Carbanions and Their Reactions
OiH O-pH O HA® f H-t-CHj3-C—Me ===== AH+CH,==C—Me —> CH,—C—Me
slow _< J fast |
Br Br
Br^ Br®
Here the effect of substitution by R and halogen on the rate and
position of attack is exactly opposite to that observed in base-induced
halogenation. Thus with the ketone R-CH 2 -CGteCH 3 , the enol
(XLIV), rather than (XLV), will be stabilised by hyperconjugation
arising from the a-hydrogens of the methyl group in addition to any in
the R group, whereas only the methylene group will be operative in
(XLV):
OH O
I Br, ||
RCH=C—CH, > R CH—C—CH,
I
Br
(XLIV) (XLVI)
OH O. I Br, ||
u RCH,—C=CH, > R • CH,—C—CH, • Br
' (XLV) (XLVII)
This leads to the formation of (XLVI) rather than (XLVII), which
would have been obtained in the presence of base. In the bromina
tion of acetone the effect of the bromine atom in the first-formed
MeCOCH 2 Br is to withdraw electrons, thus making the initial
uptake of proton by the /C=0, in forming the enol, less ready in
bromoacetone than in acetone itself, resulting in preferential attack
on the acetone rather than the bromoacetone in the system. The net
effect is that under acid conditions MeCO-CH 2 Br can actually be
isolated whereas under alkaline conditions of bromination it cannot
for, as we have seen above, it brominates more readily than does
acetone itself when base is present. Further bromination of
Me-CO-CH 8 Br under acid conditions results in preferential attack
on the methyl rather than the methylene group.
(iv) Decarboxylation
Another reaction involving carbanions is the decarboxylation of a
number of carboxylic acids via their anions^228