The thermal and photochemical decompositions of succinic anhydride and 2,3-dimethyl succinic anhydride in the gas phase

The absorption spectrum of succinic anhydride shows a broad maximum near 220 nm, attributed to the lowest π*–n− transition. Photolysis in this region (220–270 nm) gives CO2, CO, and C2H4, but not in the equimolar quantities expected from a simple molecular dissociation. Production of CO also shows a time dependence, increasing with time, and a free-radical mechanism is tentatively suggested to account for this. The thermal decomposition of succinic anhydride at 625–775 K yields the same products, but is more complex, with larger deviations from the simple stoichiometry, and product formation non-linear with time and pressure. At short times, production of CO, the most abundant product, is described by first-order Arrhenius parameters of log A (s−1) = 11.6 and E = 53 kcal/mol, apparently independent of surface.The photolysis of the cis and trans isomers of 2,3-dimethylsuccinic anhydride at 250 and 230 nm is simpler than that of succinic anhydride, giving equimolar CO and CO2, but with butene-2 still falling short of a stoichiometric yield by from 10 to 40%. Both cis- and trans-butene-2 were produced, with the latter always in excess and with no retention of the configuration of the original anhydride. The thermal decomposition of 2,3-dimethylsuccinic anhydride is more complex than the photolysis, with much less butene-2 produced (again with no retention of cis–trans configuration) and methane an important product. Activation energies for CO formation were about 48 and 45 kcal/mol and log A (s−1) was 10.4 and 9.5 for the cis and trans isomers, respectively.