Kinetic study of the reactions of methoxy-substituted phenacyl radicals

Abstract

The photochemistry of various mono- and dimethoxy-substituted α-bromoacetophenones has been investigated by laser flash photolysis in organic solvents. The short-lived excited singlet states cleave to yield bromine atoms and the corresponding methoxyphenacyl radicals with quantum yields ranging from 0.13 to 0.35. With the exception of 4-methoxy-α-bromoacetophenone (6), all other substrates yield readily detectable triplet states; these have π,π* character and are very poor hydrogen abstractors. Triplet decay does not contribute to methoxyphenacyl radical formation. While methoxyphenacyl radicals may have a spin density as high as 0.3 at the carbonyl oxygen, which accounts for the alkoxy-like visible band in their absorption spectrum, their chemical reactivity is dominated by the radical character at the carbon site. Methoxyphenacyl radicals are modest hydrogen abstractors; for example, reaction of 6 with 1,4-cyclohexadiene occurs with a rate constant of 2.6 × 10 5 M −1 s −1 , while for addition to the double bond in 1,1-diphenylethylene the rate constant is 9.4 × 10 7 M −1 s −1 . Additions to other double bonds are likely to be slower (e.g., 12 + 1,3-cyclohexadiene, k ≤ 10 5 M −1 s −1 ). In contrast, reaction with oxygen occurs with k q = 2.5 × 10 9 M −1 s −1 . Under the conditions relevant for their participation in paper yellowing, the methoxyphenacyl radicals will be converted to peroxyl radicals. These probably play a key role in the oxidative photodegradation of lignin. Keywords: methoxyphenacyl radicals, α-bromoacetophenones, laser flash photolysis.