Chapter 20 Unsaturated Hydrocarbons Name_________________Unsaturated hydrocarbons enhance our lives in many ways:

The unsaturated hydrocarbons consist of three families of homologous compounds that contain multiple bonds between carbon atoms.

Alkenes

Alkynes

Aromatic compounds

The four orbitals available for bonding in alkenes are three sp2 orbitals and one p orbital.

Figure 20.1: Schematic hybridization of 2s22px

12py1 orbitals

of carbon to form three sp2 electron orbitals and one

p electron orbital

Figure 20.2 (a) A single sp2 electron orbital and (b) a side view of three sp2 orbitals all lying in the same plane with a p orbital

perpendicular to the three sp2 orbitals.

The carbon-carbon pi () bond is much weaker and, as a consequence, much more reactive than the carbon-carbon sigma () bond.

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The formation of a triple bond between carbon atoms, as in acetylene, CHCH, may be visualized as shown below.

• These pi bond electrons are not as tightly held by the carbon nuclei as the sigma bond electrons. Acetylene, consequently, is a very reactive substance.

Nomenclature of AlkenesThe general formula for alkenes is:

IUPAC Rules for Naming Alkenes1. Select the longest continuous carbon-carbon chain that contains the double bond.

EX:

2. Name this parent compound as you would an alkane, but change the –ane ending to –ene.EX:

3. Number the carbon chain of the parent compound starting with the end nearer to the double bond. Use the smaller of the two numbers on the double-bonded carbon atoms to indicate the position of the double bond. Place this number in front of the alkene name.EX:

4. Branch chains and other groups are treated as in naming alkanes, by numbering and assigning them to the carbon atom to which they are bonded.EX:

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How would we write the structural formula for 4-methyl-2-pentene?

How would we write the structural formula for 4-methyl-2-pentene?

Write a structural formula for 7-methyl-2-octene

Name these compounds:

Cycloalkenes The carbons of the double bond are assigned numbers 1 and 2

3

H3C

H2C

C

H2C

CH2

CH3

CH2

1

2

1

2

CH3

CH3

CH3

3

4

5 4

5

6

3

1-methylcyclopentene

1,3-dimethylcyclohexene

1

2

1

2

CH3

CH3

CH3

3

4

5 4

5

6

3

1-methylcyclopentene

1,3-dimethylcyclohexene

Dienes

Trienes

conjugated vs cumulated

Common Names

Smaller alkenes:

ethylene propylene isobutylene

Substituted alkenes

vinyl allyl

Disubstituted alkenes (bivalent groups)

Ethylene dichloride propylene dichloride isobutylene dichloride

4

H2C

HC

CH

CH2

1,3-butadiene

Geometric Isomerism in Alkenes

Compounds containing a carbon-carbon double bond (pi bond) have

This restricted rotation in a molecule gives rise to a type of isomerism known as

Isomers that differ from each other only in the geometry of their molecules and not in the order of their atoms are known as geometric isomers.

– They are also called cis-trans isomers.

To be “cis”,

To be “trans”,

** An alkene shows cis-trans isomerism when each carbon atom of the double bond has two different kinds of groups attached to it.

**An alkene does not show cis-trans isomerism if one carbon of the double bond has two identical groups attached to it.

Draw a structure for cis-5-chloro-2-hexene

Is the compound shown the cis or trans isomer?

5

Cl

C

H

C

H

Cl H

C

Cl

C

H

Cl

cis-1,2-dichloroethene(bp = 60.1 C)

trans-1,2-dichloroethene(bp = 48.4 C)

H

C C

CH3

H2C

H3C

CH3

Draw out 1-bromo-2-chloropropene. Does it exhibit geometric isomerism?

Z vs E is a more general way of determining geometric isomers.

Z comes from “zusammen” meaning

E comes from “entgegen” meaning

Priorities: Groups around the double bond are assigned priority, based on atomic numbers/massRules:1. Higher the atomic number,

2. If isotopes, the higher the mass,

3. If first attached atom is same, keep going

Preparation of Alkenes (ie, an alkene is a product of each)1. Cracking

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2. Dehydration of Alcohol3. Dehydrohalogenation of Alkyl Halides4. Hydrogenation of alkynes5. Dehalogenation of vicinal dihalides

1. Cracking, or pyrolysis, is the process in which saturated hydrocarbons are heated to very high temperatures in the presence of a catalyst (usually silica-alumina):

Alkane (CnH2n+2) Mixture of alkenes + Alkanes + H2 (g)2CH3CH2CH3 CH3CH=CH2 + CH2=CH2 + CH4 + H2

This is a very general reaction and this is the only form you will need to know

2. Dehydration of Alcohols involves the elimination of a molecule of water from a reactant molecule.

Can form intermediate “carbocations” [a positive carbon ion] during the reaction mechanism that can rearrange to be more stable. Tertiary is most stable, followed by secondary, and then primary.

Mechanism step 1 & 2: The first step involves the protonation of the alcohol by an acid, followed by a loss of water to give a carbocation

Mechanism step 3: Elimination occurs when the negative part of the acid plucks off a hydrogen

Example of dehydration: Dehydration of 2-methyl-2-butanol:

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This carbocation will rearrange if it can! Primary and secondary carbocations may.

This is a tertiary carbocation; it will not rearrange!

Other examples of dehydrations

3. Dehydrohalogenation of alkyl halides involves eliminating a hydrogen and a halogen

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May form up to three products (NOT including geometric isomers!) during a concerted, one-step mechanism. [Everything happens at once; it is NOT stepwise]

4. Hydrogenation of alkynes involves adding hydrogen (H2) to an alkyne

This is a stereospecific reaction if possible.

5. Dehalogenation of vicinal dihalide involves eliminating a halogen molecule (X2) from adjacent carbon atoms

Physical Properties of Alkenes.

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• Alkenes have physical properties very similar to the corresponding alkanes.

Chemical Properties of Alkenes

Addition Reactions of Alkenes• Addition at the C=C bond is the most common reaction of alkenes.

1. H2 – hydrogenation2. Br2 and Cl2 – halogenation 3. HBr, HCl – addition of hydrogen halide4. H2SO4 – addition of sulfuric acid5. H2O – addition of water 6. H2O/X2 – halohydrin formation

1. Hydrogenation; addition of hydrogen

2. Halogenation; addition of X2

3. Addition of hydrogen halide

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For addition of “HZ” to an unsymmetrical alkene, strange products can be obtained due to a carbocation intermediate formed during the mechanism.

Mechanism step 1: A proton (H+) from HCl bonds to carbon 1 of propene by utilizing the pi bond electrons. The intermediate formed is a positively charged alkyl group, or carbocation. The positive charge is localized on carbon 2 of this carbocation

Mechanism step 2: The chloride ion then adds to the positively charged carbon atom to form a molecule of 2-chloropropane.

4. Addition of sulfuric acid

5. Addition of water

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H3CHC CH2 + H-Cl H3C

HC CH2

H123

+ Cl-

isopropyl carbocation

H3CHC CH2

H

H3CHC CH2

HCl

+ Cl-

2-chloropropane

6. Halohydrin formation

Carbocations

An ion in which a carbon atom has a positive charge is known as a carbocation.

As stated before, the order of stability of carbocations and hence the ease with which they are formed is:

A primary or secondary carbocation will rearrange if they could become MORE STABLE. Tertiary carbocations won’t rearrange.

Markovnikov’s rule

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H

CH

H

H

CC

H

C

CC

H

C

CC

C

methyl carbocation

primary (1o)carbocation

secondary (2o)carbocation tertiary (3o)

carbocation

H

CH

H

H

CC

H

C

CC

H

C

CC

C

methyl carbocation

primary (1o)carbocation

secondary (2o)carbocation tertiary (3o)

carbocation

increasing stability

When an unsymmetrical molecule such as HZ (HCl) adds to a carbon-carbon double bond, the hydrogen from HZ goes to the carbon atom that has the greater number of hydrogen atoms.

Why?

In other words:

Ex:

Exception: addition of HBr in the presence of peroxides:

ADDITION REACTION PROBLEMS:Write formulas for the organic products formed when

2-methyl-1-butene reacts with:

1. H2, Pt/25°C

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2. Cl2

3. HCl

4. H2O, H+

5. H2O / Cl2

6. HI

7. HBr/H2O2

Oxidation reactions Oxidation (adding oxygen) occurs at the double bond1. Glycol formation:

Baeyer test: reactions potassium permanganate (cold) becomes a visual “simple chemical test” to test for unsaturation due to a visual color change.

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2. Ozonolysis

3. Strong oxidation with hot KMnO4

Practice problems:

Substitution reaction:Allylic Substitution

.

Alkynes: Nomenclature and PreparationThe rules for naming alkynes are the same as those for alkenes, but the ending –yne is used to indicate the presence of a triple bond.

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Preparation of alkynes

Acetylene can be prepared from calcium carbide and water.

Acetylene is also prepared by the cracking of methane at 1500 °C.

Physical properties of alkynes

Acetylene is a colorless gas with little odor when pure.Acetylene is insoluble in water and is a gas at normal temperature and pressure.

Chemical properties of alkynesAlkynes undergo addition reactions rather similar to those of alkenes.

Cl2 and Br2

HCl and HBrPositive reaction with Baeyer’s test.

Bromination of Acetylene

HCl Addition to Unsymmetrical Alkynes

Aromatic Hydrocarbons: Benzene compounds

Benzene and all substances with structures and chemical properties that resemble benzene are classified as aromatic compounds

16Representations of Benzene

Bonding in benzene:

The electrons are not attached to particular carbon atoms, but are delocalized and associated with the entire molecule.

This electronic structure imparts unusual stability to benzene and is responsible for many of the characteristic properties of aromatic compounds.

Naming Aromatic Compounds

Naming Substituted Benzene CompoundsA substituted benzene is derived by replacing one or more hydrogen atoms of benzene by another atom or group of atoms.Monosubstituted benzene has the formula C6H5G, where G is the group replacing a hydrogen atom.

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Some monosubstituted benzenes are named by adding the name of the substituent group as a prefix to the word benzene

O2N

CH2CH3

Cl

Br

nitrobenzene ethylbenzene chlorobenzene bromobenzene

Certain monosubstituted benzenes have special names.

The C6H5- group [or the benzene ring as a substituent) is known as the phenyl group, and the name phenyl is used to name compounds that cannot easily be named as benzene derivatives

The Benzyl substituent:

Disubstituted benzenes

The prefixes ortho-, meta-, and para- (abbreviated o-, m-, and p-) are used to name disubstituted benzenes

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CH3

OH

H2N

C

O

HC

O

OH

benzoic acid benzaldehyde

CH=CH2

styrene phenol aniline

tolueneCH3

OH

H2N

C

O

HC

O

OH

benzoic acid benzaldehyde

CH=CH2

styrene phenol aniline

toluene

CH3

OH

H2N

C

O

HC

O

OH

benzoic acid benzaldehyde

CH=CH2

styrene phenol aniline

toluene

CH

CH3

CHCH2CH3

H2C

diphenylmethane

Cl

3-chloro-2-phenylpentane

1

23 4 5

G

ortho

meta

para

meta

ortho

CH

CH3

CHCH2CH3

H2C

diphenylmethane

Cl

3-chloro-2-phenylpentane

1

23 4 5

Dichlorobenzenes, C6H4Cl2

The three isomers of dichlorobenzene have different physical properties

• When the two substituents are different and neither is part of a compound with a special name, the names of the two substituents are given in alphabetical order, followed by the word benzene.

XYLENESThe dimethylbenzenes have the special name xylene

When one of the substituents corresponds to a monosubstituted benzene that has a special name, the disubstituted compound is named as a derivative of that parent compound.

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Cl

ortho-bromochlorobenzene

CH2CH3

Br

NO2para-ethylnitrobenzene

Cl

ortho-bromochlorobenzene

CH2CH3

Br

NO2para-ethylnitrobenzene

NO2

ortho-nitrophenol

NH3CH3

HO

BrNO2

para-nitrotoluenemeta-bromoaniline

NO2

ortho-nitrophenol

NH3CH3

HO

BrNO2

para-nitrotoluenemeta-bromoaniline

Polysubstituted Benzenes

• When there are more than two substituents on a benzene ring, the carbon atoms in the ring are numbered starting at one of the substituted groups.

• Numbering must be done in the direction that gives the lowest possible numbers to the substituent groups.

Polycyclic structures

naphthalene phenanthrene

anthracene

Sources of Aromatic Hydrocarbons

• The aromatic hydrocarbons, such as benzene, toluene, xylene, naphthalene, and anthracene, were first obtained in significant quantities from coal tar.

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naphthalene phenanthrene

anthracene

CH3

O2N NO2

NO2

2,4,6-trinitrotoluene (TNT)

12

3

45

6

OH

Cl

5-bromo-2-chlorophenol

12

35

6

Br 4

CH3

O2N NO2

NO2

2,4,6-trinitrotoluene (TNT)

12

3

45

6

OH

Cl

5-bromo-2-chlorophenol

12

35

6

Br 4

• Coal Coke + Coal gas + Coal tar• Because of the great demand for aromatic hydrocarbons, processes were devised to obtain them from

petroleum.

Chemical Properties of aromatic compoundsBenzene compounds do NOT undergo addition reactions to break the double bond. This is evidence of resonance.

Benzene compounds DO undergo substitution reactions, whereby a hydrogen on the ring is substituted for something else

There are three major SUBSTITUTION reactions:

1. Halogenation When benzene reacts with chlorine or bromine in the presence of a catalyst such as iron (III) chloride or iron (III) bromide, a Cl or Br atom replaces an H atom to form the products

2. Nitration When benzene reacts with a mixture of concentrated nitric acid and concentrated sulfuric acid at about 50C, nitrobenzene is formed.

3. Alkylation Alkylation of benzene is known as the Friedel-Crafts reaction.

The alkyl group from an alkyl halide (RX), in the presence of AlCl3 catalyst, substitutes for an H atom on the benzene ring.

OXIDATION of a side chain

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+ X2

X

+ HXFeX3

benzene

bromine orchlorine

bromobenzene orchlorobenzene

+ HO-NO2

NO2

+ H2OH2SO4

benzene

nitric acid

nitrobenzene

+ CH3CH2Cl

CH2CH3

+ HCl

benzene

chloro-ethane

ethylbenzene

AlCl3

• Carbon chains attached to an aromatic ring are fairly easy to oxidize.• Any and/or all side chains oxidize to acid groups.

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CH2CH3

ethylbenzene

K2Cr2O7/H2SO4

heat

COOH

+ CO2

benzoic acid