Question

Which of the following compounds undergo aldol condensation?
(A) Formaldehyde
(B) Trichloroacetaldehyde
(C) Trimethyl acetaldehyde
(D) Acetaldehyde

Answer 1

Hint:Aldehydes having $\alpha $ -hydrogen undergo self-condensation on warming with dilute or mild base to give $\beta $ -hydroxy aldehydes, called aldols (aldehyde + alcohol). This reaction is known as aldol condensation.
A typical example is the reaction of acetaldehyde with base under mild condition.

Complete step by step answer:
Aldol condensation can be of two types . One is self aldol condensation in which reaction takes place between two identical ketones or identical aldehydes , the other is cross aldol condensation where reaction can take place between two different aldehydes , two different ketones or one aldehyde and one ketone .
Mechanism:-
The first step involves the formation of a resonance-stabilized enolate anion by the removal of an $\alpha $ hydrogen from the aldehyde by the base. In the second step the enolate anion attacks the carbonyl carbon of the second molecule of the aldehyde to form an alkoxide ion.

Various basic reagents such as dilute sodium hydroxide, aqueous alkali carbonate, alkali metal alkoxides, etc. may be used. The reaction is not favorable for ketones.
So the correct answer is (D).

Additional information:-
(A) Aldols are easily dehydrated to $\alpha $,$\beta $-unsaturated compounds on heating alone or with acid or base
(B) The condensation is promoted by $ - I$ effect and reduced by $ + I$ effect on the carbonyl carbon
(C) The reaction equilibrium is favorable for aldehyde but much less favorable for ketones


Note:Aldol condensation of glyceraldehyde gives mono-saccharides. In the presence of acid Aldol condensation affected. Acetone with dry hydrochloric acid gas slowly gives methyl oxide and phorone.
Mesitylene is obtained on distilling acetone with sulphuric acid condensation of acetylene with aldehyde or Ketones gives acetylenic alcohol, a valuable commercial product.