Organic Chemistry

Section 12.4 Converting Alcohols into Sulfonate Esters 443

Table 12.1 Commonly Used Methods for Converting Alcohols into Alkyl

Halides

ROH + HBr RBr

ROH + HI RI

ROH + HCl RCl

ROH + PBr 3 RBr

ROH + PCl 3 RCl

ROH + SOCl (^2) pyridine RCl
pyridine
pyridine
∆
∆
∆
The foregoing reactions work well for primary and secondary alcohols, but tertiary
alcohols give poor yields because the intermediate formed by a tertiary alcohol is
sterically hindered to attack by the halide ion.
Table 12.1 summarizes some of the commonly used methods for converting alco-
hols into alkyl halides.
12.4 Converting Alcohols into Sulfonate Esters
Another way an alcohol can be activated for subsequent reaction with a nucleophile, be-
sides being converted into an alkyl halide, is to be converted into a sulfonate ester. A
sulfonate esteris formed when an alcohol reacts with a sulfonyl chloride.
The reaction is a nucleophilic substitution reaction. The alcohol displaces the chloride
ion. Pyridine is used both as the solvent and to prevent HCl from building up.
A sulfonic acid is a strong acid because its conjugate base is particu-
larly stable due to delocalization of its negative charge over three oxygen atoms.
(Recall from Section 7.6 that electron delocalization stabilizes a charged species.)
Since a sulfonic acid is a strong acid, its conjugate base is weak, giving the sulfonate
ester an excellent leaving group. (Notice that sulfur has an expanded valence shell—it
is surrounded by 12 electrons.)
resonance contributors
RSOH
O
O
RS
O
O
RSO
O
RSO H+
O−
O−
O−
O

• 1 pKa ' - 12


• 
  


• 
  


• S OR + Cl−
  O
  O
  N
  H
  R′ S OR
  O
  O
  R′
  N
  H
  S Cl
  O
  O
  R′ ROH
  R′ S
  O
  O
  Cl ROH pyridine
  a sulfonyl chloride a sulfonate ester


• R′ S pyridine
  O
  O
  OR ++Cl− –H+
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