A Study of the Norrish Type II Reaction in the Solid State¹

Abstract

The solid state photochemistry of six α-cycloalkyl-p-chloroacetophenone derivatives (cyclobutyl, cyclopentyl, cyclohexyl, cycloheptyl, exo-2-norbornyl and 1-adamantyl) is reported. All six undergo smooth type II photochemistry in the crystalline phase. The cyclization-to-cleavage ratios and the cis-to-trans cyclobutanol ratios are tabulated for each ketone, and the results are compared with the corresponding data from the solution photolyses. In general, the solid state medium was found to exert a relatively small effect on the product ratios. The γ-hydrogen atom to carbonyl oxygen abstraction distances, as well as the angular relationships between these two atoms, were determined form the X-ray crystal structure data for each ketone. The data showed that (1) six atom abstraction geometries other than chairlike can be accommodated, (2) abstraction can occur over distances much longer than previously supposed (up to 3.10 Å), and (3) there is no strict requirement that the hydrogen undergoing abstraction be in the plane of the carbonyl oxygen n-orbital. Attempts were made to correlate the solid state structural data with the rate constants for hydrogen atom abstraction as determined in solution from Stern-Volmer quenching plots. The lack of any such correlation is interpreted as indicating a significant contribution to reaction in solution from non-minimum energy ketone conformations.