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, ³1, encounters a state symmetry-imposed barrier, because the electronic structure of ³1 is n,π*, while that of its conjugate acid, ³1H⁺, is π,π*. Hence, the rate of protonation of ³1, k_H⁺ = 6.8 × 10⁸ M^-1 s^-1, is well below the diffusion-controlled limit. The short-lived transient intermediate formed by protonation of ³1 in 0.1−1 M aqueous HClO₄ (λ_max = 320 and 500 nm, τ = 50 ns) is not ³1H⁺, 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 HClO₄), where water activity is low. In moderately concentrated aqueous acids, adiabatic protonation of ³1 is the rate-limiting step preceding rapid adiabatic hydration of a phenyl ring, ³1H⁺ + H₂O → ³1·H₂O, k₀ = 1.5 × 10⁹ s^-1. These findings lead to a revised value for the acidity constant of protonated ³1, pK_a(³1H⁺) = −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 ³1H⁺ occurs predominantly at the meta position. A long-lived transient (λ_max = 315 nm, τ = 5.4 s) left after the decay of ³1·H₂O is attributed to a small amount of ortho-1·H₂O that regenerates 1 more slowly.