| Abstract
| - Absolute rate constants for hydrogen atom abstraction by alkoxylradicals from phenol, aniline,and diphenylamine have been measured in 14 solvents at room temperatureby laser flash photolysis.For all three substrates the rate constants decline as the solventbecomes a stronger hydrogen-bond acceptor (HBA). Thus, on changing the solvent fromCCl4 to CH3CN the rate constantsdeclineby factors of 148, 7.1, and 4.7 for PhOH, PhNH2, andPh2NH, respectively. The kineticsolventeffect for phenol correlates rather well with Abraham'sscale of relative HBA activities ofthesolvents we have employed as measured as solutes in CCl4solvent. This correlation is not quite sogood with aniline and it is almost nonexistent for diphenylamine.With all three substrates the“deviant” solvents produce higher rate constants than would beexpected from thevalue of thesolvent and, generally, these are the solvents in which sterichindrance to hydrogen-bond formationwould appear probable. The kinetic data for the three substratescan be most readily interpretedin terms of specific hydrogen bonding rather than being due to somegeneralized weakly dipolarinteraction. It is concluded, for example, that hydrogen bondinginvolves the π-electron cloud inbenzene but the chlorine atom in chlorobenzene.
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