Which of the following cannot act as the enolate (nucleophile) component in an aldol reaction? (One or more answers)

A $HCHO$
B $C X_{6} H X_{5} CHO$
C $(CH X_{3}) X_{2} CHCHO$
D $C X_{6} H X_{5} COC X_{6} H X_{5}$

Answer: A, B, D

Methanal, benzaldehyde and benzophenone have no alpha-hydrogens, so no enolate can form. 2-Methylpropanal has one alpha-hydrogen on the CH carbon and can form an enolate.

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