Photohydration of Benzophenone in Aqueous Acid†
2003-01-09T00:00:00Z (GMT) by
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, kH+ = 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 (λmax = 320 and 500 nm, τ = 50 ns) is not 31H+, as was assumed in previous studies. The latter (λmax = 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, pKa(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 (λmax = 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.