Which of the following compounds will not undergo Aldol condensation…

The correct answer is Option (1) → A and D only

Aldehydes and ketones undergo aldol condensation in dilute alkali only if they have at least one α-hydrogen (a hydrogen on the carbon next to C=O). The base removes this hydrogen to form a carbanion (enolate), which adds to a second carbonyl molecule to give a β-hydroxy aldehyde or ketone. Aldehydes with no α-hydrogen undergo the Cannizzaro reaction with concentrated alkali instead.

• (A) Benzaldehyde, \( \ce{C6H5CHO} \) — Does not undergo aldol. The CHO is attached to a ring carbon that carries no hydrogen; it gives the Cannizzaro reaction:

      \( \displaystyle \ce{2C6H5CHO} \xrightarrow[\Delta]{\text{conc. }\ce{NaOH}} \ce{C6H5CH2OH + C6H5COONa} \)

• (B) Acetone, \( \ce{CH3COCH3} \) — Undergoes aldol. Six α-hydrogens on the two \( \ce{CH3} \) groups:

      \( \displaystyle \ce{2CH3COCH3} \xrightarrow{\text{dil. }\ce{NaOH}} \ce{(CH3)2C(OH)CH2COCH3} \)

• (C) Propanal, \( \ce{CH3CH2CHO} \) — Undergoes aldol. Two α-hydrogens on the \( \ce{CH2} \) group:

      \( \displaystyle \ce{2CH3CH2CHO} \xrightarrow{\text{dil. }\ce{NaOH}} \ce{CH3CH2CH(OH)CH(CH3)CHO} \)

• (D) Formaldehyde, HCHO — Does not undergo aldol. It has no α-carbon at all; it gives the Cannizzaro reaction:

      \( \displaystyle \ce{2HCHO + NaOH} \xrightarrow[\Delta]{\text{conc.}} \ce{CH3OH + HCOONa} \)

• (E) Acetophenone, \( \ce{C6H5COCH3} \) — Undergoes aldol. Three α-hydrogens on the \( \ce{CH3} \) group:

      \( \displaystyle \ce{2C6H5COCH3} \xrightarrow{\text{dil. }\ce{NaOH}} \ce{C6H5C(OH)(CH3)CH2COC6H5} \)

NCERT (2026-27 edition):
Aldol Condensation – “Aldehydes and ketones having at least one α-hydrogen undergo a reaction in the presence of dilute alkali as catalyst to form β-hydroxy aldehydes (aldol) or β-hydroxy ketones (ketol), respectively. This is known as Aldol reaction.”

Cannizzaro Reaction – “Aldehydes which do not have an α-hydrogen atom, undergo self oxidation and reduction (disproportionation) reaction on heating with concentrated alkali. In this reaction, one molecule of the aldehyde is reduced to alcohol while another is oxidised to carboxylic acid salt.”