Oxygen versus carbon acidity in the side-chain fragmentation of 2-, 3-, and 4-arylalkanol radical cations in aqueous solution: The influence of the distance between the OH group and the aromatic ring

The decay of 2-, 3-, and 4-(4-methoxyphenyl) alkanol radical cations in water has been kinetically investigated by pulse radiolysis, the reaction products being determined either by steady-slate gamma-radiolysis experiments or by reactions promoted by potassium 12-tungstocobalt(III)ate, a bona fide one-electron oxidant. It was found that all 2-arylalkanol radical cations react with -OH at a diffusion-controlled rate leading to C-alpha-C-beta bond cleavage products. This suggests a reaction induced by deprotonation at the alcoholic OH group. In acidic medium (pH = 4), the rates of decay of these radical cations are much lower leading to C-alpha-H deprotonation (for 2-(4-methoxyphenyl)ethanol (1(.+)) and 1-(4-methoxyphenyl)-2-propanol (4(.+))) or C-alpha-C-beta bond cleavage produces (for 1-phenyl-2-(4-methoxyphenyl)ethanol (5(.+)) and 2-methyl-1-phenyl-3-(4-methoxyphenyl)-2-propanol (6(.+))). The 3-(4- methoxyphenyl)propanol radical cation (2(.+)) reacts in acidic medium (pH = 4) at a rate close to that of 1(.+), undergoing C-alpha-H deprotonation. In contrast, in basic medium (pH = 10) 2(.+) produces 3-(4-methoxyphenyl)propanal, with a rate similar to 5-fold lower than that of 1(.+), again indicating a reaction promoted by O-H deprotonation. With 4-(4-methoxyphenyl)-1-butanol radical cation (3(.+)), products of C-alpha-H deprotonation were observed both in the presence and in the absence of -OH. These results are discussed in terms of a mechanistic dichotomy, that is, carbon versus oxygen acidify, which appears to be operating for 2- and 3-arylalkanols whereas with 4-arylalkanol radical cations only carbon acidity is observed.