Organic Chemistry

acetyl-CoA

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19

Carbonyl Compounds III

Reactions at the -Carbona

W

hen you studied the reac-

tions of carbonyl compounds

in Chapters 17 and 18, you saw

that their site of reactivity is the partially

positively charged carbonyl carbon, which is

attacked by nucleophiles.

Aldehydes, ketones, esters, and N,N-disubstituted amides have a second site of

reactivity. A hydrogen bonded to a carbon adjacent to a carbonyl carbonis sufficiently

acidic to be removed by a strong base. The carbon adjacent to a carbonyl carbon is called

an A hydrogen bonded to an is called an

In Section 19.1, you will find out why a hydrogen bonded to a carbon adjacent to a

carbonyl carbon is more acidic than other hydrogens attached to hybridized car-

bons, and you will look at some reactions that result from this acidity. At the end of

this chapter, you will see that a proton is not the only substituent that can be removed

from an A carboxyl group bonded to an can be removed as

Finally, you will be introduced to some synthetic schemes that rely on being able to re-

move protons and carboxyl groups from a-carbons.

a-carbon: a-carbon CO 2.

sp^3

RCHC

O

R

base

H

RCHC

O

R

−

baseH

an -hydrogen

an -carbon

A-carbon. a-carbon A-hydrogen.

+

−

RCH 2 C

O

R RCH 2 CR

O−

Nu− Nu