10.1021/jp026663f.s001 Markus Ramseier Markus Ramseier Paul Senn Paul Senn Jakob Wirz Jakob Wirz Photohydration of Benzophenone in Aqueous Acid<sup>†</sup> American Chemical Society 2003 adiabatic HClO acid triplet protonation benzophenone 3 1 H 3 1 protonated 3 1 derivative nm max hydration 2003-01-09 00:00:00 Journal contribution https://acs.figshare.com/articles/journal_contribution/Photohydration_of_Benzophenone_in_Aqueous_Acid_sup_sup_/3724551 Why is the triplet state of aromatic ketones quenched by protons? The long-known but unexplained quenching process was investigated in detail for benzophenone (<b>1</b>). Adiabatic protonation of triplet benzophenone, <sup>3</sup><b>1</b>, encounters a state symmetry-imposed barrier, because the electronic structure of <sup>3</sup><b>1</b> is n,π*, while that of its conjugate acid, <sup>3</sup><b>1</b>H<sup>+</sup>, is π,π*. Hence, the rate of protonation of <sup>3</sup><b>1</b>, <i>k</i><sub>H</sub><sup><sub>+</sub></sup> = 6.8 × 10<sup>8</sup> M<sup>-1</sup> s<sup>-1</sup>, is well below the diffusion-controlled limit. The short-lived transient intermediate formed by protonation of <sup>3</sup><b>1</b> in 0.1−1 M aqueous HClO<sub>4</sub> (λ<sub>max</sub> = 320 and 500 nm, τ = 50 ns) is not <sup>3</sup><b>1</b>H<sup>+</sup>, as was assumed in previous studies. The latter (λ<sub>max</sub> = 385 nm) is detectable only in acidified acetonitrile or in highly concentrated aqueous acid (>5 M HClO<sub>4</sub>), where water activity is low. In moderately concentrated aqueous acids, adiabatic protonation of <sup>3</sup><b>1</b> is the rate-limiting step preceding rapid adiabatic hydration of a phenyl ring, <sup>3</sup><b>1</b>H<sup>+</sup> + H<sub>2</sub>O → <sup>3</sup><b>1</b>·H<sub>2</sub>O, <i>k</i><sub>0</sub> = 1.5 × 10<sup>9</sup> s<sup>-1</sup>. These findings lead to a revised value for the acidity constant of protonated <sup>3</sup><b>1</b>, p<i>K</i><sub>a</sub>(<sup>3</sup><b>1</b>H<sup>+</sup>) = −0.4 ± 0.1. Acetophenone (<b>2</b>) and several derivatives of <b>1</b> and <b>2</b> undergo a similar reaction sequence in aqueous acid. The acid-catalyzed photohydration of parent <b>1</b> and <b>2</b> 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 <sup>3</sup><b>1</b>H<sup>+</sup> occurs predominantly at the meta position. A long-lived transient (λ<sub>max</sub> = 315 nm, τ = 5.4 s) left after the decay of <sup>3</sup><b>1</b>·H<sub>2</sub>O is attributed to a small amount of <i>ortho</i>-<b>1</b>·H<sub>2</sub>O that regenerates <b>1</b> more slowly.