Gas and Solution Phase Reactivities of Triplet Aromatic Carbonyls

Abstract

AbstractAbsolute rate studies pertinent to the gas and solution phase photochemistry of triplet benzaldehyde, acetophenone and benzophenone have been carried out, and it has been confirmed that, in the gas phase, attack on a hydrocarbon by the carbonyl results in the expected hydrogen abstraction. Also absolute rates in both the gas phase and solution (acetonitrile) are quite comparable. For a given carbonyl the differences in the rates of abstraction are largely accounted for by activation energy changes. The activation energies vary from a high of 4.8 kcal mol−1 for abstraction of a primary hydrogen by acetophenone to a low of 1.4 kcal mol−1 for abstraction of tertiary hydrogen by benzaldehyde. At room temperature the relative reactivities of primary, secondary and tertiary C—H bonds for all the carbonyls are 1:23:190. In solution benzophenone is two times more reactive than acetophenone, while benzaldehyde is ten times more reactive. In the gas phase benzaldehyde is about 18 times more reactive than acetophenone. Molecular orbital calculations indicate that the greater reactivity of benzaldehyde as compared to acetophenone stems from steric hindrance in the latter. The quenching of acetophenone triplets by various other substrates including naphthalene, benzene, isopropanol and triethylamine is also discussed.