Question 9.2: The Friedel–Crafts reaction of benzene with 2-chloro3methylb...

Predicting the Product of a Carbocation Rearrangement

The Friedel–Crafts reaction of benzene with 2-chloro3methylbutane in the presence of AlCl_3 occurs with a carbocation rearrangement. What is the structure of the product?

S t r a t e g y
A Friedel–Crafts reaction involves initial formation of a carbocation, which can rearrange by either a hydride shift or an alkyl shift to give a more stable carbocation. Draw the initial carbocation, assess its stability, and see if the shift of a hydride ion or an alkyl group from a neighboring carbon will result in increased stability. In the present instance, the initial carbocation is a secondary one that can rearrange to a more stable tertiary one by a hydride shift:

Use this more stable tertiary carbocation to complete the Friedel–Crafts reaction.