Organic Chemistry

Section 15.15 Alkylation of Benzene by Acylation–Reduction 615

PROBLEM 22

Show the mechanism for alkylation of benzene by an alkene.

PROBLEM 23

What would be the major product of a Friedel–Crafts alkylation reaction using the follow-

ing alkyl halides?

a. d.

b. e.

c. f.

15.15 Alkylation of Benzene by Acylation–Reduction

It is not possible to obtain a good yield of an alkylbenzene containing a straight-chain
alkyl group via a Friedel–Crafts alkylation reaction, because the incipient primary car-
bocation will rearrange to a more stable carbocation.

Acylium ions, however, do not rearrange. Consequently, a straight-chain alkyl group
can be placed on a benzene ring by means of a Friedel–Crafts acylation reaction, fol-
lowed by reduction of the carbonyl group to a methylene group. It is called a reduction
reaction because the two bonds are replaced by two bonds (Section 4.8).
Only a ketone carbonyl group that is adjacent to a benzene ring can be reduced to a
methylene group by catalytic hydrogenation(H 2 /Pd).

C¬O C¬H

CH 3 CH 2 CH(Cl)CH 3 CH 2 “CHCH 2 Cl

CH 3 CH 2 CH 2 Cl (CH 3 ) 2 CHCH 2 Cl

CH 3 CH 2 Cl (CH 3 ) 3 CCH 2 Cl

CHCH 2 CH 3

CH 3

++CH 3 CH 2 CH 2 CH 2 Cl

AlCl 3

major product

CH 2 CH 2 CH 2 CH 3

minor product

In addition to reacting with carbocations generated from alkyl halides, benzene
can react with carbocations generated from the reaction of an alkene (Section 4.1) or
an alcohol (Section 12.1) with an acid.

CHCH 3

OH

+ CH 3 CHCH 3

H 2 SO 4

∆

isopropylbenzene

cumene

CH 3

alkylation of benzene by an alcohol

alkylation of benzene by an alkene

CHCH 2 CH 3

+ CH 3 CH CHCH 3

HF

sec-butylbenzene

CH 3

CCH 2 CH 2 CH 3

O

+ CH 3 CH 2 CH 2 CCl

O

1. AlCl 3

acyl-substituted benzene alkyl-substituted benzene

CH 2 CH 2 CH 2 CH 3

2. H 2 O

H 2

Pd