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