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Structure of Alkenes

Alkenes (and alkynes) are unsaturated hydrocarbons

Alkenes have one or more double bonds

The two bonds in a double bond are different:

- one bond is a sigma (W) bond; these are cylindrical

in shape and are very strong- the other is a pi () bond; these involve sideways

overlap of p-orbitals and are weaker than Wbonds

Alkenes are flat and have a trigonal planar shape

around each of the two Cs in a double bond

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Hydrogenation of Alkenes and Alkynes

H2 can be added to alkenes or alkynes to form alkanes

Usually a metal catalyst (Pt, Pd or Ni) is used to speed up the

reaction (the reaction generally doesnt work without a catalyst) Because these reactions take place on a surface, hydrogenation

of substituted cycloalkenes produces cis products.

H 2C C H 2 + H 2

Examples:

H C C H + 2H 2

C H 3

C H 3

+ H2

H

H

H

H

H

H

H

H

H

H

H

H

C H 3

C H 3H

H

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Addition of H2 - Reduction

Virtually all alkenes add H2 in the

presence of a transition metal

catalyst, commonly Pd, Pt, or Ni

HH3 C

C C

H CH3

PdCH3 CH2 CH2 CH3

trans-2-Butene

+ H225C, 3 atm

Butane

Pd+ H2

Cyclohexene Cyclohexane

25C, 3 atm

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Hydrohalogenation of Alkenes

Hydrogen halides (HCl, HBr or HI) can add to alkenes to formhaloalkanes

When a hydrogen halide adds to a substituted alkene, the halidegoes to the more substituted C (Markovnikovs rule)

Examples:

C C

H

H H

H

+ HBr C C

H

H

H

B r

H

H

C C

H

H H

C H 3

+ HCl C C

H

H

H

C l

C H 3

H

C H 3

H

I

C H 3

H

H

+ HI

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Addition of HX reaction is regioselectiveregioselective

Markovnikovs rule:Markovnikovs rule: H adds to the less

substituted carbon and X to the moresubstituted carbon

CH3 CH= CH2 HCl CH3 CH-CH2

HCl

CH3 CH-CH2

ClH

1-Chloropropane(not formed)

2-ChloropropanePropene

+

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Mechanism of hydrohalogenation

Hydrohalogenation takes place in two steps

In the first step, H+

is transferred from HBr to the alkene toform a carbocation and bromide ion

Second, Br- reacts with the carbocation to form a bromoalkane

Example:

C C

H

H H

C H 3

+ C C

H

H

H

C H 3

H

+ Br

C C

H

H

H

C H 3

H

+ Br C C

H

H

H

B r

C H 3

H

H B r

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Addition of Water to Alkenes In the presence of a strong acid catalyst (HCl, H2SO4 etc.)

alkenes react with H2O to form alcohols

Recall that acids form H3O+

in water; it is the H3O+

that reactswith the alkene

Hydration reactions follow Markovnikovs ruleE m l :

C C

H

H H

H

AcidC t.

C C

H

H

H

O H

H

H+ H O

C C

H

H C H 3

H

AcidC t.

C C

H

H

H

O H

H

C H 3+ H O

C H 3

H

AcidC t.

+ H O

C H 3

O H

H

H

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Mechanism of Acid-Catalyzed Alkene Hydration First, the alkene reacts with H3O

+ to form a carbocation

Next an H2O quickly reacts with the carbocation to form a protonated alcohol

In the last step the proton is removed by an H2O to form an alcohol

C C

H

H C H 3

H

+

H

H

H

C C

H

H

H

C H 3

H

C C

H

H

H

C H 3

H

+

H HC C

H

H

H

O

H

C H 3

H

H

C C

H

H

H

O

H

C H 3

H

H

+O

H HC C

H

H

H

O

H

C H 3

H

+ OH

H

H

+O

H H

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Halogenation of Alkenes and Alkynes

Halogens (Cl2 or Br2) can add to alkenes or alkynes to formhaloalkanes

Alkenes form dihaloalkanes; alkynes form tetrahaloalkanes Reaction with cycloalkenes produces a trans product

Exampl :

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Addition of Cl2 and Br2 Addition takes place readily at room temperature reaction is generally carried out using pure reagents, or mixing

them in a nonreactive organic solvent

addition of Br2 is a useful qualitative test for the presence of acarbon-carbon double bond

Br 2 has a deep red color; dibromoalkanes are colorless

CH3

CH=CHCH3

Br2 CH

2Cl

2

CH3

CH- CHCH3

Br Br

2,3-Dibromobutane2-Butene

+

Br2CH2Cl2

Br

Br

+

1,2-DibromocyclohexaneCyclohexene

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Mechanism of Bromonation of Ethene

First, a Br+ is transferred from Br2 to the alkene to form abromonium ion and a bromide ion

Next, the bromide ion reacts with the bromonium ion to formthe product

C C

H

H H

H

+ B r B r C C

B rH

H

H

H

C C

B rH

H

H

H

+ B r

+ B r C C

H

B r

H

H

B r

H

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Polymers

A polymer is a long chain of repeating subunits called

monomers

- examples of natural polymers: DNA, protein, starch

- example of synthetic polymers: polyethylene

Many synthetic polymers are made from alkenes,although other functional groups are also used

The monomers are added to the chain through a seriesof addition reactions

Polymerization reactions usually require hightemperature and pressure and are often radicalreactions carried out with a catalyst

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Polymerization

From the perspective of the organicchemical industry, the single most

important reaction of alkenes is

polymerization polymer:polymer: Greek:poly, many and meros, part

monomer:monomer: Greek: mono, single and meros, part

nCH2

= CH2 CH2 CH2

in itiat

Eth l n P l th l n

n(p l m ization)

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Polymerization

show the structure of a polymer by placing parenthesesaround the repeating monomer unit

place a subscript, n, outside the parentheses to indicate thatthis unit repeats n times

the structure of a polymer chain can be reproduced byrepeating the enclosed structure in both directions

following a section of polypropene (polypropylene)

CH2CH-CH2CH-CH2CH-CH2CH

CH3 CH3 CH3 CH3

CH2CH

CH3

The repeating unitPart of an extended po ly er chain

n

monomer units show n in red

n

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Polyethylene

Low-density polyethylene (LDPE) a highly branched polymer; polymer chains do not pack well and

London dispersion forces between them are weak

softens and melts above 115C

approximately 65% used for the production of films for packagingand for trash bags

High-density polyethylene (HDPE)

only minimal chain branching; chains pack well and Londondispersion forces between then are strong

has higher melting point than LDPE and is stronger

can be blow molded to squeezable jugs and bottles

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Aromatic Compounds Aromatic compound:Aromatic compound: a hydrocarbon that contains

one or more benzene-like rings arene:arene: a term used to describe aromatic compounds

Ar Ar--:: a symbol for an aromatic group derived by removingan -H from an arene

Kekul structure for benzene (1872)

C

CCC

C

C

H

H

H

H

H

H

A K k ul st uctushowing all atoms

A K kul st uctuas a lin -angl f o mula

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Conjugated Alkenes and Aromatic Compounds

Recall that a double bond consists of one Wbond and one T

bond; aT

bond is formed by sideways overlap of two porbitals (one electron comes from each orbital)

A conjugated alkene has alternating double and single bonds

The p orbitals overlap in a conjugated system (the T electronsare delocalized throughout the system), making conjugated

alkenes more stable than non-conjugated alkenes An aromatic hydrocarbon consists of alternating double and

single bonds in a flat ring system

Benzene (C6H6) is the most common aromatic hydrocarbon

In benzene all the double bonds are conjugated, and so the Telectrons can circulate around the ring, making benzene morestable than 1,3,5-hexatriene (the p orbitals on the end of achain can not overlap)

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1,3,5-hexa t ene

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Resonance Structures

There are two ways to write the structure of benzene

These are called resonance structures

However, neither of these represents the true structure ofbenzene since benzene has only one structure, with all C-Cbonds being equivalent

The true structure is a hybrid of the the two resonancestructures; this can be represented by drawing the Tbonds as acircle

We use the individual resonance structures when we writereaction mechanisms involving benzene to show more clearlythe bond formation and bond breaking in the reaction

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Benzene

Delocalized electrons are not confinedbetween two adjacent bonding atoms, but

actually extend over three or more atoms.

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Naming Monosubstituted Benzene Compounds Benzene compounds with a single substituent are named by

writing the substituent name followed by benzene

Many of these compounds also have common names that areaccepted by IUPAC (you should know those listed here)

C H 3 O H N H 2 O C H 3

C

O H

C

O O H

Toluene(methylbenzene)

Phenol(hydroxybenzene)

Analine(aminobenzene)

Anisole(methoxybenzene)

Benzaldehyde

(benzenecarbaldehyde

Benzoic Acid

(benzenecarboxylic acid)

Styrene

(phenylethene)

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Naming Multisubstituted Benzene Compounds When there are 2 or more substituents, they are numbered to

give the lowest numbers (alphabetical if same both ways)

Disubsituted benzenes are also named by the common prefixesortho, meta andpara

B r

B r

3

l

F

B r

meta -di romobenzene(1,3-dibromobenzene)

ortho -chlorotol ene(1-chloro-2-methylbenzene)

ara -ethyl henol

(1-hydroxy-4-ethylbenzene)

4-bromo-2-fl oroanaline

(1-amino-4-bromo-2-fl orobenzene)

Examples:

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Nomenclature For three or more substituents:

if one of the substituents imparts a special name, name the

molecule as a derivative of that parent

if none of the substituents imparts a special name, number

the substituents to give the smallest set of numbers, and

list them in alphabetical order before the ending "benzene"

CH3NO2

Cl

OH

Br

BrBr

NO2

CH2 CH3

Br

4

3

2

1

5

6

4

3

21

4

3

12

4-Ch loro-2-nitrotol ne

2,4,6-Tribromophenol 2-Bromo-1-ethy l-4-n itrobenzene

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Nomenclature phenyl group (Cphenyl group (C66HH55-- or Phor Ph--):): the substituent

group derived by loss of an H from benzene

C6H5

1- e y l y l exe e 4- e yl-1-b te ee yl r

1

243

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PAHs Polynuclear aromatic hydrocarbon (PAH)Polynuclear aromatic hydrocarbon (PAH)

a hydrocarbon that contain two or more

benzene rings, with each pair of rings sharingtwo adjacent carbon atoms

PhenanthreneAnthraceneNaphthalene Benzo[a]pyrene

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Physical Properties of Aromatic Compounds

Because aromatic compounds (like benzene) are flat, theystack well, and so have higher melting and boiling points thancorresponding alkanes and alkenes (similar to cycloalkanes)

Substituted aromatic compounds can have higher or lowermelting and boiling points than benzene

- para-xylene has a higher m.p. than benzene

- ortho and meta-xylene have lower m.p.s than benzene Aromatic compounds are more dense than other hydrocarbons,

but less dense than water (halogenated aromatics can be moredense than water, as can haloalkanes)

Aromatic compounds are insoluble in water, and arecommonly used as solvents for organic reactions

Aromatic compounds are also flammable, and many arecarcinogenic

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Chemical Reactivity of Aromatic Compounds

Aromatic compounds do not undergo addition reactions because

they would lose their special stability (aromaticity)

Instead, they undergo substitution reactions, which allow themto retain their aromaticity

We will study three types of substitution reactions of benzene:

halogenation, nitration and sulfonation

+ Br 2

B r

B r

Aromati Loses aromati ity

+ Br 2

B rFeBr 3

Aromati Retains Aromati ity

+ HBr

A ition:

Substitution:

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Halogenation of Benzene and Toluene Br2 or Cl2 can react with benzene, using a catalyst, to form

bromobenzene or chlorobenzene

Only the monohalogenation product is produced When Br 2 or Cl2 reacts with toluene, a mixture of isomersis produced

- Ortho and para isomers are the major products, and metaisomer is the minor product

+ Cl 2

C le C l3

+ H Cl

C H 3

+ r2

e r3

C H 3

r

C H

3

r

C H 3

r

+ +

(Minor)

xa ples:

+ H Br

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Mechanism of Bromonation of Benzene

First, a Br+ is transferred from Br2 to benzene, forming acarbocation and a chloride ion

Next, the chloride ion removes an H+ from the carbocationto form chlorobenzene and HBr

+ B r B r F B r

3 + Br

+ Br

H

B r

H

B r

+ H Br

B r

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Nitration and Sulfonation of Benzene

Nitric acid can react with benzene, using sulfuric acid as acatalyst, to form nitrobenzene plus water

First H2SO4 donates a proton to HNO3, which thendecomposes to form H2O and NO2+ (the reactive species)

Sulfur trioxide plus sulfuric acid (fuming sulfuric acid) canreact with benzene to produce benzenesulfonic acid

First H2SO4 donates a proton to SO3 to produce HSO3+ (the

reactive species)

+ HNO 3

N O 2H 2 O 4

+ H 2 O

+ O 3

O 3 H

H 2 O 4