Organic Chemistry

156 CHAPTER 4 Reactions of Alkenes

Carbocation rearrangements also can occur by ring expansion, another type of

1,2-shift. In the following example, a secondary carbocation is formed initially:

Ring expansion leads to a more stable carbocation—it is tertiary rather than secondary,

and a five-membered ring has less angle strain than a four-membered ring (Section 2.11).

In subsequent chapters, you will study other reactions that involve the formation

of carbocation intermediates. Keep in mind that whenever a reaction leads to the

formation of a carbocation, you must check its structure for the possibility of

rearrangement.

PROBLEM 13 SOLVED

Which of the following carbocations would you expect to rearrange?

a. c. e.

b. d.

SOLUTION

a. This carbocation will rearrange because a 1,2-hydride shift will convert a primary car-

bocation into a tertiary carbocation.

b. This carbocation will not rearrange because it is tertiary and its stability cannot be im-

proved by a carbocation rearrangement.

c. This carbocation will rearrange because a 1,2-hydride shift will convert a secondary

carbocation into a tertiary carbocation.

CH 3 H

+

+

CH 3

H CH 2

+

+

CH 3

CH 3 CH 2 C

+

HCH 3

CH 3

+

CH 3

CH 3 CHCHCH 3

+

CH 3

+

CH 2

+

CH

CH 3

CH 2

4

3 21

4

3

CH (^21)
CH 3
CH 3
CH 3
H+ + Br−

• CH 3
  CH 3
  Br
  CH 3
  4-methyl-1-pentene
  CH 3 CHCH 2 CH CH 2 HBr CH 3 CHCH 2 CHCH 3
  CH 3 CH 3 CH 3


• 
  

  CH 3 CHCH 2 CHCH 3
  CH 3
  Br

  
  


• 
  

  CH 3 CHCHCH 2 CH 3

  
  


• 
  

  the carbocation does
  not rearrange
  Br−