Organic Chemistry

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

When an ether reacts with a sulfonyl chloride, however, the oxygen atom does not have

a proton that can dissociate. The alkyl group (R) cannot dissociate, so a stable sulfonate

ester cannot be formed. Instead, the more stable starting materials are reformed.

However, like alcohols, ethers can be activated by protonation. Ethers, therefore,

can undergo nucleophilic substitution reactions with HBr or HI. As with alcohols, the

reaction of ethers with hydrogen halides is slow, and the reaction mixture must be

heated in order for the reaction to occur at a reasonable rate.

The first step in the cleavage of an ether by HI or HBr is protonation of the ether

oxygen. This converts the very basic leaving group into the less basic ROH leaving

group. What happens next in the mechanism depends on the structure of the ether.

If departure of the leaving group creates a relatively stable carbocation (e.g., a tertiary

carbocation), an reaction occurs—the leaving group departs, and the halide ion

combines with the carbocation.

However, if departure of the leaving group would create an unstable carbocation

(e.g., a methyl, vinyl, aryl, or primary carbocation), the leaving group cannot depart. It

has to be displaced by the halide ion. In other words, an reaction occurs. The

halide ion preferentially attacks the less sterically hindered of the two alkyl groups.

The cleavage of ethers by HI or HBr occurs more rapidly if the reaction can take

place by an pathway. If the instability of the carbocation requires the reaction to

follow an SN 2 pathway, cleavage will be more rapid with HI than with HBr because I-

SN 1

SN 2

SN 1

RO-

ROR′ ++HI RO+ R′ R IR′ OH

H

I−

∆

S Cl Cl−

O

O

ROR R′ S

O

O

RO

R

R′

an ether

+ +

+

S

O

O

ROH R′ Cl S

O

O

RO

H

R′ SH+

O

O

RO R′

an alcohol

a sulfonate ester

++

+ Cl−

+

SN 1 I

+

CH 3 C OCH 3 H+

CH 3

CH 3

CH 3 C OCH 3

H

CH 3

CH 3

CH 3 CI

CH 3

CH 3

CH 3 C+

CH 3

CH 3 OH

CH 3

+ +

−

protonation carbocation

formation

attack by a nucleophile

I

SN 2

H+

H

CH 3 OCH 2 CH 2 CH 3 + CH 3 OCH 2 CH 2 CH 3 CH 3 I + CH 3 CH 2 CH 2 OH

−

+

protonation nucleophile attacks the less

sterically hindered carbon

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