Organic Chemistry

Section 19.3 How Enols and Enolate Ions React 793

In an acidic solution, the carbonyl oxygen of the keto tautomer is protonated and water
removes a proton from the forming the enol.

Notice that the steps are reversed in the base- and acid-catalyzed reactions. In the
base-catalyzed reaction, the base removes the in the first step and the
oxygen is protonated in the second step. In the acid-catalyzed reaction, the acid
protonates the oxygen in the first step and the is removed in the second
step. Notice also how the catalyst is regenerated in both the acid- and base-catalyzed
mechanisms.

PROBLEM 5

Draw the enol tautomers for each of the following compounds. For those compounds that

have more than one enol tautomer, indicate which is more stable.

19.3 How Enols and Enolate Ions React

The carbon–carbon double bond of an enol suggests that it is a nucleophile—like an
alkene. An enol is more electron rich than an alkene because the oxygen atom donates
electrons by resonance. An enol, therefore, is a better nucleophile than an alkene.

Carbonyl compounds that form enols undergo substitution reactions at the
When an reaction takes place under acidic conditions, water removes a
proton from the of the protonated carbonyl compound. The nucleophilic enol
then reacts with an electrophile. The overall reaction is an —
one electrophile (E+)is substituted for another (H+).

A-substitution reaction

a-carbon

a-substitution

a-carbon.

RCH C

OH

R RCH C

+OH

R

resonance contributors of an enol

−

electron-rich -carbon

f. CH 2 CCH 3

O

d.

O

O

CCH 3

O

b.

e. CH 3 CH 2 CCH 2 CCH 2 CH 3

O OO

CH 3 CH 2 CCH 2 CH 3 c.

O

a.

a-proton

a-proton

protonation

of oxygen

removal of a proton

from the -carbon

RCH 2 C

O

R

OH

H

H

C

OH

R RCH

enol tautomer

C

OH

RCH R + H 3 O+

H

H 2 O

keto tautomer

acid-catalyzed keto–enol interconversion

+

+

a-carbon,