KINETIC-STUDY OF THE REACTIONS OF METHOXY SUBSTITUTED PHENACYL RADICALS

The photochemistry of various mono- and dimethoxy-substituted alpha-br omoacetophenones has been investigated by laser flash photolysis in or ganic solvents. The short-lived excited singlet states cleave to yield bromine atoms and the corresponding methoxyphenacyl radicals with qua ntum yields ranging from 0.13 to 0.35. With the exception of 4-methoxy -alpha-bromoacetophenone (6), all other substrates yield readily detec table triplet states; these have pi pi character and are very poor hy drogen abstractors. Triplet decay does not contribute to methoxyphenac yl radical formation. While methoxyphenacyl radicals may have a spin d ensity as high as 0.3 at the carbonyl oxygen, which accounts for the a lkoxy-like visible band in their absorption spectrum, their chemical r eactivity is dominated by the radical character at the carbon site. Me thoxyphenacyl radicals are modest hydrogen abstractors; for example, r eaction 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-di phenylethylene 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-cyclohe xadiene, k less than or equal to 10(5) M(-1) s(-1)). In contrast, reac tion 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 m ethoxyphenacyl radicals will be converted to peroxyl radicals. These p robably play a key role in the oxidative photodegradation of Lignin.