Ionization of mercapto- and hydroxypyridines by free electron transfer to alkyl chloride parent radical cations

Abstract

In the pulse radiolysis of solutions of 2-mercaptopyridine (2-PySH) and 2-hydroxypyridine (2-PyOH) in n-butyl chloride, as well as 4-mercaptopyridine (4-PySH) and 3-hydroxypyridine (3-PyOH) in 1,2-dichloroethane, radical cations (PySH˙+ and PyOH˙+) are generated in the electron transfer reaction from solute to parent solvent radical cations exhibiting rate constants (1–2) × 1010 dm3 mol−1 s−1. The solute radical cations decay within less than 100 up to 200 ns, yielding pyridinethiyl radicals (PyS˙) or pyridoxyl radicals (PyO˙), respectively. Quenching the solvent cation radicals with ethanol, it is observed that the resulting radicals, e.g., Cl-CH2-CH˙ -Cl radicals, react with 3-PyOH by addition. In the presence of ethanol, radicals formed by deprotonation of the solvent radical cations react with mercaptopyridine resulting in pyridinethiyl radicals PyS˙(k about 1× 109 dm3 mol−1 s−1). With parent 4-PySH, 4-mercaptopyridinethiyl radicals undergo an equilibrium-like dimerization reaction where k = 900 dm3 mol−1 was determined. Quantum chemical calculations support our mechanistic interpretations and provide information about the charge distribution and the reactivity of the radical cations and radicals described.