Organic Chemistry

PROBLEM 21

Propose a mechanism for the following reaction:

The synthesis of benzaldehyde from benzene poses a problem because formyl chlo-
ride, the acyl halide required for the reaction, is unstable and cannot be purchased.
Formyl chloride can be prepared, however, by means of the Gatterman–Kochformyla-
tion reaction. This reaction uses a high-pressure mixture of carbon monoxide and HCl
to generate formyl chloride, along with an aluminum chloride–cuprous chloride cata-
lyst to carry out the acylation reaction.

15.14 Friedel–Crafts Alkylation of Benzene

The Friedel–Crafts alkylation reaction substitutes an alkyl group for a hydrogen.

In the first step of the reaction, a carbocation is formed from the reaction of an alkyl
halide with Alkyl fluorides, alkyl chlorides, alkyl bromides, and alkyl iodides
can all be used. Vinyl halides and aryl halides cannot be used because their carbocations
are too unstable to be formed (Section 10.8).

In Section 16.3, we will see that an alkyl-substituted benzene is more reactive than
benzene. Therefore, to prevent further alkylation of the alkyl-substituted benzene, a
large excess of benzene is used in Friedel–Crafts alkylation reactions. This approach
ensures that the electrophile is more likely to encounter a molecule of benzene than a
molecule of alkyl-substituted benzene.
Recall that a carbocation will rearrange if rearrangement leads to a more stable car-
bocation (Section 4.6). When the carbocation can rearrange in a Friedel–Crafts alkyla-
tion reaction, the major product will be the product with the rearranged alkyl group on

R

+ R+ + HB+

mechanism for Friedel–Crafts alkylation

RRCl ++AlCl 3 + −AlCl 4

B

H

R

+

AlCl 3.

++HCl

Friedel–Crafts alkylation

R

RCl

AlCl 3

high

pressure

benzaldehyde

AlCl 3 /CuCl

formyl chloride

unstable

CO C

HCl

O

+ HCl

C

O

H

AlCl 3

O

Cl

O

Section 15.14 Friedel–Crafts Alkylation of Benzene 613