A Guidebook to Mechanism in Organic Chemistry

Aldol Reactions

O

OH

O

RCH,-C* rCHv-N O 1

H

t

Oe

CH,=N—Or

O

R CH,—C—CH.,—N—Oe

H

H,0

R • CH 4 • CH(OH) • CH 2 • NO,

in the adjacent /?-position that can be readily expelled as an anion,
results in an easy 'attack from the back' by an electron pair:

OH *-OH

I ©OH H O ©OH

Me • CH—CH, • CHO Me • CH-^CH • CHO >

Me CH=CHCHO

Thus the dehydration of aldols is subject to bagg-catalysis and car­

banion additions are often followed by elimination of water resulting

in an overall condensation reaction. The successive additions of car­

banion, followed by elimination of water induced by strong base,

result in the formation of low molecular weight polymers from simple

aliphatic aldehydes; if the process is to be halted at the simple aldol,

a weak base such as K 2 C0 3 is used. A preparative use of carbanion

addition followed by elimination is seen in the Claisen-Schmidt

condensation of aromatic aldehydes with aliphatic aldehydes or

ketones in the presence of 10 per cent mineral alkali:

Ph—C—H

+ CH, CO Me > Ph CH=CH CO • Me

+ CH, CHO*^ Ph CH=CH CHO

With aliphatic aldehydes, self-condensation can, of course, consti­

tute an important side reaction. The presence of electron-donating

groups in the aromatic nucleus will reduce the positive nature of the