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 Ring1

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-state γ-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 α −C β 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 α −H deprotonation (for 2-(4-methoxyphenyl)ethanol ( 1 • + ) and 1-(4-methoxyphenyl)-2-propanol ( 4 • + )) or C α −C β bond cleavage products (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 α −H deprotonation. In contrast, in basic medium (pH = 10) 2 • + produces 3-(4-methoxyphenyl)propanal, with a rate ∼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 α −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 acidity, which appears to be operating for 2- and 3-arylalkanols whereas with 4-arylalkanol radical cations only carbon acidity is observed.