Studies in pyrolysis. Part XXIV. Competitive routes in the pyrolysis of carboxylic acid anhydrides

Abstract

Where the open-chain anhydride of a monobasic acid contains an α-hydrogen atom, the known major primary scission is a non-radical acyl–oxygen scission, yielding a keten and a carboxylic acid as discrete molecules. If, in addition, the anhydride is αβ-unsaturated, primary intra-acyl scission competes with the above acyl–oxygen scission: however, the latter is precluded if there is no α-hydrogen atom, so that intra-acyl scission is here the only primary breakdown route. The pyrolysis of cyclic anhydrides of dibasic acids is explicable on similar lines: here, acyl–oxygen scission would yield a single open-chain ‘keten-acid’, terminated by a keten group (CH·CH:CO) and a carboxyl group, the former rearranging from its cumulative to its conjugated form (C:CH·CHO), followed by decarbonylation to C:CH₂ and other competing and secondary reactions. All the observed products are explicable on this basis, coupled with the known competing formation of a cyclic ketone if the anhydride is of sufficient ring size. Cyclic transition states in the acyl–oxygen scissions are suggested. All pyrolyses were at ca. 500°.