Photochemical α Cleavage of Ketones in Solution. VI. Substituent Effects on the Photochemical α Cleavage of Deoxybenzoin

The effects of aromatic substituents which are not conjugated with the carbonyl on the photochemical behavior of deoxybenzoin have been investigated. Photochemical α cleavage is the exclusive primary photoprocess observed in benzene solution. Room temperature phosphorescence is observed for several deoxybenzoins and provides a convenient method of measuring triplet lifetimes. Substituents affect the rate constant for α cleavage without altering the triplet energy or radiative lifetime. The rate constants for α cleavage fit the Hammett equation with the use of σ⁺ (ρ = -1.1). It is concluded that the transition state for α cleavage lies early on the reaction coordinate and has moderate ionic character.