Photohydration of Benzophenone in Aqueous Acid

Why is the triplet state of aromatic ketones quenched by protons? The long-known but unexplained quenching process was investigated in detail for benzophenone ( 1 ). Adiabatic protonation of triplet benzophenone, 3 1, encounters a state symmetry-imposed barrier, because the electronic structure of 3 1 is n,π*, while that of its conjugate acid, 3 1 H +, is π,π*. Hence, the rate of protonation of 3 1, k H + = 6.8 × 10 8 M -1 s -1, is well below the diffusion-controlled limit. The short-lived transient intermediate formed by protonation of 3 1 in 0.1−1 M aqueous HClO 4 (λ max = 320 and 500 nm, τ = 50 ns) is not 3 1 H +, as was assumed in previous studies. The latter (λ max = 385 nm) is detectable only in acidified acetonitrile or in highly concentrated aqueous acid (>5 M HClO 4 ), where water activity is low. In moderately concentrated aqueous acids, adiabatic protonation of 3 1 is the rate-limiting step preceding rapid adiabatic hydration of a phenyl ring, 3 1 H + + H 2 O → 3 1 ·H 2 O, k 0 = 1.5 × 10 9 s -1 . These findings lead to a revised value for the acidity constant of protonated 3 1, p K a ( 3 1 H + ) = −0.4 ± 0.1. Acetophenone ( 2 ) and several derivatives of 1 and 2 undergo a similar reaction sequence in aqueous acid. The acid-catalyzed photohydration of parent 1 and 2 is reversible. In meta-fluorinated derivatives, the reaction results in a clean and efficient formation of the corresponding phenols, a novel aromatic photosubstitution reaction. This indicates that hydration of 3 1 H + occurs predominantly at the meta position. A long-lived transient (λ max = 315 nm, τ = 5.4 s) left after the decay of 3 1 ·H 2 O is attributed to a small amount of ortho - 1 ·H 2 O that regenerates 1 more slowly.