Organic Chemistry

Section 18.13 Nucleophilic Addition to a,b-Unsaturated Aldehydes and Ketones 769

18.13 Nucleophilic Addition to -Unsaturated

Aldehydes and Ketones

The resonance contributors for an carbonyl compound show that the
molecule has two electrophilic sites: the carbonyl carbon and the

This means that if an aldehyde or a ketone has a double bond in the -position, a
nucleophile can add either to the carbonyl carbon or to the -carbon.
Nucleophilic addition to the carbonyl carbon is called direct additionor 1,2-
addition.

Nucleophilic addition to the is called conjugate additionor 1,4-addition,
because addition occurs at the 1- and 4-positions (i.e., across the conjugated system).
After 1,4-addition has occurred, the product—an enol—tautomerizes to a ketone (or to
an aldehyde, Section 6.6), so the overall reaction amounts to addition to the car-
bon–carbon double bond, with the nucleophile adding to the and a proton
from the reaction mixture adding to the Compare these reactions with the
1,2- and 1,4-addition reactions you studied in Section 8.7.

Whether the product obtained from nucleophilic addition to an -unsaturated
aldehyde or ketone is the direct addition product or the conjugate addition product de-
pends on the nature of the nucleophile, the structure of the carbonyl compound, and
the conditions under which the reaction is carried out.
Nucleophiles that form unstable addition products—that is, nucleophiles that are
weak bases, allowing direct addition to be reversible—form conjugate addition prod-
ucts because conjugate addition is notreversible, and the conjugate addition product is
more stable. Nucleophiles in this group include halide ions, cyanide ion, thiols, alco-
hols, and amines.

a,b

O

Y− + RCH CH CR

O

Y

RCH CH CR

−

O−

Y

RCH CH CR

O

Y

keto tautomer enol tautomer

RCH CH 2 CR

OH

Y

RCH CH CR

H 3 O+

conjugate addition

resonance contributors

a-carbon.

b-carbon

b-carbon

O

Y− + RCH CH CR

direct addition

O−

RCH CH CR

Y

OH

RCH CH CR

Y

H 3 O+

b

a,b

O

RCH CH

an , -unsaturated

carbonyl compound

CR

O−

RCH CH CR

+

O−

RCH CH CR

+

electrophilic site electrophilic site

b-carbon.

a,b-unsaturated

A,B