chapter 12 reactions of arenes electrophilic aromatic substitution l.
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H. E. d +. d –. +. +. E. Y. H. Y. Chapter 12 Reactions of Arenes: Electrophilic Aromatic Substitution. H. E. d +. d –. +. +. E. Y. H. Y. 12.1 Representative Electrophilic Aromatic Substitution Reactions of Benzene. H. E. d +. d –. +. +. E. Y. H. Y.

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electrophilic aromatic substitutions include

H

E

d+

d–

+

+

E

Y

H

Y

Electrophilic aromatic substitutions include:

Nitration

Sulfonation

Halogenation

Friedel-Crafts Alkylation

Friedel-Crafts Acylation

table 12 1 nitration of benzene

H

NO2

Table 12.1: Nitration of Benzene

H2SO4

+

HONO2

+

H2O

Nitrobenzene(95%)

table 12 1 sulfonation of benzene

H

SO2OH

Table 12.1: Sulfonation of Benzene

heat

+

HOSO2OH

+

H2O

Benzenesulfonic acid(100%)

table 12 1 halogenation of benzene

H

Br2

Table 12.1: Halogenation of Benzene

FeBr3

+

Br2

+

HBr

Bromobenzene(65-75%)

table 12 1 friedel crafts alkylation of benzene

H

C(CH3)3

Table 12.1: Friedel-Crafts Alkylation of Benzene

AlCl3

+

(CH3)3CCl

+

HCl

tert-Butylbenzene(60%)

table 12 1 friedel crafts acylation of benzene

O

O

H

CCH2CH3

CH3CH2CCl

Table 12.1: Friedel-Crafts Acylation of Benzene

AlCl3

+

+

HCl

1-Phenyl-1-propanone(88%)

step 1 attack of electrophile on p electron system of aromatic ring

E+

H

H

E

H

H

H

H

H

+

H

H

H

H

H

Step 1: attack of electrophileon p-electron system of aromatic ring

highly endothermic

carbocation is allylic, but not aromatic

step 2 loss of a proton from the carbocation intermediate
Step 2: loss of a proton from the carbocationintermediate

highly exothermic

this step restores aromaticity of ring

H

H

E

H

H

H

H

E

+

H

H

H

H

H

H+

slide12

H

H

E

H

+

H

H

H

H

H

H

H

+ E+

+ H+

H

H

H

E

H

H

H

H

based on this general mechanism
Based on this general mechanism:

what remains is to identify the electrophile in nitration, sulfonation, halogenation, Friedel-Crafts alkylation, and Friedel-Crafts acylation to establish the mechanism of specific electrophilic aromatic substitutions

nitration of benzene

H

NO2

+

O

N

O

••

••

Nitration of Benzene

H2SO4

+

HONO2

+

H2O

Electrophile isnitronium ion

where does nitronium ion come from

••

••

••

••

O

O

O

O

+

+

••

••

N

N

+

O

O

••

••

H

H

H

••

+

O

O

N

O

••

H

H

••

••

Where does nitronium ion come from?

H2SO4

+

sulfonation of benzene

H

SO2OH

••

••

O

O

+

••

S

O

••

Sulfonation of Benzene

heat

+

HOSO2OH

+

H2O

Several electrophiles present:

a major one is sulfur trioxide

halogenation of benzene

H

Br2

Halogenation of Benzene

FeBr3

+

Br2

+

HBr

Electrophile is a Lewis acid-Lewis basecomplex between FeBr3 and Br2.

the br 2 febr 3 complex

+

••

••

••

••

Br

Br

Br

Br

FeBr3

••

••

••

••

The Br2-FeBr3 Complex

The Br2-FeBr3 complex is more electrophilic than Br2 alone.

+

FeBr3

Lewis base

Lewis acid

Complex

step 1 attack of br 2 febr 3 complex on p electron system of aromatic ring

H

H

Br

H

+

H

H

H

Step 1: attack of Br2-FeBr3 complex on p-electron system of aromatic ring

+

Br

Br

FeBr3

H

H

H

H

H

H

+ FeBr4–

friedel crafts alkylation of benzene

H

C(CH3)3

H3C

+

CH3

C

H3C

Friedel-Crafts Alkylation of Benzene

AlCl3

+

(CH3)3CCl

+

HCl

Electrophile is tert-butyl cation

role of alcl 3

+

••

AlCl3

(CH3)3C

Cl

••

Role of AlCl3

acts as a Lewis acid to promote ionizationof the alkyl halide

••

+

(CH3)3C

Cl

AlCl3

••

role of alcl 332

+

••

AlCl3

(CH3)3C

Cl

••

••

AlCl3

Cl

••

Role of AlCl3

acts as a Lewis acid to promote ionizationof the alkyl halide

••

+

(CH3)3C

Cl

AlCl3

••

+

+

(CH3)3C

step 1 attack of tert butyl cation on p electron system of aromatic ring

+

C(CH3)3

H

H

C(CH3)3

H

+

H

H

H

Step 1: attack of tert-butyl cationon p-electron system of aromatic ring

H

H

H

H

H

H

rearrangements in friedel crafts alkylation

H

C(CH3)3

AlCl3

Rearrangements in Friedel-Crafts Alkylation

Carbocations are intermediates.

Therefore, rearrangements can occur

+

(CH3)2CHCH2Cl

Isobutyl chloride

tert-Butylbenzene(66%)

rearrangements in friedel crafts alkylation36

H

C(CH3)3

AlCl3

Rearrangements in Friedel-Crafts Alkylation

Isobutyl chloride is the alkyl halide.

But tert-butyl cation is the electrophile.

+

(CH3)2CHCH2Cl

Isobutyl chloride

tert-Butylbenzene(66%)

rearrangements in friedel crafts alkylation37

H

••

AlCl3

H3C

Cl

C

CH2

••

CH3

••

AlCl3

Cl

••

Rearrangements in Friedel-Crafts Alkylation

+

H

+

+

H3C

C

CH2

CH3

reactions related to friedel crafts alkylation

H

Reactions Related to Friedel-Crafts Alkylation

Cyclohexene is protonated by sulfuric acid, giving cyclohexyl cation which attacks the benzene ring

H2SO4

+

Cyclohexylbenzene(65-68%)

friedel crafts acylation of benzene

H

+

+

••

CH3CH2C

O

CH3CH2C

O

Friedel-Crafts Acylation of Benzene

O

O

CCH2CH3

AlCl3

+

CH3CH2CCl

+

HCl

Electrophile is an acyl cation

step 1 attack of the acyl cation on p electron system of aromatic ring

O

O

CCH2CH3

+

CCH2CH3

Step 1: attack of the acyl cationon p-electron system of aromatic ring

H

H

H

H

H

H

H

+

H

H

H

H

H

acid anhydrides

O

O

O

H

CCH3

CH3COCCH3

O

+

CH3COH

Acid Anhydrides

can be used instead of acyl chlorides

AlCl3

+

Acetophenone(76-83%)

acylation reduction

O

O

H

CR

Acylation-Reduction

permits primary alkyl groups to be attachedto an aromatic ring

Reduction of aldehyde and ketonecarbonyl groups using Zn(Hg) and HCl is called the Clemmensen reduction.

RCCl

AlCl3

Zn(Hg), HCl

CH2R

acylation reduction46

O

O

H

CR

Acylation-Reduction

permits primary alkyl groups to be attachedto an aromatic ring

Reduction of aldehyde and ketonecarbonyl groups by heating with H2NNH2and KOH is called theWolff-Kishner reduction.

RCCl

H2NNH2, KOH,triethylene glycol,heat

AlCl3

CH2R

example prepare isobutylbenzene
Example: Prepare isobutylbenzene

No! Friedel-Crafts alkylation of benzene using isobutyl chloride fails because of rearrangement.

(CH3)2CHCH2Cl

CH2CH(CH3)3

AlCl3

recall

C(CH3)3

AlCl3

Recall

+

(CH3)2CHCH2Cl

Isobutyl chloride

tert-Butylbenzene(66%)

use acylation reduction instead

O

(CH3)2CHCCl

CH2CH(CH3)3

O

CCH(CH3)2

Use Acylation-Reduction Instead

+

AlCl3

Zn(Hg)HCl