Organic Chemistry

792 CHAPTER 19 Carbonyl Compounds III

For most ketones, the enol tautomer is much less stable than the keto tautomer. For

example, an aqueous solution of acetone exists as an equilibrium mixture of more than

99.9% keto tautomer and less than 0.1% enol tautomer.

The fraction of the enol tautomer in an aqueous solution is considerably greater for a

because the enol tautomer is stabilized by intramolecular hydrogen bonding

and by conjugation of the carbon–carbon double bond with the second carbonyl group.

Phenol is unusual in that its enol tautomer is morestable than its keto tautomer

because the enol tautomer is aromatic, but the keto tautomer is not.

PROBLEM 4

Only 15% of 2,4-pentanedione exists as the enol tautomer in water, but 92% exists as the

enol tautomer in hexane. Explain why this is so.

Now that we know that a hydrogen on a carbon adjacent to a carbonyl carbon is

somewhat acidic, we can understand why keto and enol tautomers interconvert as we

first saw in Chapter 6. Keto–enol interconversionis also called keto–enol tautomer-

izationor enolization. The interconversion of the tautomers can be catalyzed by either

acids or bases.

In a basic solution, hydroxide ion removes a proton from the of the keto

tautomer. The anion that is formed has two resonance contributors: a carbanion and an

enolate ion. The enolate ion contributes more to the resonance hybrid because the

negative charge is better accommodated by oxygen than by carbon. Protonation on

oxygen forms the enol tautomer, whereas protonation on the reforms the

keto tautomer.

a-carbon

a-carbon

keto tautomer

OH

enol tautomer

O

a hydrogen bond

15%

enol tautomer

85%

keto tautomer

C

CH 2 CH 3

O O

C

H 3 C

C

C CH 3

H

O

C

H 3 C

H

O

b-diketone

CH 3 CCH 3

OH

CH 3

> 99.9%

keto tautomer

< 0.1%

enol tautomer

O

CH 2 C

3-D Molecules:

Enol of acetone;

Enol of a -diketoneb

protonation of oxygen

removal of a proton

from the -carbon

RCH C

O

R RCH C

O

R

H

RCH C

O

R RCH

enol tautomer

C

OH

R + HO−

H

H

O

HO−

−

keto tautomer enolate ion

base-catalyzed keto–enol interconversion

−