Organic Chemistry

614 CHAPTER 15 Aromaticity • Reactions of Benzene

INCIPIENT PRIMARY

CARBOCATIONS

For simplicity, we have shown the formation of a

primary carbocation in the two preceding reactions. However, as

we saw in Section 10.5, primary carbocations are too unstable to

be formed in solution. The fact is that a true primary carbocation

is never formed in a Friedel–Crafts alkylation reaction. Instead,

the carbocation remains complexed with the Lewis acid—it is

called an “incipient”carbocation. A carbocation rearrangement

occurs because the incipient carbocation has sufficient carbo-

cation character to permit the rearrangement.

CH 3 CHCH 2 CH 3 CHCH 3

H

CH 3 CH 2 CH 2 Cl + AlCl 3 Cl AlCl 3

1,2-hydride

shift

incipient primary carbocation

Cl

AlCl 3

δ+

δ−

δ+ δ−

the benzene ring. The relative amounts of rearranged and unrearranged product

depend on the increase in carbocation stability achieved as a result of the rearrange-

ment. For example, when benzene reacts with 1-chlorobutane, a primary carbocation

rearranges to a secondary carbocation, and 60–80% of the product (the actual percent-

age depends on the reaction conditions) is the rearranged product.

When benzene reacts with 1-chloro-2,2-dimethylpropane, a primary carbocation re-

arranges to a tertiary carbocation. Thus, there is a greater increase in carbocation

stability and, therefore, a greater amount of rearranged product—100% of the product

(under all reaction conditions) has the rearranged alkyl substituent.

CH 3

+

+

CH 3 CCH 2 CH 3 CCH 2 CH 3

a primary carbocation

1,2-methyl shift

CH 3

CH 3

a tertiary carbocation

rearranged alkyl

substituent

unrearranged alkyl

substituent

CH 2 CH 2 CH 2 CH 3 CHCH 2 CH 3

++CH 3 CH 2 CH 2 CH 2 Cl

+

+

CH 3

2-phenylbutane

65%

1-phenylbutane

35%

1-chlorobutane

AlCl 3

0 °C

CH 3 CH 2 CHCH 2 CH 3 CH 2 CHCH 3

a primary carbocation

1,2-hydride shift

H

a secondary carbocation

rearranged alkyl

substituent

unrearranged alkyl

substituent

CH 2 CCH 3

CH 3

CH 3

CCH 2 CH 3

++CH 3 CCH 2 Cl CH

3

CH 3

CH 3

CH 3

AlCl 3

2-methyl-2-phenylbutane

100%

2,2-dimethyl-1-phenylpropane

0%

1-chloro-2,2-dimethylpropane