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

Abstract

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 Cα−Cβ 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 Cα−H deprotonation (for 2-(4-methoxyphenyl)ethanol (1•+) and 1-(4-methoxyphenyl)-2-propanol (4•+)) or Cα−Cβ 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 Cα−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 Cα−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.