Question

: Assertion: p-N, N- dimethylaminobenzaldehyde undergoes benzoin condensation.
Reason: The aldehyde group is meta directing.
A.Both assertion and reason are correct and reason is the correct explanation for assertion
B.Both assertion and reason are correct but reason is not the correct explanation for assertion
C.Assertion is correct but reason is incorrect
D.Both assertion and reason are incorrect

Answer 1

Hint: To answer this question you must recall the mechanism of benzoin condensation reaction. Benzoin condensation is an Addition reaction between two aromatic aldehydes like benzaldehydes in the presence of an alcoholic cyanide as catalyst.

Complete step by step solution:
The benzoin condensation in simple words is a coupling reaction between aromatic aldehydes resulting in the formation of benzoin or its derivatives. In the first step, the cyanide ions react with the benzaldehyde forming a cyanohydrin. The cyanide ion takes part in a nucleophilic addition reaction. It helps to carry the reaction forward by acting as a nucleophile and easing the loss of protons, thus resulting in the formation of cyanohydrin.
The cyanohydrin then attacks on the second benzaldehyde molecule resulting in a condensation reaction between the two molecules.
The third and the final step involves the rearrangement of the intermediate and removal of the cyanide ion to form benzoin as the product. The rearrangement results in the reversal of polarity of the carbonyl group.
p-N, N- dimethylaminobenzaldehyde is a benzaldehyde as well with a tertiary amine group at the para position. Thus, it undergoes the benzoin condensation reaction. We can say that assertion is correct.
Aldehyde group contains an oxygen atom bonded to a carbon atom. It is an electron withdrawing group and withdraws electrons through –M effect forming an electron deficiency at the ortho and para positions. Thus it deactivates the ortho and para positions and is thus meta directing. Thus the reason is correct but it has nothing to do with the benzoin condensation of p-N,N- dimethylaminobenzaldehyde.

Thus, the correct answer is B.

Note:
Overall in the reaction, the carbonyl carbon atom is deprotonated by a strong base, that is, the cyanide ion, and the second aldehyde reacts with this carbanion intermediate regenerating the carbonyl group and eliminating the catalyst and a water molecule is lost.