Could anybody explain why the alpha hydrogen of the Carbon of acetophenone takes part in the aldol condensation reaction and not that of the one conjoined to the phenyl ring?

1 Answer

Because of the (relative) acidity of hydrogen on alfa-carbon


A strong base extracts a proton from the methyl Group in Alpha position with respect to carbonyl. The resulting carbanion is stabilized by resonance mainly with the relatively stable enolate form having the negative charge on the more electronegative oxygen atom.

#C_6H_5-CO-CH_3 + B: -> BH^+ +C_6H_5-CO-CH_2:^-#

#C_6H_5-COCH_2:^(-) rightleftharpoons C_6H_5-CO:^(-)=CH_2^#

The carbanion resulting from the extraction of hydrogen in ortho position of the ring is only in resonance with other forms having the negative charge on ring carbon atoms (more unstable)

#C_6H_4:^(-)- CO-CH_3 rightleftharpoons C_6H_4:^(-)- CO-CH_3#