Organic Chemistry

442 CHAPTER 12 Reactions of Alcohols, Ethers, Epoxides, and Sulfur-Containing Compounds

3-D Molecules:

Thionyl chloride;

Pyridine;

Pyridinium ion

(^1) Because of its instability, is generated in situ (in the reaction mixture) from the reaction of
phosphorus with iodine.
PI 3
12.3 Other Methods for Converting Alcohols
into Alkyl Halides
Alcohols are inexpensive and readily available compounds. As we have just seen, they
do not undergo nucleophilic substitution because the group is too basic to be dis-
placed by a nucleophile. Chemists, therefore, need ways to convert readily available
but unreactive alcohols into reactive alkyl halides that can be used as starting materials
for the preparation of many organic compounds (Section 10.4).
We have just seen that an alcohol can be converted into an alkyl halide by treating it
with a hydrogen halide. However, better yields are obtained and carbocation
rearrangements can be avoided if a phosphorus trihalide^1 or
thionyl chloride is used instead. These reagents all act in the same way: They
convert an alcohol into an alkyl halide by converting the alcohol into an intermediate
with a leaving group that is readily displaced by a halide ion. For example, phosphorus
tribromide converts the OH group of an alcohol into a bromophosphite group that can
be readily displaced by a bromide ion.
Thionyl chloride converts an OH group into a chlorosulfite group that can be
displaced by
Pyridine is generally used as the solvent in these reactions because it prevents the
buildup of HBr or HCl and it is a relatively poor nucleophile.
pyridine
HCl
N

• Cl−
  N
  H


• 
  


• 
  

  Cl-.
  (SOCl 2 )
  (PCl 3 ,PBr 3 ,or PI 3 )
  ROH RX RNu
  HX −Nu
  alkyl halide
  X = Cl, Br, I
  alcohol
  ∆

  
  

• OH

CH 3 CH 2 OH CH 3 CH 2 OPBr 2

−OPBr

+ 2

N

N

H

PBr

Br

Br

+

+

+

−

CH 3 CH 2 OPBr 2 CH 3 CH 2 Br

phosphorus Br

tribromide

H

a bromophosphite

group

CH 3 OH

+ Cl−

+ SO 2

N

N

H

+ Cl SCl

+

+

−

CH 3 Cl

Cl

thionyl

chloride

H

a chlorosulfite

group

O

CH 3 OClS

O

CH 3 OClS

O

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