A 14C, 18O, and mass spectral study of the pyrolytic formation of carbon suboxide from diacetyltartaric anhydride and acetoxymaleic anhydride

Abstract

In order to examine the proposal that carbon suboxide originates by collapse of a cyclic heteryne formed when diacetyltartaric anhydride is pyrolysed at 600–700°, the anhydride has been prepared with 1,4-¹⁴C and 2,3-¹⁴C labelling. Its elimination product, acetoxymaleic anhydride, has also been made with 1,2-¹⁴C, 3-¹⁴C, and 4-¹⁴C labelling. These five compounds have been pyrolysed and the corresponding positions of the labelling in carbon suboxide traced by conversion of the latter into malonic acid. Decarboxylation, together with Schmidt degradation of the acetic acid formed, gives complete information for this. A symmetrical intermediate such as the heteryne is not involved, and C-1 of acetoxymaleic anhydride is eliminated. Despite the side reactions, deposition of carbon, and low yield in this type of reaction, a sharply defined labelling situation is maintained.

As the full stoicheiometric position is difficult to ascertain, diacetyltartaric anhydride has been prepared with ¹⁸O labelling at the 2,3-ester ether positions. After pyrolysis, 78% of one of the ¹⁸O labels is found in the carbon suboxide, as determined mass spectrometrically; figures are also obtained for the acetic acid. In the light of these findings pyrolytic mechanisms are discussed and it is concluded that there may be an initiating gas-phase heterolytic rupture of the C(4)-ether oxygen in acetoxymaleic anhydride. The relevance of mass spectral information on diacetyltartaric anhydride and acetoxymaleic anhydride is considered.