Organic Chemistry

420 CHAPTER 11 Elimination Reactions of Alkyl Halides • Competition Between Substitution and Elimination

Because a carbocation is formed in an E1 reaction, you must check for the possibil-

ity of a carbocation rearrangement before you use Zaitsev’s rule to determine the elim-

ination product. In the following reaction, the secondary carbocation undergoes a

1,2-hydride shift, in order to form a more stable tertiary carbocation.

Table 11.4 summarizes the stereochemical outcome of substitution and elimination

reactions.

H

H

CH 3

CH 3 + Br−

+

+

H

CH (^3) CH 3 OH
H+
E1 conditions
CH 3
H
Br
H
H
CH 3
CH 3

• H
  H
  H
  1,2-hydride shift
  secondary carbocation
  tertiary carbocation
  CH 3
  CH 3
  Table 11.4 Stereochemistry of Substitution and Elimination Reactions
  Mechanism Products
  Both stereosiomers (Rand S) are formed (more inverted than retained).
  E1 Both Eand Zstereoisomers are formed (more of the stereoisomer with
  the bulkiest groups on opposite sides of the double bond).
  Only the inverted product is formed.
  E2 Both Eand Zstereoisomers are formed (more of the stereoisomer with
  the bulkiest groups on opposite sides of the double bond is formed)
  unless the -carbon of the reactant is bonded to only one hydrogen, in
  which case only one stereoisomer is formed, with a configuration that
  depends on the configuration of the reactant.
  b
  SN 2
  SN 1
  PROBLEM 14
  Give the substitution and elimination products for the following reactions, showing the
  configuration of each product:
  a.
  b.
  c.
  d.
  e. f. CH^3 OH
  SN1/E1 conditions
  CH 3
  H
  Br
  CH 3 CH 2
  C
  CH 3
  H
  Br
  CH 3 CH 2
  CH 3 O−
  SN2/E2 conditions
  C
  trans-1-chloro-2-methylcyclohexane
  CH 3 OH
  SN1/E1 conditions
  trans-1-chloro-2-methylcyclohexane
  CH 3 O−
  SN2/E2 conditions
  (S)-2-chlorohexane
  CH 3 OH
  SN1/E1 conditions
  (S)-2-chlorohexane
  CH 3 O−
  SN2/E2 conditions
  BRUI11-400_436r3 26-03-2003 10:20 AM Page 420