P and Q are isomers of dicarboxylic acid C₄H₄O₄. Both decolorise Br₂/H₂O. On heating, P forms the cyclic anhydride. Upon treatment with dilute alkaline KMnO₄, P as well as Q could produce one or more than one from S, T and U.

In the following reaction sequences V and W are, respectively,

The major product H of the given reaction sequence is

$$C{H_3} - C{H_2} - CO - C{H_3}\buildrel {\overline C N} \over \longrightarrow G\mathrel{\mathop{\kern0pt\longrightarrow} \limits_{Heat}^{95\% \,{H_2}S{O_4}}} H$$

The compound that undergoes decarboxylation most readily under mild condition is

In the following reaction sequence, the compound J is an intermediate.

J(C₉H₈O₂) gives effervescence on treatment with NaHCO₃ and a positive Baeyer's test.