JEE Main · 2021 · 25 Jul 2021 · Shift 2 · Aldehydes Ketones and Carboxylic Acids

A cyclic anhydride is formed when two carboxyl groups of the same molecule lose a molecule of water between them, which demands that the two $-\mathrm{COOH}$ groups be able to reach each other. The double bond of but-2-enedioic acid fixes their positions and cannot rotate. In the cis acid, maleic acid, both carboxyl groups lie on the same side and are close enough for heating to close a five-membered ring with loss of water, while in the trans acid, fumaric acid, they point away from each other and heating cannot bring them together. Treatment with an alcohol and an acid catalyst esterifies a carboxyl group and gives no anhydride at all. Maleic anhydride is therefore prepared by heating cis-but-2-enedioic acid.