The interaction of the carboxyl groups in the fragmentation of the molecular ions of cis-cyclobutane-1,2-dicarboxylic acid helps to distinguish it from trans-cyclobutane-1,2-dicarboxylic acid.
Crystallise the acid from *C6H6 or *C6H6/EtOAc. The diphenacyl ester has m 98o (from EtOH) and the p-bromodiphenacyl ester has m 158o (from EtOH). The cis-acid isomerizes to the trans-acid on heating in conc HCl at 190o. [Reed J Chem Soc 685 1951, Fison et al. J Am Chem Soc 51 1536 1929, Fison et al. J Am Chem Soc 56 1774 1934, pK: Bode Chem Ber 67 332 1934, Beilstein 9 IV 2788.]
