Q.Explain Knoevenagal reaction.

A weak base (piperidine) deprotonates the acidic methylene of a compound like diethyl malonate, and the resulting carbanion adds to an aldehyde/ketone; base-catalysed dehydration of the aldol-type product then gives an $\alpha,\beta$-unsaturated compound — the Knoevenagel reaction.

The Knoevenagel reaction is a condensation between a carbonyl compound (aldehyde or ketone, $R-CHO$) and an 'active methylene' compound — one where the $CH_2$ group is flanked by two electron-withdrawing groups (such as two ester groups in diethyl malonate, $CH_2(COOC_2H_5)_2$, or a nitrile and ester in cyanoacetic ester), catalysed by a weak base such as piperidine or pyridine. The mechanism proceeds via: (i) the weak base removes one of the acidic methylene protons (acidified by the two flanking electron-withdrawing groups), generating a stabilised carbanion;

(ii) this carbanion performs a nucleophilic addition to the carbonyl carbon of the aldehyde/ketone, giving an aldol-type (beta-hydroxy) intermediate; …