It is remarkable that the position of the halogen in the molecule has no effect on the heat of formation; for example, chlorpropylene and allylchloride, and also ethylene dichloride and ethylidene dichloride, have equal heats of formation.
Two acids corresponding to this empirical formula are known - namely ethylene succinic acid, H0 2 C CH 2 CH 2 CO 2 H and ethylidene succinic acid CH3 CH(C02H)2.
Ethylidene succinic acid or isosuccinic acid, CH3 CH(C02H)2, is produced by the hydrolysis of a-cyanpropionic acid and by the action of methyl iodide on sodio-malonic ester.