Question

Which of the following compound(s)cannot undergo self-aldol condensation in the presence of cold dilute alkali?

• A. $C_6{H}_{5}CHO$
• B. $C{H}_{3}C{H}_{2}CHO$
• C. $HC\equiv CCHO$
• D. $C{H}_2=CHCHO$

Answer 1

Answer: (A), (C), (D)

$C{H}_2=CHCHO$

An aldol condensation is a condensation reaction in which an enol or an enolate ion reacts with a carbonyl compound to form a $\beta$-hydroxyaldehyde or $\beta$-hydroxyketone, followed by dehydration to an $\alpha ,\beta$-unsaturated aldehyde. When aldol condensation is carried out using same aldehyde or ketone, it is called Self-aldol condensation.
Aldol condensation occurs in aldehydes having $\alpha$-hydrogen.
Compounds (a) and (c) do not have $\alpha$-hydrogen , so they don't undergo self aldol condensation.
In compound (d) $\alpha$-hydrogen is bonded to double bond, and negative charge doubly bonded carbon is unstable so it wont undergo self aldol.
Compound (b) has $\alpha$-hydrogen thus it will undergo self aldol condensation.