A reaction in which an aldehyde RCHO reacts with malonic acid (CH2(COOH)2) with subsequent loss of CO2 to give an unsaturated carboxylic acid RCH=CHCOOH. Thus, the chain length is increased by 2. The reaction is base-catalysed; usually pyridine is used. The reaction is named after Emil Knoevenagel (1865–1921), who discovered it in 1898.