The photolysis and thermal decomposition of pyruvic acid in the gas phase

The photolysis of pyruvic acid vapour has been studied at wavelengths of 366, 345, and 320 nm, at a temperature of 340 K and pressures from about 1 to 10 Torr. Products observed were CO 2 and CH 3 CHO, with the former always in excess, and a quantum yield of CO 2 of 0.9 ± 0.1 at 366 nm. Light emission was also observed, and from the effect of added O 2 it was concluded that emission occurred from both singlet and triplet states. It is suggested that the primary process in the photolysis is an internal hydrogen-atom transfer followed by dissociation into CO 2 and CH 3 —C—OH, with the latter then rearranging to give CH 3 CHO and other products.The absorption spectrum is reported; the first system begins at about 380 nm, rising to a maximum at about 350 nm, with ε max = 10 M −1 cm −1 , and is attributed to the first excited π* ← n + singlet state. Weak absorption below 300 nm is probably due to the π* ← n − state and much stronger absorption below 220 nm to the π* ← π state.The thermal decomposition was studied briefly at temperatures from 455 to 584 K and pressures from about 0.8 to 7 Torr. Products were the same as in the photolysis, and Arrhenius parameters of log A (s −1 ) = 7.19 and E = 27.7 kcal/mol were obtained, based on the formation of CO 2 , which was apparently a simple, homogeneous unimolecular process.