JEE Main · 2023 · 12 Apr 2023 · Shift 1 · Aldehydes Ketones and Carboxylic Acids

Compound A is squaric acid, $3,4$-dihydroxycyclobut-$3$-ene-$1,2$-dione. Two equivalents of hydroxide remove both acidic hydroxyl protons to give the squarate dianion B. In this dianion the two negative charges and the $\pi$ system are delocalised symmetrically over the four-membered carbon ring, so B is aromatic and all four carbon-carbon bonds become equal in length. Squaric acid owes its marked acidity to this stabilisation and exists through keto-enol (enediol) forms, and the removal of its protons by hydroxide is fast rather than slow. Thus the statements that B is aromatic, that A shows keto-enol tautomerism, and that all the carbon-carbon bonds of B are equal are the correct ones.