The Photolysis of Benzoic Acid in the Vapor Phase

The vapor phase photolysis of benzoic acid has been investigated in a flow system at temperatures ranging from 110° to 305°C, pressures from approximately 0.06 to 1.73 torr, and at various incident light intensities. The main identified products are carbon dioxide and benzene, and the minor identified products are carbon monoxide together with much smaller amounts of hydrogen. -- At 150° and 205° the rate of formation of carbon dioxide is directly proportional to the light intensity absorbed. For a constant incident light intensity, the rate of carbon dioxide formation was found to be independent of the benzoic acid concentration at pressures from 0.29 to 1.73 torr. The rate of formation of carbon dioxide increases with temperature and the activation energy is 3.5 ± 0.4 kcal mole⁻¹. -- A mechanism for carbon dioxide formation is suggested in which reaction occurs from a vibrationally excited ground state molecule, C₆H₅COOH', originating from the initially formed singlet state, C₆H₅COOH*, by internal conversion. The phenyl radical so formed initiates a free radical chain reaction. -- C₆H₅COOH + h v → C₆H₅COOH* -- C₆H₅COOH* → C₆H₅COOH' -- C₆H₅COOH' + M → C₆H₅COOH + M -- C₆H₅COOH' → C₆H₅ + COOH -> C₆H₅ + C₆H₅COOH → C₆H₅ + C₆H₆COO -- C₆H₅COO → C₆H₅ + CO₂ -- C₆H₅ + Wall → X