KINETIC-STUDY OF THE REACTIONS OF METHOXY SUBSTITUTED PHENACYL RADICALS

The photochemistry of various mono- and dimethoxy-substituted alpha-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-alpha-bromoacetophenone (6), all other substrates yield readily detectable triplet states; these have pi pi* 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 x 10(5) M(-1) s(-1), while for addition to the double bond in 1,1-diphenylethylene the rate constant is 9.4 x 10(7) M(-1) s(-1). Additions to other double bonds are likely to be slower (e.g., 12 + 1,3-cyclohexadiene, k less than or equal to 10(5) M(-1) s(-1)). In contrast, reaction with oxygen occurs with k(q) = 2.5 x 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.