Kinetic Solvent Effects on Hydrogen Atom Abstraction from Phenol, Aniline, and Diphenylamine. The Importance of Hydrogen Bonding on Their Radical-Trapping (Antioxidant) Activities1

Abstract

Absolute rate constants for hydrogen atom abstraction by alkoxyl radicals from phenol, aniline, and diphenylamine have been measured in 14 solvents at room temperature by laser flash photolysis. For all three substrates the rate constants decline as the solvent becomes a stronger hydrogen-bond acceptor (HBA). Thus, on changing the solvent from CCl4 to CH3CN the rate constants decline by factors of 148, 7.1, and 4.7 for PhOH, PhNH2, and Ph2NH, respectively. The kinetic solvent effect for phenol correlates rather well with Abraham's scale of relative HBA activities of the solvents we have employed as measured as solutes in CCl4 solvent. This correlation is not quite so good with aniline and it is almost nonexistent for diphenylamine. With all three substrates the “deviant” solvents produce higher rate constants than would be expected from the value of the solvent and, generally, these are the solvents in which steric hindrance to hydrogen-bond formation would appear probable. The kinetic data for the three substrates can be most readily interpreted in terms of specific hydrogen bonding rather than being due to some generalized weakly dipolar interaction. It is concluded, for example, that hydrogen bonding involves the π-electron cloud in benzene but the chlorine atom in chlorobenzene.