Organic Chemistry

I

n Chapters 10 and 11, we saw that alkyl

halides undergo substitution and elimina-

tion reactions because of their electron-

withdrawing halogen atoms. Compounds with

other electron-withdrawing groups also under-

go substitution and elimination reactions. The

relative reactivity of these compounds depends

on the electron-withdrawing group.

Alcohols and ethers have leaving groups that are stronger bases than

halide ions Alcohols and ethers, therefore, are less reactive than alkyl halides in

substitution and elimination reactions. We will see that because their leaving groups

are strongly basic, alcohols and ethers have to be “activated” before they can undergo

a substitution or an elimination reaction. In contrast, sulfonates and sulfonium salts

have weakly basic leaving groups so they undergo substitution reactions with ease.

12.1 Substitution Reactions of Alcohols

An alcoholcannot undergo a nucleophilic substitution reaction because it has a strongly

basic leaving group that cannot be displaced by a nucleophile.

An alcohol is able to undergo a nucleophilic substitution reaction if its OH group is

converted into a group that is a weaker base (i.e., a better leaving group). One way to

CH 3 OH Br−

strong base

+ CH 3 Br + HO−

(-OH)

RX ROH

an alkyl halide an alcohol an ether a sulfonium salt

X = F, Cl, Br, I

ROR

a sulfonate ester

ROS

O

O

RRSR

R

+

(-X).

(-OH, -OR)

12

Reactions of Alcohols,

Ethers, Epoxides,

and Sulfur-Containing

Compounds •

Organometallic Compounds

437

CH 3 OH CH 3 OCH 3

The weaker the base, the more easily it

can be displaced.

The stronger the acid, the weaker is its

conjugate base.

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