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.
Purify the acid by crystallisation from H2O or ligroin, or by hydrolysis of the anhydride [b 120-150o/40mm, m 77-77.5o (from *C6H6, 74-75o from H2O or ligroin)] with H2O. The diphenacyl ester has m 113o (from EtOH) and the p-bromodiphenacyl ester has m 153o (from EtOH/Me2CO). [Vogel Justus Liebigs Ann Chem 615 13 1958, Reed J Chem Soc 685 1951, Fison et al. J Am Chem Soc 56 1774 1934, pK: Bode Chem Ber 67 332 1934, Beilstein 9 IV 2788.]
