Organic Chemistry

CCH 3 RO−

CH 3

Base More substituted

product

Less substituted

product

CH 3 Br

CH 3 CH +

CH 3 CH 2 O− 79% 21%

27% 73%

19% 81%

8% 92%

CCH 3

CH 3

CH 3

CH 3 C + CH 2

CH 3

CH 3

2,3-dimethyl-

1-butene

2,3-dimethyl-

2-butene

2-bromo-2,3-dimethyl-

butane

CH 3 CHC

CH 3 CO−

CH 3

CH 3

CH 3 CO−

CH 3

CH 2 CH 3

CH 3 CH 2 CO−

CH 2 CH 3

CH 2 CH 3

Table 11.1 Effect of the Steric Properties of the Base on the Distribution of

Products in an E2 Reaction

Section 11.2 The Regioselectivity of the E2 Reaction 405

Zaitsev’s rule cannot be used to predict the major products of the foregoing reactions

because it does not take into account the fact that conjugated double bonds are more

stable than isolated double bonds (Section 8.3). Therefore, if there is a double bond or

a benzene ring in the alkyl halide, do not use Zaitsev’s rule to predict the major (most

stable) product of an elimination reaction.

In some elimination reactions, the less stable alkeneis the major product. For ex-

ample, if the base in an E2 reaction is sterically bulky and the approach to the alkyl

halide is sterically hindered, the base will preferentially remove the most accessible

hydrogen. In the following reaction, it is easier for the bulky tert-butoxide ion to re-

move one of the more exposed terminal hydrogens, which leads to formation of the

less substituted alkene. Because the less substituted alkene is more easily formed, it is

the major product of the reaction.

The data in Table 11.1 show that when a sterically hindered alkyl halide undergoes an

E2 reaction with a variety of alkoxide ions, the percentage of the less substituted

alkene increases as the size of the base increases.

If the alkyl halide is not sterically hindered and the base is only moderately hin-

dered, the major product will still be the more stable product. For example, the major

approach to the hydrogen

is sterically hindered bulky base

CH 3 CCH 2 CH (^3) (CH
3 ) 3 COH

• CH 3 C CHCH 3 + CH 2 CCH 2 CH 3 + CH 3 COH + Br−
  Br
  CH 3 CH 3 CH 3 CH 3
  CH 3
  2-bromo-2-methylbutane
  CH 3 CO−
  CH 3
  CH 3
  tert-butoxide ion
  2-methyl-2-butene
  28%
  2-methyl-1-butene
  72%
  BRUI11-400_436r3 26-03-2003 10:20 AM Page 405