Phorone and isomeric forms

Phorone, 2,6‐dimethyl‐2,5‐heptadien‐4‐one (I), when prepared by the dehydration of acetone condensation products with mineral acid catalysts, is accompanied by an isomer, purer samples of which were prepared by isomerisation of pure phorone with an acid catalyst and subsequent distillation; by this method scanty but definite evidence of a second isomer was obtained. The isomer obtained by dehydration is shown to be 2,6‐dimethyl‐1,5‐heptadiene‐4‐one (II) and the equilibrium mixture of phorone and isomer (II) at room temperature contains about 7% of isomer (II). Both phorone and isomer (II) can be rearranged to the equilibrium mixture by acid or alkali treatment but are reasonably stable, even to distillation at atmospheric pressure, under neutral conditions. The second isomer found in isomerisation experiments is believed, from spectroscopic and chemical evidence on its mixtures with isomer (II), to be 2,6‐dimethyl‐1,6‐heptadien‐4‐one (III). Attempts to prepare the 2,4‐dinitrophenylhydrazone of phorone yielded mixtures. The properties of purified derivatives of phorone, phorone isomer (II) and related ketones are described.