bonds).

CH3CH2CH2CH2

8 7 6 5

CPC

CH3CH2CH2

CH2CH3

G D

GD

CH3

2 1

4 3

CH3CH2OCqCOCOCH3

6 5 4 3 2 1

CH3

A

CH3A

Learning Goal

2

Learning Goal

2

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Draw a complete structural formula for each of the following compounds:

a. 1-Bromo-3-hexyneb. 2-Butynec. Dichloroethyned. 9-Iodo-1-nonyne

Name the following compounds using the I.U.P.A.C. Nomenclature System:

a. CH3—CqC—CH2CH3

b. CH3CH2CHCHCH2CqCH| |Br Br

c.

Br CH3| |

CH3CH—CPC—CHCH3| |CH3 CH3

d.

CH2CH3|

CH3CH—CqC—CHCH3|Br

12.3 Geometric Isomers: A Consequence ofUnsaturation

The carbon-carbon double bond is rigid because of the shapes of the orbitals in-volved in its formation. The electrons of one of the two carbon-carbon bonds lie ina line between the two nuclei. This is called a sigma (�) bond. The second bond isformed between two p orbital electrons and is called a pi (�) bond. The two elec-trons of the � bond lie in the region above and below the two carbon atoms asshown in the following diagram.

Rotation around the double bond is restricted because the � bond would have tobe broken to allow rotation. Thus, the double bond is rigid.

In Section 11.3, we observed that the rotation around the carbon-carbon bondsof cycloalkanes was restricted. The consequence of the absence of free rotation wasthe formation of geometric or cis-trans isomers. The cis isomers of cycloalkanes hadsubstituent groups on the same side of the ring (L., cis, “on the same side”). Thetrans isomers of cycloalkanes had substituent groups located on opposite sides ofthe ring (L., trans, “across from”).

H

H

H

H

C C

π bond

σ bond

π bond

Q u e s t i o n 12.2


12.3 Geometric Isomers: A Consequence of Unsaturation 333

12-9

Restricted rotation around doublebonds is partially responsible for theconformation and hence the activity ofmany biological molecules that we willstudy later.

The electron charge cloud associatedwith the two electrons making up the �bond (in red) is concentrated betweenthe two nuclei. The electron chargecloud associated with the two electronsof the � bond (in blue) is concentratedin two regions above and below the �bond framework of the molecule.

Learning Goal

3

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EXAMPLE 12.4

EXAMPLE 12.3

In the alkenes, geometric isomers differ from one another by the location ofgroups on the same or opposite sides of the double bond. Because the double bondof the alkenes is also rigid and there is no free rotation around it, geometric iso-mers are formed when there are two different groups on each of the carbon atomsattached by the double bond. If both groups are on the same side of the doublebond, the molecule is a cis isomer. If the groups are on opposite sides of the doublebond, the molecule is a trans isomer.

Consider the two isomers of 1,2-dichloroethene:

If one of the two carbon atoms of the double bond has two identical sub-stituents, there are no cis-trans isomers for that molecule. Consider the example of1,1-dichloroethene:

Identifying cis and trans Isomers of Alkenes

Two isomers of 2-butene are shown below. Which is the cis isomer andwhich is the trans isomer?

Solution

As we saw with cycloalkanes, the prefixes cis and trans refer to theplacement of the substituents attached to a bond that cannot undergo freerotation. In the case of alkenes, it is the groups attached to the carbon-carbon double bond (in this example, the H and CH3 groups). When thegroups are on the same side of the double bond, as in the structure on theleft, the prefix cis is used. When the groups are on the opposite sides of thedouble bond, as in the structure on the right, trans is the appropriate prefix.

Naming cis and trans Compounds

Name the following geometric isomers.

HCPC

H3C

HG D

GD

CH3

HCPC

H3C

CH3G D

GD

H

cis-2-Butene trans-2-Butene

HCPC

H3C

HG D

GD

CH3

HCPC

H3C

CH3G D

GD

H

HCPC

H

ClG D

GD

Cl

1, 1-Dichloroethene

HCPC

Cl

HG D

GD

Cl

cis-1, 2-Dichloroethene

HCPC

Cl

ClG D

GD

H

trans-1, 2-Dichloroethene


12-10

Continued—

Learning Goal

3

Learning Goal

3

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EXAMPLE 12.5

Solution

The longest chain of carbon atoms in each of the following molecules ishighlighted in yellow. The chain must also contain the carbon-carbon doublebond. The location of functional groups relative to the double bond is usedin determining the appropriate prefix, cis or trans, to be used in namingeach of the molecules.

Parent chain: heptene

Position of double bond: 3-

Configuration: trans

Substituents: 3,4-dichloro

Name: trans-3,4-Dichloro-3-heptene

Parent chain: octene

Position of double bond: 3-

Configuration: cis

Substituents: 3,4-dimethyl

Name: cis-3,4-Dimethyl-3-octene

In each of the following pairs of molecules, identify the cis isomer and the transisomer.

a.

b.

Provide the complete I.U.P.A.C. name for each of the compounds in Question 12.3.

Identifying Geometric Isomers

Determine whether each of the following molecules can exist as cis-transisomers: (1) 1-pentene, (b) 3-ethyl-3-hexene, and (c) 3-methyl-2-pentene.

Solution

a. Examine the structure of 1-pentene,

HCPC

H

CH2CH2CH3G D

GD

H


BrCPC

H3C

CH3G D

GD

Br

BrCPC

H3C

BrG D

GD

CH3

HCPC

CH3CH2

HG D

GD

CH2CH3

HCPC

CH3CH2

CH2CH3G D

GD

H


CH3CH2

CPCCH3

CH2CH2CH2CH3G D

GD

CH3

Cl

CPC

CH3CH2CH2

CH2CH3G D

GD

Cl

12.3 Geometric Isomers: A Consequence of Unsaturation 335

12-11

EXAMPLE 12.4 —Continued

Continued—

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We see that carbon-1 is bonded to two hydrogen atoms, rather than to twodifferent substituents. In this case there can be no cis-trans isomers.

b. Examine the structure of 3-ethyl-3-hexene:

We see that one of the carbons of the carbon-carbon double bond is bondedto two ethyl groups. As in example (a), because this carbon is bonded totwo identical groups, there can be no cis or trans isomers of this compound.

c. Finally, examination of the structure of 3-methyl-2-pentene reveals thatboth a cis and trans isomer can be drawn.

Each of the carbon atoms involved in the double bond is attached to twodifferent groups. As a result, we can determine which is the cis isomer andwhich is the trans isomer based on the positions of the methyl groupsrelative to the double bond.

Draw condensed formulas for each of the following compounds:

a. cis-3-Octeneb. trans-5-Chloro-2-hexenec. trans-2,3-Dichloro-2-butene

Name each of the following compounds, using the I.U.P.A.C. system. Be sure toindicate cis or trans where applicable.

a. c.

b.

12.4 Alkenes in NatureFolklore tells us that placing a ripe banana among green tomatoes will speed upthe ripening process. In fact, this phenomenon has been demonstrated experimen-tally. The key to the reaction is ethene, the simplest alkene. Ethene, produced byripening fruit, is a plant growth substance. It is produced in the greatest abun-dance in areas of the plant where cell division is occurring. It is produced during

CH3CH2

CPC

CH3

CH2CH3G D

GD

H

CH3

CPC

H

HG D

GD

CH2CCH3

CH3A

CH3

A

CH3

CPC

H

CH3G D

GD

CH3



H3CCPC

CH3CH2

CH3G D

GD

H

H3CCPC

CH3CH2

HG D

GD

CH3

CH3CH2

CPC

CH3CH2

CH2CH3G D

GD

H


12-12


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fruit ripening, during leaf fall and flower senescence, as well as under conditionsof stress, including wounding, heat, cold, or water stress, and disease.

There are a surprising number of polyenes, alkenes with several double bonds,found in nature. These molecules, which have wildly different properties andfunctions, are built from one or more five-carbon units called isoprene.

CH3|

CH2PC—CHPCH2

Isoprene

The molecules that are produced are called isoprenoids, or terpenes (Figure 12.3).Terpenes include the steroids; chlorophyll and carotenoid pigments that functionin photosynthesis, and the lipid soluble vitamins A, D, and K (Figure 12.1).

Many other terpenes are plant products familiar to us because of their distinc-tive aromas. Geraniol, the familiar scent of geraniums, is a molecule made up oftwo isoprene units. Purified from plant sources, geraniol is the active ingredient inseveral natural insect repellants. These can be applied directly to the skin to pro-vide four hours of protection against a variety of insects, including mosquitoes,ticks, and fire ants.

D-Limonene is the most abundant component of the oil extracted from the rindof citrus fruits. Because of its pleasing orange aroma, D-limonene is used as a flavorand fragrance additive in foods. However, the most rapidly expanding use of thecompound is as a solvent. In this role, D-limonene can be used in place of more toxicsolvents, such as mineral spirits, methyl ethyl ketone, acetone, toluene, and fluori-nated and chlorinated organic solvents. It can also be formulated as a water-basedcleaning product, such as Orange Glo, that can be used in place of more causticcleaning solutions. There is a form of limonene that is a molecular mirror image ofD-limonene. It is called L-limonene and has a pine or turpentine aroma.

The terpene myrcene is found in bayberry. It is used in perfumes and scentedcandles because it adds a refreshing, spicy aroma to them. Trace amounts ofmyrcene may be used as a flavor component in root beer.

Farnesol is a terpene found in roses, orange blossom, wild cyclamen, and lily ofthe valley. Cosmetics companies began to use farnesol in skin care products in theearly 1990s. It is claimed that farnesol smoothes wrinkles and increases skin elas-ticity. It is also thought to reduce skin aging by promoting regeneration of cells andactivation of the synthesis of molecules, such as collagen, that are required forhealthy skin.

Another terpene, retinol, is a form of vitamin A (Figure 12.1). It is able to pene-trate the outer layers of skin and stimulate the formation of collagen and elastin.This reduces wrinkles by creating skin that is firmer and smoother.

12.5 Reactions Involving AlkenesReactions of alkenes involve the carbon-carbon double bond. The key reaction ofthe double bond is the addition reaction. This involves the addition of two atomsor groups of atoms to a double bond. The major alkene addition reactions includeaddition of hydrogen (H2), halogens (Cl2 or Br2), water (HOH), or hydrogenhalides (HBr or HCl). A generalized addition reaction is:

R RG D

ROCOA

RA

A

CBC GD

R R

�AAB

RA

ROCOB

12.5 Reactions Involving Alkenes 337

12-13

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12-14

Figure 12.3Many plant products, familiar to usbecause of their distinctive aromas, areisoprenoids, which are alkenes havingseveral double bonds.

C

Geraniol(Rose and geraniums)

CHCH2

CH2

CH OH2

CH3

CH

CH3

C

CH3

C

Limonene(Oil of lemon and orange)

CH

C

CH2

CH2 H2

CH3

CH

CH2

C

CH3

Myrcene(Oil of bayberry)

C

CH

C

CH2

CH2 H2

CH2

CH

CH3

C

CH3

C

Farnesol(Lily of the valley)

CHCH2

CH2

C CH2

CH

H OH2

CH3

C

CH3

CH

CH3

C

CH3

CH2

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EXAMPLE 12.6

Note that the double bond is replaced by a single bond. The former double bondcarbons receive a new single bond to a new atom, producing either an alkane or asubstituted alkane. This involves breaking the � bond between the two carbons ofthe double bond and forming a new � bond to each of these carbons.

Hydrogenation: Addition of H2 to an AlkeneHydrogenation is the addition of a molecule of hydrogen (H2) to a carbon-carbondouble bond to give an alkane. In this reaction the double bond is broken, and twonew C—H single bonds result. Platinum, palladium, or nickel is needed as a cata-lyst to speed up the reaction. Heat and/or pressure may also be required.

Writing Equations for the Hydrogenation of Alkenes

Write a balanced equation showing the hydrogenation of (a) 1-pentene and(b) trans-2-pentene.

Solution

(a) Begin by drawing the structure of 1-pentene and of diatomic hydrogen(H2) and indicating the catalyst.

Knowing that one hydrogen atom will form a covalent bond with each ofthe carbon atoms of the carbon-carbon double bond, we can write theproduct and complete the equation.

(b) Begin by drawing the structure of trans-2-pentene and of diatomichydrogen (H2) and indicating the catalyst.

CPC

CH3CH2

CH3H

HG

G D

D

trans-2-Pentene

HOH

Hydrogen

�Ni

CPC

CH3CH2CH2

HH

HG

G D

D

1-Pentene

HOH

Hydrogen

�Ni

CH3CH2CH2OCOCOH

HA

HA

HA

HA

Pentane

CPC

CH3CH2CH2

HH

HG

G D

D

1-Pentene

HOH

Hydrogen

�Ni

R RG D

ROCOH

RA

A

CBC GD

R R

�HAH

RA

ROCOH Pt, Pd, or NiHeat or pressure

Alkene Hydrogen Alkane


12-15

Recall that a catalyst itself undergoes nonet change in the course of a chemicalreaction (see Section 8.3).

Remember that the R in these generalformulas represents an alkyl group.

Note that the alkene is gaining twohydrogens. Thus, hydrogenation is areduction reaction (see Sections 9.5and 13.6).

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Knowing that one hydrogen atom will form a covalent bond with each ofthe carbon atoms of the carbon-carbon double bond, we can write theproduct and complete the equation.

The trans isomer of 2-pentene was used in Example 12.6. Would the result be anydifferent if the cis isomer had been used?

Write balanced equations for the hydrogenation of 1-butene and cis-2-butene.

Hydrogenation is used in the food industry to produce margarine, which is amixture of hydrogenated vegetable oils (Figure 12.4). Vegetable oils are unsatu-rated, that is, they contain many double bonds and as a result have low meltingpoints and are liquid at room temperature. The hydrogenation of these doublebonds to single bonds increases the melting point of these oils and results in a fat,such as Crisco, that remains solid at room temperature. Through further processingthey may be converted to margarine, such as corn oil or sunflower oil margarines.

Halogenation: Addition of X2 to an AlkeneChlorine (Cl2) or bromine (Br2) can be added to a double bond. This reaction, calledhalogenation, proceeds readily and does not require a catalyst:

R RG D

ROCOX

RA

A

CBC GD

R R

�XAX

RA

ROCOX

Alkene Halogen Alkyl dihalide



CPC

CH3CH2

CH3H

HG

G D

D

trans-2-Pentene

HOH

Hydrogen

�Ni

CH3CH2OCOCOCH3

HA

HA

HA

HA

Pentane


12-16


Saturated and unsaturated dietary fats arediscussed in Section 18.2.

H2, 200�C,25 psi,metal catalyst

CHPCH

CHPCH

CHP CH

(CH2)7 CH2CH3

O

COB

OO O(CH2)7O

(CH2)7 CH2CH3

O

COB

OO O(CH2)7O

(CH2)7CH3

O

COB

OO O(CH2)7O

An oil

(CH2)7CH2CH3 OCOB

OO O(CH2)7O

(CH2)7CH2CH3 OCOB

OO O(CH2)7O

(CH2)7CH2CH3 OCOB

OO O(CH2)7O

A fat

CH2

CH2

CH2O

CH2O

CH2O

O

O

O

O

O

O

O

O

O

O

O

O

CH CH

Figure 12.4The conversion of a typical oil to a fatinvolves hydrogenation. In this example,triolein (an oil) is converted to tristearin(a fat).

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12-17

EXAMPLE 12.7Writing Equations for the Halogenation of Alkenes

Write a balanced equation showing (a) the chlorination of 1-pentene and(b) the bromination of trans-2-butene.

Solution

(a) Begin by drawing the structure of 1-pentene and of diatomic chlorine (Cl2).

Knowing that one chlorine atom will form a covalent bond with each of thecarbon atoms of the carbon-carbon double bond, we can write the productand complete the equation.

(b) Begin by drawing the structure of trans-2-butene and of diatomicbromine (Br2).

Knowing that one bromine atom will form a covalent bond with each of thecarbon atoms of the carbon-carbon double bond, we can write the productand complete the equation.

Below we see an equation representing the bromination of 1-pentene. Notice thatthe solution of reactants is red because of the presence of bromine. However, theproduct is colorless (Figure 12.5).

This bromination reaction can be used to show the presence of double bondsin an organic compound. The reaction mixture is red because of the presence ofdissolved bromine. If the red color is lost, the bromine has been consumed. Thusbromination has occurred, and the compound must have had a carbon-carbon

1-Pentene(colorless)

Bromine(red)

CH3CH2CH2CHCH2

BrA

BrA

1,2-Dibromopentane(colorless)

CH3CH2CH2CHPCH2 � Br2

CPC

H3C

CH3H

HG

G D

D

trans-2-Butene

BrOBr

Bromine

� CH3OCOCOCH3

HA

HA

BrA

BrA

2,3-Dibromobutane

CPC

H3C

CH3H

HG

G D

D

trans-2-Butene

BrOBr

Bromine

�

CPC

CH3CH2CH2

HH

HG

G D

D

1-Pentene

ClOCl

Chlorine

� CH3CH2CH2OCOCOH

HA

HA

ClA

ClA

1,2-Dichloropentane

CPC

CH3CH2CH2

HH

HG

G D

D

1-Pentene

ClOCl

Chlorine

�

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double bond. The greater the amount of bromine that must be added to the reac-tion, the more unsaturated the compound is, that is, the greater the number ofcarbon-carbon double bonds.

Hydration: Addition of H2O to an AlkeneA water molecule can be added to an alkene. This reaction, termed hydration, re-quires a trace of acid (H�) as a catalyst. The product is an alcohol, as shown in thefollowing reaction:

The following equation shows the hydration of ethene to produce ethanol.

With alkenes in which the groups attached to the two carbons of the doublebond are different (unsymmetrical alkenes), two products are possible. For example:

HOCOCPCOH � HOOH HOCOCOOCOH � HOCOCOCOH

3 2 1 3 2 13 2 1

HA

HA

HA

HA

H�

HA

HA

OHA

HA

HA

HA

HA

HA

HA

HA

OHA

HA

Propene(propylene)

Major product2-Propanol(isopropyl alcohol)

Minor product1-Propanol(propyl alcohol)

H HG D

HOCOH

HA

A

CBC GD

H H

�HAOH

HA

HOCOOH

Ethene Water Ethanol(ethyl alcohol)

H�

R RG D

ROCOH

RA

A

CBC GD

R R

�HAOH

RA

ROCOOH

Alkene Water Alcohol

H�


12-18

Figure 12.5Bromination of an alkene. The solution ofreactants is red because of the presenceof bromine. When the bromine has beenused in the reaction, the solutionbecomes colorless.

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When hydration of an unsymmetrical alkene, such as propene, is carried outin the laboratory, one product (2-propanol) is favored over the other. The Russianchemist Vladimir Markovnikov studied many such reactions and came up witha rule that can be used to predict the major product of such a reaction.Markovnikov’s rule tells us that the carbon of the carbon-carbon double bond thatoriginally has more hydrogen atoms receives the hydrogen atom being added tothe double bond. The remaining carbon forms a bond with the —OH. Simplystated, “the rich get richer”—the carbon with the most hydrogens gets the new oneas well. In the preceding example, carbon-1 has two C—H bonds originally, andcarbon-2 has only one. The major product, 2-propanol, results from the new C—Hbond forming on carbon-1 and the new C—OH bond on carbon-2.

Addition of water to a double bond is a reaction that we find in several bio-chemical pathways. For instance, the citric acid cycle is a key metabolic pathwayfor the complete oxidation of the sugar glucose and the release of the majority ofthe energy used by the body. It is also the source of starting materials for the syn-thesis of the biological molecules needed for life. The next-to-last reaction in thecitric acid cycle is the hydration of a molecule of fumarate to produce a moleculecalled malate.


12-19

Folklore, Science, and Technology

For many years it was suspected that there existed a gas thatstimulated fruit ripening and had other effects on plants. Theancient Chinese observed that their fruit ripened more quicklyif incense was burned in the room. Early in this century, ship-pers realized that they could not store oranges and bananas onthe same ships because some “emanation” given off by the or-anges caused the bananas to ripen too early.

Puerto Rican pineapple growers and Philippine mangogrowers independently developed a traditional practice ofbuilding bonfires near their crops. They believed that thesmoke caused the plants to bloom synchronously.

In the mid–nineteenth century, streetlights were fueled withnatural gas. Occasionally the pipes leaked, releasing gas intothe atmosphere. On some of these occasions, the leaves fellfrom all the shade trees in the region surrounding the gas leak.

What is the gas responsible for these diverse effects onplants? In 1934, R. Gane demonstrated that the simple alkeneethylene was the “emanation” responsible for fruit ripening.More recently, it has been shown that ethylene induces andsynchronizes flowering in pineapples and mangos, inducessenescence (aging) and loss of leaves in trees, and effects a widevariety of other responses in various plants.

We can be grateful to ethylene for the fresh, unbruised fruitsthat we can purchase at the grocery store. These fruits arepicked when they are not yet ripe, while they are still firm.They then can be shipped great distances and gassed with eth-ylene when they reach their destination. Under the influence ofthe ethylene, the fruit ripens and is displayed in the store.

The history of the use of ethylene to bring fresh ripe fruitsand vegetables to markets thousands of miles from the farms isan interesting example of the scientific process and its applica-tion for the benefit of society. Scientists began with the curiousobservations of Chinese, Puerto Rican, and Filipino farmers.Through experimentation they came to understand the phe-nomenon that caused the observations. Finally, through tech-nology, scientists have made it possible to harness the power ofethylene so that grocers can “artificially” ripen the fruits andvegetables they sell to us.

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EXAMPLE 12.8

Hydration of a double bond also occurs in the �-oxidation pathway (see Section23.2). This pathway carries out the oxidation of dietary fatty acids. Like the citricacid cycle, �-oxidation harvests the energy of the food molecules to use as fuel forbody functions.

Writing Equations for the Hydration of Alkenes

Write an equation showing all the products of the hydration of 1-pentene.

Solution

Begin by drawing the structure of 1-pentene and of water and indicatingthe catalyst.

Markovnikov’s rule tells us that the carbon atom that is already bonded tothe greater number of hydrogen atoms is more likely to receive the hydrogenatom from the water molecule. The other carbon atom is more likely tobecome bonded to the hydroxyl group. Thus we can predict that the majorproduct of this reaction will be 2-pentanol and that the minor product will be1-pentanol. Now we write the equation showing the reactants and products:

Write a balanced equation for the hydration of each of the following alkenes.Predict the major product of each of the reactions.

a. CH3CHPCHCH3 c. CH3CH2CH2CHPCHCH2CH3

b. CH2PCHCH2CH2CHCH3 d. CH3CHClCHPCHCHClCH3|CH3

Write a balanced equation for the hydration of each of the following alkenes.Predict the major product of each of the reactions.

a. CH2PCHCH2CHPCH2 c. CH3CHBrCH2CHPCHCH2Cl

b. CH3CH2CH2CHPCHCH3 d. CH3CH2CH2CH2CH2CHPCHCH3

Q u e s t i o n 12.10


CPC

CH3CH2CH2

HH

HG

G D

D

1-Pentene

HOOH

Water

�H�

CH3CH2CH2OCOCOH � CH3CH2CH2OCOCOH

HA

HA

OHA

HA

HA

HA

HA

OHA

2-Pentanol(major product)

1-Pentanol(minor product)

CPC

CH3CH2CH2

HH

HG

G D

D

1-Pentene

HOOH

Water

�H�

C—H A

COO�A

Fumarase

Fumarate Malate

H—C � H2O

COO� A

HO—C—H A

COO�A

H—C—H

COO� A

A


12-20

We have seen that hydration of a doublebond requires a trace of acid as acatalyst. In the cell, this reaction iscatalyzed by an enzyme, or biologicalcatalyst, called fumarase.

Learning Goal

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Learning Goal

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EXAMPLE 12.9

Hydrohalogenation: Addition of HX to an AlkeneA hydrogen halide (HBr, HCl, or HI) also can be added to an alkene. The productof this reaction, called hydrohalogenation, is an alkyl halide:

This reaction also follows Markovnikov’s rule. That is, if HX is added to anunsymmetrical alkene, the hydrogen atom will be added preferentially to the car-bon atom that originally had the most hydrogen atoms. Consider the followingexample:

Writing Equations for the Hydrohalogenation of Alkenes

Write an equation showing all the products of the hydrohalogenation of 1-pentene with HCl.

Solution

Begin by drawing the structure of 1-pentene and of hydrochloric acid.

Markovnikov’s rule tells us that the carbon atom that is already bondedto the greater number of hydrogen atoms is more likely to receive thehydrogen atom of the hydrochloric acid molecule. The other carbon atom ismore likely to become bonded to the chlorine atom. Thus we can predict

CPC

CH3CH2CH2

HH

HG

G D

D

1-Pentene

HOCl

Hydrochloricacid

�

HOCOCPCOH � HOBr HOCOCOCOH � HOCOCOCOH

HA

HA

HA

HA

HA

HA

BrA

HA

HA

HA

HA

HA

HA

HA

BrA

HA

Propene Major product2-Bromopropane

Minor product1-Bromopropane

H HG D

HOCOH

HA

A

CBC GD

H H

�HABr

HA

HOCOBr

Ethene Hydrogen bromide Bromoethane

R RG D

ROCOH

RA

A

CBC GD

R R

�HAX

RA

ROCOX

Alkene Hydrogen halide Alkyl halide


12-21

Continued—

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Learning Goal

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that the major product of this reaction will be 2-chloropentane and that theminor product will be 1-chloropentane. Now we write the equationshowing the reactants and products:

Predict the major product in each of the following reactions. Name the alkenereactant and the product, using I.U.P.A.C. nomenclature.

a.

b. CH3CH2CHPCH2 � H2O H� ?c. CH3CHPCHCH3 � Cl2 ?d. CH3CH2CH2CHPCH2 � HBr ?

Predict the major product in each of the following reactions. Name the alkenereactant and the product, using I.U.P.A.C. names.

a.

b.

c.

d.

Addition Polymers of AlkenesPolymers are macromolecules composed of repeating structural units calledmonomers. A polymer may be made up of several thousand monomers. Manycommercially important plastics and fibers are addition polymers made fromalkenes or substituted alkenes. They are called addition polymers because theyare made by the sequential addition of the alkene monomer. The general formulafor this addition reaction follows:

CH3OCOCHPCH2 � HCl ?

CH3

A

CH3A

CH3CPCHCH3 � Br2 ?

CH3

A

CH3OCPCHCH2CH2CH3 � H2O ?

CH3

A

H�

CPC

H

HCH3

CH3

G

G D

DH2 ?�

Ni

Q u e s t i o n 12.12

CPC

H

CH3CH3

HG

G D

DH2 ?�

Pd

Q u e s t i o n 12.11

CPC

CH3CH2CH2

HH

HG

G D

D

1-Pentene

HOCl

Hydrochloricacid

� CH3CH2CH2OCOCOH � CH3CH2CH2OCOCOH

HA

HA

ClA

HA

HA

HA

HA

ClA

2-Chloropentane(major product)

1-Chloropentane(minor product)


12-22


Learning Goal

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The product of the reaction is generally represented in a simplified manner:

COC

RA

RA

AR

AR n

RA

RA

RA

RA

RA

RA

RA

RA

HeatPressure

Alkene monomerR � H, X, or an alkyl group

Addition polymer

RCPC

R

RG D

GD

R

n etc.OCOCOCOCOCOCOetc.

RA

RA

RA

RA


12-23

Life without Polymers?

What do Nike Air-Sole shoes, Saturn automobiles, disposablediapers, tires, shampoo, and artificial joints and skin share incommon? These products and a great many other items we useevery day are composed of synthetic or natural polymers. In-deed, the field of polymer chemistry has come a long way sincethe 1920s and 1930s when DuPont chemists invented nylon andTeflon.

Consider the disposable diaper. The outer, waterproof layeris composed of polyethylene. The polymerization reaction thatproduces polyethylene is shown in Section 12.4. The diapershave elastic to prevent leaking. The elastic is made of a naturalpolymer, rubber. The monomer from which natural rubber isformed is 2-methyl-1,3-butadiene. The common name of thismonomer is isoprene. As we will see in coming chapters, iso-prene is an important monomer in the synthesis of many nat-ural polymers.

The diaper is filled with a synthetic polymer called poly(acrylicacid). This polymer has the remarkable ability to absorb manytimes its own weight in liquid. Polymers that have this abilityare called superabsorbers, but polymer chemists have no ideawhy they have this property! The acrylate monomer and re-sulting poly(acrylic acid) polymer are shown here:

Another example of a useful polymer is Gore-Tex. This amaz-ing polymer is made by stretching Teflon. Teflon is producedfrom the monomer tetrafluoroethene, as seen in the followingreaction:

Clothing made from this fabric is used to protect firefighters be-cause of its fire resistance. Because it also insulates, Gore-Texclothing is used by military forces and by many amateur ath-letes, for protection during strenuous activity in the cold. In ad-dition to its use in protective clothing, Gore-Tex has been usedin millions of medical procedures for sutures, synthetic bloodvessels, and tissue reconstruction.

To learn more about the fascinating topic of polymerchemistry, visit The Macrogalleria, www.psrc.usm.edu/macrog/index.html, an Internet site maintained by the Department ofPolymer Science of the University of Southern Mississippi.

nCH2PCOCHPCH2 CH2OCPCHOCH2 n

CH3A

CH3A

2-Methyl-1, 3,-butadiene(isoprene)

Rubber polymer

n CPC CH2OCH

nCPOA

Acrylate monomer Poly(acrylic acid)

OA

HA

H

HG D

H

GD

CPO

OA

HA

CPC

Tetrafluoroethene Teflon

F

FG D

F

GD

F

n COC

FA

FA

AF

AF n

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Polyethylene is a polymer made from the monomer ethylene (ethene):

It is used to make bottles, injection-molded toys and housewares, and wirecoverings.

Polypropylene is a plastic made from propylene (propene). It is used to makeindoor-outdoor carpeting, packaging materials, toys, and housewares. When

nCH2PCH2 CH2OCH2 n

Ethene(ethylene)

Polyethylene


12-24

Plastic Recycling

Plastics, first developed by British inventor Alexander Parkesin 1862, are amazing substances. Some serve as containers formany of our foods and drinks, keeping them fresh for long pe-riods of time. Other plastics serve as containers for detergentsand cleansers or are formed into pipes for our plumbing sys-tems. We have learned to make strong, clear sheets of plasticthat can be used as windows, and feather-light plastics that canbe used as packaging materials. In the United States alone,seventy-five billion pounds of plastics are produced each year.

But plastics, amazing in their versatility, are a mixed bless-ing. One characteristic that makes them so useful, their stabil-ity, has created an environmental problem. It may take forty tofifty years for plastics discarded into landfill sites to degrade.Concern that we could soon be knee-deep in plastic worldwidehas resulted in a creative new industry: plastic recycling.

Since there are so many types of plastics, it is necessary toidentify, sort, and recycle them separately. To help with thissorting process, manufacturers place recycling symbols on theirplastic wares. As you can see in the accompanying table, eachsymbol corresponds to a different type of plastic.

Polyethylene terephthalate, also known as PETE or simply#1, is a form of polyester often used to make bottles and jars tocontain food. When collected, it is ground up into flakes andformed into pellets. The most common use for recycled PETE isthe manufacture of polyester carpets. But it may also be spuninto a cotton-candy-like form that can be used as a fiber fillingfor pillows or sleeping bags. It may also be rolled into thinsheets or ribbons and used as tapes for VCRs or tape decks.Reuse to produce bottles and jars is also common.

HDPE, or #2, is high-density polyethylene. Originally usedfor milk and detergent bottles, recycled HDPE is used to pro-duce pipes, plastic lumber, trash cans, or bottles for storage ofmaterials other than food. LDPE, #4, is identical to HDPEchemically, but it is produced in a less-dense, more flexibleform. Originally used to produce plastic bags, recycled LDPE isalso used to make trash bags, grocery bags, and plastic tubingand lumber.

PVC, or #3, is one of the less commonly recycled plastics inthe United States, although it is actively recycled in Europe. Therecycled material is used to make non-food-bearing containers,shoe soles, flooring, sweaters, and pipes. Polypropylene, PP or

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propylene polymerizes, a methyl group is located on every other carbon of themain chain:

or

CH2OCHOCH2OCHOCH2OCH

CH3ACH3A

CH3A

nCH2PCH CH2OCH n

CH3ACH3A


12-25

#5, is found in margarine tubs, fabrics, and carpets. Recycledpolypropylene has many uses, including fabrication of garden-ing implements.

You probably come into contact with polystyrene, PS or #6,almost every day. It is used to make foam egg cartons and meattrays, serving containers for fast food chains, CD “jewel boxes,”

and “peanuts” used as packing material. At the current time,polystyrene food containers are not recycled. PS from nonfoodproducts can be melted down and converted into pellets thatare used to manufacture office desktop accessories, hangers,video and audio cassette housings, and plastic trays used tohold plants.

Code Type Name Formula Description Examples

PETE Polyethylene Usually clear Soda bottles, peanutterephthalate —CH2—CH2—O—C C—O— or green, rigid butter jars, vegetable

oil bottles

HDPE High-density —CH2—CH2— Semirigid Milk and waterpolyethylene jugs, juice and bleach

bottlesPVC Polyvinyl —CH—CH2— Semirigid Detergent and

chloride cleanser bottles,pipes

LDPE Low-density —CH2—CH2— Flexible, Six-pack rings, polyethylene not crinkly bread bags, sandwich

bagsPP Polypropylene —CH—CH2— Semirigid Margarine tubs, straws,

screw-on lids

PS Polystyrene —CH—CH2— Often brittle Styrofoam, packing peanuts, egg cartons, foam cups

Other Multilayer N/A Squeezable Ketchup and syrup plastics bottles

O OBO

OB

|Cl

|CH3

O

1

2

3

4

5

6

7

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Polymers made from alkenes or substituted alkenes are simply very largealkanes or substituted alkanes. Like the alkanes, they are typically inert. Thischemical inertness makes these polymers ideal for making containers to holdjuices, chemicals, and fluids used medically. They are also used to make sutures,catheters, and other indwelling devices. A variety of polymers made from substi-tuted alkenes are listed in Table 12.1.

12.6 Aromatic HydrocarbonsIn the early part of the nineteenth century chemists began to discover organic com-pounds having chemical properties quite distinct from the alkanes, alkenes, andalkynes. They called these substances aromatic compounds because many of the firstexamples were isolated from the pleasant-smelling resins of tropical trees. The car-bon:hydrogen ratio of these compounds suggested a very high degree of unsatu-ration, similar to the alkenes and alkynes. Imagine, then, how puzzled these earlyorganic chemists must have been when they discovered that these compounds donot undergo the kinds of addition reactions common for the alkenes and alkynes.

We no longer define aromatic compounds as those having a pleasant aroma; infact, many do not. We now recognize aromatic hydrocarbons as those that exhibita much higher degree of chemical stability than their chemical composition wouldpredict. The most common group of aromatic compounds is based on the six-member aromatic ring, the benzene ring. The structure of the benzene ring is rep-resented in various ways in Figure 12.6.

No reactionC C

C C

C

C D

J

BA

M

G DH

DH HG

HG

� Br2

AH

AH

CH2PCH2 � Br2 CH2OCH2

BrA

BrA


12-26

Table 12.1 Some Important Addition Polymers of Alkenes

Monomer name Formula Polymer Uses

Styrene CH2PCH Polystyrene Styrofoam containers

Acrylonitrile CH2PCHCN Polyacrylonitrile Clothing (Orlon)

OB

Methyl methacrylate CH2PC(CH3)—COCH3 Polymethyl Basketballmethacrylate backboards(Plexiglas, Lucite)

Vinyl chloride CH2PCHCl Polyvinyl chloride Plastic pipe, (PVC) credit cards

Tetrafluoroethene CF2PCF2 Polytetrafluoro- Nonstickethylene (Teflon) surfaces

O

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Structure and PropertiesThe benzene ring consists of six carbon atoms joined in a planar hexagonalarrangement. Each carbon atom is bonded to one hydrogen atom. FriedrichKekulè proposed a model for the structure of benzene in 1865. He proposed thatsingle and double bonds alternated around the ring (a conjugated system of dou-ble bonds). To explain why benzene did not decolorize bromine—in other words,didn’t react like an unsaturated compound—he suggested that the double and sin-gle bonds shift positions rapidly. We show this as a resonance model today.

The current model of the structure of benzene is based on the idea of overlap-ping orbitals. Each carbon is bonded to two others by sharing a pair of electrons (�bonds). Each carbon atom also shares a pair of electrons with a hydrogen atom.The remaining six electrons are located in p orbitals that are perpendicular to theplane of the ring. These p orbitals overlap laterally to form pi (�) orbitals that forma cloud of electrons above and below the ring. These � orbitals are shaped likedoughnuts, as shown in Figure 12.7.

Two symbols are commonly used to represent the benzene ring. The represen-tation in Figure 12.6b is the structure proposed by Kekulé. The structure in Figure12.6d represents the � clouds.

The equal sharing of the six electrons of the p orbitals results in a rigid, flat ringstructure, in contrast to the relatively flexible, nonaromatic cyclohexane ring. Themodel also explains the unusual chemical stability of benzene and its resistance toaddition reactions. The electrons of the � cloud are said to be delocalized. Thatmeans they have much more space and freedom of movement than they wouldhave if they were restricted to individual double bonds. Because electrons repelone another, the system is more stable when the electrons have more space to oc-cupy. As a result, benzene is unusually stable and resists addition reactions typicalof alkenes.

Benzene as a resonance hybrid

A

DH

DH HG

HG

H

AH

A

DH

DH HG

HG

H

AH

12.6 Aromatic Hydrocarbons 351

12-27

Figure 12.6Four ways to represent the benzenemolecule. Structure (b) is a simplifieddiagram of structure (a). Structure (d), asimplified diagram of structure (c), is themost commonly used representation.

H

C

(a)

(b)

(c)

(d)

H

H

HH

H

H H

H

HH

H

C

C

CC

C

C C

C

C C

C

Resonance models are described in Section 4.4.

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NomenclatureMost simple aromatic compounds are named as derivatives of benzene. Thus ben-zene is the parent compound, and the name of any atom or group bonded to ben-zene is used as a prefix, as in these examples:

Other members of this family have unique names based on history rather thanlogic:

When two groups are present on the ring, three possible orientations exist, andthey may be named by either the I.U.P.A.C. Nomenclature System or the commonsystem of nomenclature. If the groups or atoms are located on two adjacent carbons,

meta-Xylene

ACH3

meta-Cresol

AOH

para-Xylene

ACH3

para-Cresol

AOH

GCH3

GCH3 A

CH3

ACH3

Benzoic acid

ACOOHBO

Benzaldehyde

ACOHBO

ortho-Xylene

ACH3

ortho-Cresol

AOH

DCH3 D

CH3

Toluene Phenol Aniline

ACH3 A

OHANH2

Anisole

AOCH3

Nitrobenzene Ethylbenzene Bromobenzene

ANO2 A

CH2CH3 ABr


12-28

Figure 12.7The current model of the bonding inbenzene.

C CH H

H H

HHC C

C C

π cloud

σ bond

C CHH

H H

H H

C CC C

p orbitals

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EXAMPLE 12.11

ACH3 A

NO2

DCl A

CH2CH3

AOH

GNH2

EXAMPLE 12.10

they are referred to as ortho (o) in the common system or with the prefix 1,2- in theI.U.P.A.C. system. If they are on carbons separated by one carbon atom, they aretermed meta (m) in the common system or 1,3- in the I.U.P.A.C. system. Finally, if thesubstituents are on carbons separated by two carbon atoms, they are said to be para(p) in the common system or 1,4- in the I.U.P.A.C. system. The following examplesdemonstrate both of these systems:

If three or more groups are attached to the benzene ring, numbers must be usedto describe their location. The names of the substituents are given in alphabeticalorder.

Naming Derivatives of Benzene

Name the following compounds using the I.U.P.A.C. Nomenclature System.

a. b. c.

Solution

Parentcompound: toluene phenol anilineSubstituents: 2-chloro 4-nitro 3-ethylName: 2-Chlorotoluene 4-Nitrophenol 3-Ethylaniline

Naming Derivatives of Benzene

Name the following compounds using the common system of nomenclature.

a. b. c.

Solution

Parent compound: toluene phenol anilineSubstituents: ortho-chloro para-nitro meta-ethyl

Two groups 1,2 or ortho

AG

Two groups 1,3 or metaG � Any group

AG

DG

GG

Two groups 1,4 or para

AG

AG


12-29

ACH3 A

NO2

DCl A

CH2CH3

AOH

GNH2

Continued—

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Name: ortho-Chlorotoluene para-Nitrophenol meta-EthylanilineAbbreviatedName: o-Chlorotoluene p-Nitrophenol m-Ethylaniline

In I.U.P.A.C. nomenclature, the group—C6H5 derived by removing one hydro-gen from benzene, is called the phenyl group. An aromatic hydrocarbon with analiphatic side chain is named as a phenyl substituted hydrocarbon. For example:

One final special name that occurs frequently in aromatic compounds is thebenzyl group:

The use of this group name is illustrated by:

Draw each of the following compounds:

a. 1,3,5-Trichlorobenzeneb. ortho-Cresolc. 2,5-Dibromophenold. para-Dinitrobenzenee. 2-Nitroanilinef. meta-Nitrotoluene

Draw each of the following compounds:

a. 2,3-Dichlorotoluene d. o-Nitrotolueneb. 3-Bromoaniline e. p-Xylenec. 1-Bromo-3-ethylbenzene f. o-Dibromobenzene

Reactions Involving BenzeneAs we have noted, benzene does not readily undergo addition reactions. The typ-ical reactions of benzene are substitution reactions, in which a hydrogen atom isreplaced by another atom or group of atoms.

Q u e s t i o n 12.14

Q u e s t i o n 12.13

Benzyl chloride Benzyl alcohol

OCH2Cl OCH2OH

OCH2OorC6H5CH2O

ACH3CHCH2CH3 A

CH3CHCHPCH2

2-Phenylbutane 3-Phenyl-1-butene


12-30


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Benzene can react (by substitution) with Cl2 or Br2. These reactions require ei-ther iron or an iron halide as a catalyst. For example:

FeCl3C C

C C

C

C D

D

AA

G

G DH

DH HG

HG

� Cl2

HA

HA

Benzene

C C

C C

C

C D

D

AA

G

G DCl

DH HG

HG

� HCl

HA

HA

ChlorobenzeneChlorine


12-31

Aromatic Compounds and Carcinogenesis

We come into contact with many naturally occurring aro-matic compounds each day. Originally, the name aromatic wasgiven to these compounds because of the pleasant aromas ofsome members of this family. Indeed, many food flavoringsand fragrances that we enjoy contain aromatic rings. Examplesof other aromatic compounds include preservatives (such asBHT, butylated hydroxytoluene), insecticides (such as DDT),pharmaceutical drugs (such as aspirin), and toiletries.

The polynuclear aromatic hydrocarbons (PAH) are an impor-tant family of aromatic hydrocarbons that generally have toxiceffects. They have also been shown to be carcinogenic, that is,they cause cancer. PAH are formed from the joining of the ringsso that they share a common bond (edge). Three common ex-amples are shown:

The more complex members of this family (typically con-sisting of five or six rings at a minimum) are among the mostpotent carcinogens known. It has been shown that the carcino-genic nature of these chemicals results from their ability to bindto the nucleic acid (DNA) in cells. As we will see in Chapter 24,the ability of the DNA to guide the cell faithfully from genera-tion to generation is dependent on the proper expression of thegenetic information, a process called transcription, and the ac-curate copying or replication of the DNA. Accurate DNA repli-cation is essential so that every new cell inherits a completecopy of all the genetic information that is identical to that of theoriginal parent cell. If a mistake is made in the DNA replicationprocess, the result is an error, or mutation, in the new DNAmolecule. Some of these errors may cause the new cell to growout of control, resulting in cancer.

Polynuclear aromatic hydrocarbons are thought to causecancer by covalently binding to the DNA in cells and interfer-ing with the correct replication of the DNA. Some of the muta-tions that result may cause a cell to begin to divide in anuncontrolled fashion, giving rise to a cancerous tumor.

Benzopyrene (shown below) is found in tobacco smoke,smokestack effluents, charcoal-grilled meat, and automobileexhaust. It is one of the strongest carcinogens known. It is esti-mated that a wide variety of all cancers are caused by chemicalcarcinogens, such as PAH, in the environment.

Naphthalene

Anthracene

Phenanthrene Benzopyrene

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When a second equivalent of the halogen is added, three isomers—para, ortho, andmeta—are formed.

Benzene also reacts with sulfur trioxide by substitution. Concentrated sulfuricacid is required as the catalyst. Benzenesulfonic acid, a strong acid, is the product:

Benzene can also undergo nitration with concentrated nitric acid dissolved inconcentrated sulfuric acid. This reaction requires temperatures in the range of50–55�C.

12.7 Heterocyclic Aromatic CompoundsHeterocyclic aromatic compounds are those having at least one atom other thancarbon as part of the structure of the aromatic ring. The structures and commonnames of several heterocyclic aromatic compounds are shown:

All these compounds are more similar to benzene in stability and chemical behav-ior than they are to the alkenes. Many of these compounds are components of mol-ecules that have significant effects on biological systems. For instance, the purinesand pyrimidines are components of DNA (deoxyribonucleic acid) and RNA (ri-bonucleic acid). DNA and RNA are the molecules responsible for storing and ex-pressing the genetic information of an organism. The pyridine ring is found in

Imidazole Furan Pyrrole

H

NA

H

NA

N

O

Pyridine Pyrimidine Purine

N N

N

N

N

N

HA

N

Concentrated H2SO4

50–55°C

Benzene Nitrobenzene

� HNO3 ONO2 � H2O

Nitric acid

Concentrated H2SO4

Benzene Benzenesulfonic acid

� SO3 OSOOH � H2O

Sulfur trioxide

OB

OB

Benzene Bromobenzene

� Br2 OBr � HBr

Bromine

FeBr3


12-32

See the Chemistry Connection: TheNicotine Patch in Chapter 16.

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nicotine, the addictive compound in tobacco. The pyrrole ring is a component ofthe porphyrin ring found in hemoglobin and chlorophyll.

The imidazole ring is a component of cimetidine, a drug used in the treatment ofstomach ulcers. The structure of cimetidine is shown below:

We will discuss a subset of the heterocyclic aromatic compounds, the heterocyclicamines, in Chapter 16.

Cimetidine

CH2SCH2CH2NHCNHCH3

NCNB

CH3DNG

H

N G

Porphyrin

Fe3�D

N

DN

”N

”N

Summary of Reactions 357

12-33

Summary of ReactionsAddition reactions of alkenes

Hydrogenation: Halogenation:

Hydration: Hydrohalogenation:

R RG D

ROCOH

RA

A

CBC GD

R R

�HAH

RA

ROCOH Pt, Pd, or NiHeat or pressure

Alkene Hydrogen Alkane

R RG D

ROCOX

RA

A

CBC GD

R R

�XAX

RA

ROCOX

Alkene Halogen Alkyl dihalide

R RG D

ROCOH

RA

A

CBC GD

R R

�HAOH

RA

ROCOOH

Alkene Water Alcohol

H�

R RG D

ROCOH

RA

A

CBC GD

R R

�HAX

RA

ROCOX

Alkene Hydrogen halide Alkyl halide

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12-34

Addition Polymers of Alkenes

Reactions of Benzene

Halogenation:

Sulfonation:

Nitration:

CPC

Alkene monomer Addition polymer

R

RG D

R

GD

R

n COC

RA

RA

AR

AR n

Summary

12.1 Alkenes and Alkynes: Structure andPhysical Properties

Alkenes and alkynes are unsaturated hydrocarbons. Alkenesare characterized by the presence of at least one carbon-carbon double bond and have the general molecular for-mula CnH2n. Alkynes are characterized by the presence ofat least one carbon-carbon triple bond and have the generalmolecular formula CnH2n�2. The physical properties of thealkenes and alkynes are similar to those of alkanes, buttheir chemical properties are quite different.

12.2 Alkenes and Alkynes: NomenclatureAlkenes and alkynes are named by identifying the parentcompound and replacing the -ane ending of the alkanewith -ene (for an alkene) or -yne (for an alkyne). The parentchain is numbered to give the lowest number to the first ofthe two carbons involved in the double bond (or triplebond). Finally, all groups are named and numbered.

12.3 Geometric Isomers: A Consequence ofUnsaturation

The carbon-carbon double bond is rigid. This allows theformation of geometric isomers, or isomers that differ from

one another depending on whether chemical groups are onthe same or opposite sides of the rigid double bonds. Whengroups are on the same side of a double bond, the prefix cisis used to describe the compound. When groups are on op-posite sides of a double bond, the prefix trans is used.

12.4 Alkenes in NatureAlkenes and polyenes (alkenes with several carbon-carbondouble bonds) are common in nature. Ethene, the simplestalkene, is a plant growth substance involved in fruit ripen-ing, senescence and leaf fall, and responses to environmen-tal stresses. Isoprenoids, or terpenes, are polyenes builtfrom one or more isoprene units. Isoprenoids includesteroids, chlorophyll and other photosynthetic pigments,and vitamins A, D, and K.

12.5 Reactions Involving AlkenesWhereas alkanes undergo substitution reactions, alkenes andalkynes undergo addition reactions. The principal additionreactions of the unsaturated hydrocarbons are halogenation,hydration, hydrohalogenation, and hydrogenation. Polymerscan be made from alkenes or substituted alkenes.

12.6 Aromatic HydrocarbonsAromatic hydrocarbons contain benzene rings. The ringscan be represented as having alternating double and single

Benzene Halobenzene

� X2 OX � HX

Halogen

FeX3

Concentrated H2SO4

Benzene

� SO3

Sulfur trioxide

Benzenesulfonic acid

OSOOH � H2O

OB

OB

Concentrated H2SO4

50–55°C

Benzene

� HNO3

Nitric acid

Nitrobenzene

ONO2 � H2O

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bonds. However, it is more accurate to portray � (sigma)bonds between carbons of the ring and a � (pi) cloud ofelectrons above and below the ring. Simple aromatic com-pounds are named as derivatives of benzene. Several mem-bers of this family have historical common names, such asaniline, phenol, and toluene. Aromatic compounds do notundergo addition reactions. The typical reactions of ben-zene are substitution reactions: halogenation, nitration, andsulfonation.

12.7 Heterocyclic Aromatic CompoundsHeterocyclic aromatic compounds are those having at leastone atom other than carbon as part of the structure of thearomatic ring. They are more similar to benzene in stabilityand chemical behavior than they are to the alkenes. Manyof these compounds are components of molecules thathave significant effects on biological systems, includingDNA, RNA, hemoglobin, and nicotine.

Key Terms

Questions and Problems

Alkenes and Alkynes: Structure and Physical Properties

12.15 Write the general formulas for alkanes, alkenes, and alkynes.12.16 What are the characteristic functional groups of alkenes and

alkynes?12.17 Describe the geometry of ethene.12.18 What are the bond angles in ethene?12.19 Describe the geometry of ethyne.12.20 What are the bond angles in ethyne?

Alkenes and Alkynes: Nomenclature and Geometric Isomers

12.21 Draw a condensed formula for each of the followingcompounds:a. 2-Methyl-2-hexeneb. trans-3-Heptenec. cis-1-Chloro-2-pentened. cis-2-Chloro-2-methyl-3-heptenee. trans-5-Bromo-2,6-dimethyl-3-octene

12.22 Draw a condensed formula for each of the followingcompounds:a. 2-Hexyneb. 4-Methyl-1-pentyne

c. 1-Chloro-4,4,5-trimethyl-2-heptyned. 2-Bromo-3-chloro-7,8-dimethyl-4-decyne

12.23 Name each of the following using the I.U.P.A.C.Nomenclature System:a. CH3CH2CHCHPCH2

|CH3

b. CH2CH2CH2CH2—Br|CH2CHPCH2

c. CH3CH2CHPCHCHCH2CH3|Br

d.

e. CH3CHCH2CHPCCH3| |CH3 CH3

f. Cl—CH2CHCqC—H|CH3

g. CH3CHCH2CH2CH2—CqC—H|Cl

h.

12.24 Draw each of the following compounds using condensedformulas:a. 1,3,5-Trifluoropentaneb. cis-2-Octenec. Dipropylacetylened. 3,3,5-Trimethyl-1-hexenee. 1-Bromo-3-chloro-1-heptyne

12.25 Of the following compounds, which can exist as cis-transgeometric isomers? Draw the two geometric isomers.a. 2,3-Dibromobutaneb. 2-Heptenec. 2,3-Dibromo-2-butened. Propenee. 1-Bromo-1-chloro-2-methylpropenef. 1,1-Dichloroetheneg. 1,2-Dibromoetheneh. 3-Ethyl-2-methyl-2-hexene

12.26 Which of the following alkenes would not exhibit cis-transgeometric isomerism?a.

b.

c.

d. CH3

CPCD

HD

CH2CH2CH3

G

CH3CH2G

CHCH3

CPCD

CH3

DCHCH2CH3

G

CH3CH2G

CH2CH3A

CH3

A

CH3

CPCD

CH3

DH

G

CH3G

CH3

CPCD

HD

CH2CH3

G

CH3G

ClD

BrG

OCH3CH3OCO

CH3A

CH3

A

Questions and Problems 359

12-35

addition polymer (12.5)addition reaction (12.5)alkene (12.1)alkyne (12.1)aromatic

hydrocarbon (12.6)geometric isomers (12.3)halogenation (12.5)heterocyclic aromatic

compound (12.7)

hydration (12.5)hydrogenation (12.5)hydrohalogenation (12.5)Markovnikov’s rule (12.5)monomer (12.5)phenyl group (12.6)polymer (12.5)substitution reaction (12.6)unsaturated hydrocarbon

(Intro)

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12.27 Which of the following structures have incorrect I.U.P.A.C.names? If incorrect, give the correct I.U.P.A.C. name.a.

b.

c.

d.

e.

12.28 Which of the following can exist as cis and trans isomers?a. H2CPCH2 d. ClBrCPCClBrb. CH3CHPCHCH3 e. (CH3)2CPC(CH3)2c. Cl2CPCBr2

12.29 Provide the I.U.P.A.C. name for each of the followingmolecules:a. CH2PCHCH2CH2CHPCHCH2CH2CH3b. CH2PCHCH2CHPCHCH2CHPCHCH3c. CH3CHPCHCH2CHPCHCH2CH3d. CH3CHPCHCHCHPCHCH3

|CH3

12.30 Provide the I.U.P.A.C. name for each of the followingmolecules:a. CH3CPCHCHCHPCCH3

| | |Br CH3 Br

b.

CH3 CH2CH3| |

CH2PCHCHCHPCHCHCH2CH3

c.

CH3 CH3| |

CH2PCHCCHPCHCH2CHPCHCHCH3|CH3

d.

CH2CH3 CH2CH3| |

CH3CHCHCHPCHCH2CHPCHCHCH3|CH3

Reactions Involving Alkenes

12.31 How could you distinguish between a sample of cyclohexaneand a sample of hexene (both C6H12) using a simple chemicaltest? (Hint: Refer to the subsection entitled “Halogenation:Addition of X2 to an Alkene.”)

12.32 Quantitatively, 1 mol of Br2 is consumed per mole of alkene,and 2 mol of Br2 are consumed per mole of alkyne. Howmany moles of Br2 would be consumed for 1 mol of each ofthe following:a. 2-Hexyneb. Cyclohexenec.

d.

12.33 Complete each of the following reactions by supplying themissing reactant or product(s) as indicated by a question mark:a. CH3CH2CHPCHCH2CH3 � ?

CH3CH2CH2CH2CH2CH3

b.

c.

d. 2CH3CH2CH2CH2CH2CH3 � ?O2 Heat ? � ?

(completecombustion)

e.

f.

12.34 Draw and name the product in each of the followingreactions:a. Cyclopentene � H2O (H�)b. Cyclopentene � HClc. Cyclopentene � H2d. Cyclopentene � HI

12.35 A hydrocarbon with a formula C5H10 decolorized Br2 andconsumed 1 mol of hydrogen upon hydrogenation. Draw allthe isomers of C5H10 that are possible based on the aboveinformation.

12.36 Triple bonds react in a manner analogous to that of doublebonds. The extra pair of electrons in the triple bond, however,generally allows 2 mol of a given reactant to add to the triplebond in contrast to 1 mol with the double bond. The “rich getricher” rule holds. Predict the major product in each of thefollowing reactions:a. Acetylene with 2 mol HClb. Propyne with 2 mol HBrc. 2-Butyne with 2 mol HI

12.37 Complete each of the following by supplying the missingproduct indicated by the question mark:

a. 2-Butene HBr

?

b. 3-Methyl-2-hexene HI

?c.

? HCl

? H2O, H�

OHA

? � � HClClO

? �

BrD

HG

CH2

BCH3OCOCH3 � ? CH3COOH

CH3

A

CH3A

—CqC—CH3

OCHPCH2

1-Chloro-5-methyl-2-hexene

H

CPCD

HD

CHCH2CH3

G

CICH2 G CH3A

trans-6-Chloro-3-heptene

CH2CHCH3

CPCD

HD

HG

CH3CH2G

ClA

CH3

ACH3CHCH2OCqCOCH2CHCH3

2-Ethyl-7-methyl-4-octyne

CH2CH3A

3-Ethyl-3-hexyne

CH2CH3

CPCD

CH3CH2

DH

G

CH3CH2G

CH3

ACH3CqCOCH2CHCH3

2-Methyl-4-hexyne


12-36

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12.38 Bromine is often used as a laboratory spot test forunsaturation in an aliphatic hydrocarbon. Bromine in CCl4 isred. When bromine reacts with an alkene or alkyne, the alkylhalide formed is colorless; hence a disappearance of the redcolor is a positive test for unsaturation. A student tested thecontents of two vials, A and B, both containing compoundswith a molecular formula, C6H12. Vial A decolorized bromine,but vial B did not. How may the results for vial B beexplained? What class of compound would account for this?

12.39 What is meant by the term polymer?12.40 What is meant by the term monomer?12.41 Write an equation representing the synthesis of Teflon from

tetrafluoroethene. (Hint: Refer to Table 12.1.)12.42 Write an equation representing the synthesis of polystyrene.

(Hint: Refer to Table 12.1.)12.43 Provide the I.U.P.A.C. name for each of the following

molecules. Write a balanced equation for the hydration of each.a. CH3CHPCHCH2CH3b. CH2CHPCH2

|Br

c.

12.44 Provide the I.U.P.A.C. name for each of the followingmolecules. Write a balanced equation for the hydration of each.a.

b. CH3CHPCHCH2CHPCHCH2CHPCHCH3

CH2CH3|

c. CH3CHPCHCCH3|CH3

12.45 Write an equation for the addition reaction that producedeach of the following molecules:

CH3|

a. CH2CH2CH2CHCH3|OH

b. CH3CH2CHCH2CH2CH3|Br

c.

d.

12.46 Write an equation for the addition reaction that producedeach of the following molecules:

OH|

a. CH3CH2CHCHCH3|CH2CH3

b. CH3CHCH2CH3|OH

c.

d.

12.47 Draw the structure of each of the following compounds andwrite a balanced equation for the complete hydration of each:a. 1,4-Hexadieneb. 2,4,6-Octatrienec. 1,3 Cyclohexadiened. 1,3,5-Cyclooctatriene

12.48 Draw the structure of each of the following compounds andwrite a balanced equation for the hydrobromination of each:a. 3-Methyl-1,4-hexadieneb. 4-Bromo-1,3-pentadienec. 3-Chloro-2,4-hexadiened. 3-Bromo-1,3-Cyclohexadiene

Aromatic Hydrocarbons

12.49 Draw the structure for each of the following compounds:a. 2,4-Dibromotolueneb. 1,2,4-Triethylbenzenec. Isopropylbenzened. 2-Bromo-5-chlorotoluene

12.50 Name each of the following compounds, using the I.U.P.A.C.system.a.

b. d.

c. e.

12.51 Draw each of the following compounds, using condensedformulas:a. meta-Cresolb. Propylbenzenec. 1,3,5-Trinitrobenzened. m-Chlorotoluene

12.52 Draw each of the following compounds, using condensedformulas:a. p-Xyleneb. Isopropylbenzenec. m-Nitroanisoled. p-Methylbenzaldehyde

ACH3

G DNO2O2N

ABr

ACH2CH3

ACH3

ABr

GCl

DCH3

ANO2

DNO2

ACH3

DCH3

Br

OHG

GD

OH

DBr

HO

CH3

G

G DCH3

OCH2CH3

OOH

Br

CH3

G

G

OCH2CH3

DCH3

CH3

G

Questions and Problems 361

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12.53 Describe the Kekulé model for the structure of benzene.12.54 Describe the current model for the structure of benzene.12.55 How does a substitution reaction differ from an addition

reaction?12.56 Give an example of a substitution reaction and of an addition

reaction.12.57 Write an equation showing the reaction of benzene with Cl2

and FeCl3.12.58 Write an equation showing the reaction of benzene with SO3.

Be sure to note the catalyst required.

Heterocyclic Aromatic Compounds

12.59 Draw the general structure of a pyrimidine.12.60 What biological molecules contain pyrimidine rings?12.61 Draw the general structure of a purine.12.62 What biological molecules contain purine rings?

Critical Thinking Problems

1. There is a plastic polymer called polyvinylidene difluoride(PVDF) that can be used to sense a baby’s breath and thus beused to prevent sudden infant death syndrome (SIDS). Thesecret is that this polymer can be specially processed so that itbecomes piezoelectric (produces an electrical current when it isphysically deformed) and pyroelectric (develops an electricalpotential when its temperature changes). When a PVDF film isplaced beside a sleeping baby, it will set off an alarm if the babystops breathing. The structure of this polymer is shown here:

Go to the library and investigate some of the other amazinguses of PVDF. Draw the structure of the alkene from which thiscompound is produced.

2. Isoprene is the repeating unit of the natural polymer rubber. Itis also the starting material for the synthesis of cholesterol andseveral of the lipid-soluble vitamins, including vitamin A andvitamin K. The structure of isoprene is seen below.

What is the I.U.P.A.C. name for isoprene?3. When polyacrylonitrile is burned, toxic gases are released. In

fact, in airplane fires, more passengers die from inhalation oftoxic fumes than from burns. Refer to Table 12.1 for thestructure of acrylonitrile. What toxic gas would you predict tobe the product of the combustion of these polymers?

4. If a molecule of polystyrene consists of 25,000 monomers, whatis the molar mass of the molecule?

5. A factory produces one million tons of polypropylene. Howmany moles of propene would be required to produce thisamount? What is the volume of this amount of propene at25�C and 1 atm?

CH2PCOCHPCH2

CH3A

OCOCOCOCO

FA

FA

HA

HA

FA

FA

HA

HA


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The Unsaturated Hydrocarbons: Alkenes, Alkynes, and Aromatics