Organic Chemistry

Section 12.5 Elimination Reactions of Alcohols: Dehydration 449

groups on opposite sides of the double bond is produced because it is formed more

rapidly since, being the more stable isomer, the transition state leading to its formation

is more stable (Section 11.5).

The relatively harsh conditions (acid and heat) required for alcohol dehydration and

the structural changes resulting from carbocation rearrangements may result in low

yields of the desired alkene. Dehydration, however, can be carried out under milder

conditions by using phosphorus oxychloride and pyridine.

Reaction with converts the OH group of the alcohol into a good

leaving group. The basic reaction conditions favor an E2 reaction, so a carbocation is

not formed and carbocation rearrangements do not occur. Pyridine serves as a base to

remove the proton in the elimination reaction and to prevent the buildup of HC1,

which would add to the alkene.

POCl 3 OPOCl 2 ,

OH

POCl 3

CH 3 CH 2 CHCH (^3) pyridine, 0 °C CH 3 CH CHCH 3
(POCl 3 )
OH
H 2 SO 4
CH 3 CH 2 CHCH 3 CH 3 CH 2 CHCH 3 C
H 3 CH
HCH 3
C
∆
C
H 3 CCH 3
HH

• 
  
  
  
  • C ++CH^3 CH^2 CH CH^2 H+
  
  
  
  


• H 2 O
  2-butanol trans-2-butene
  74%
  cis-2-butene
  23%
  1-butene
  3%


• 
  

  CH 3 CH
  HCl Cl
  O P
  CH 3 O
  N
  CH 3 CH
  Cl ClCl
  OH P
  CH 3 O
  CH 2
  H
  CH
  Cl Cl
  O P
  CH 3 O
  −OPOCl
  (^2) + 2

  
  


• 
  


• Cl−
  CH 2 CHCH 3
  N+
  H
  N
  phosphorus
  oxychloride
  BIOLOGICAL DEHYDRATIONS
  Dehydration reactions are known to occur in
  many important biological processes. Instead of
  being catalyzed by strong acids, which would not be available
  to a cell, they are catalyzed by enzymes. Fumarase, for exam-
  ple, is an enzyme that catalyzes the dehydration of malate in the
  citric acid cycle (p. 1038). The citric acid cycle is a series of
  reactions that oxidize compounds derived from carbohydrates,
  fatty acids, and amino acids.
  Enolase, another enzyme, catalyzes the dehydration of -phos-
  phoglycerate in glycolysis (p. 1037). Glycolysis is a series of reac-
  tions that prepare glucose for entry into the citric acid cycle.
  a
  -phosphoglycerate phosphoenolpyruvate
  enolase
  O−
  O
  CH 2 CH COCH 2 C C −
  O
  OH OPO 32 − OPO 32 −


• H 2 O
  malate fumarate
  − fumarase
  OOC −
  −O
  O−
  O
  O
  CCC
  CH
  HC
  CH 2 CH
  O O
  OH


• H 2 O
  BRUI12-437_480r3 27-03-2003 11:51 AM Page 449