Photohydration of Benzophenone in Aqueous Acid

Abstract

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 1H + , 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 1H + , 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 1H + + H 2 O f 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, pK a ( 3 1H + ) ) -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 1H + 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.

Photo‐Organocatalysis of Atom‐Transfer Radical Additions to Alkenes

“…Complete reactivity was restored when using a band-pass filter at l = 360 nm. We confirmed that trace amounts of acid were also generated during the ATRA reaction, which greatly affected the reactivity, [14] thus explaining the need for a base such as 2,6-lutidine [15] (entry 3 in Table 1). [13] In contrast, the organic halides 2, the olefins 3, and 2,6-lutidine do not absorb within this frequency region.…”

Water‐ and Acid‐Mediated Excited‐State Intramolecular Proton Transfer and Decarboxylation Reactions of Ketoprofen in Water‐Rich and Acidic Aqueous Solutions

“…[45] Our previous results also observed the 3 BP species was simultaneously protonated to generate the 3 …”

Inside/Outside: Post‐Synthetic Modification of the Zr‐Benzophenonedicarboxylate Metal–Organic Framework

“…Formation of the benzophenone diradical can be followed by the abstraction of an H atom from a substance RH and subsequent addition of R to the former keto carbon atom (path A). However, a radical electron of the photoexcited benzophenone can also become delocalized over a neighbouring aromatic ring, as exemplarily shown in path B for the reaction with water (see below) which leads to a hydroxylation of the benzene ring (path B) . Both path A and path B could also occur with benzophenone moieties which are part of the framework.…”

“…This part focusses on the reactions with simple molecules, namely methanol and ethanol as well as water. Only few studies have reported on the reactions of benzophenone in aqueous media; in the present case, these may be especially important, as in the laboratory the MOF samples are exposed to light and air humidity when no special precautions are taken. In the second part of the study, we have reacted the Zr‐ bzpdc ‐MOF with series of alkanes and alcohols of different chain length.…”