Two channels of electron transfer observed for the reaction of n-butyl chloride parent radical cations with naphthols and hydroxybiphenyls

Abstract

Pulse radiolysis of naphthols (NpOH) and hydroxybiphenyls (ByOH) in n-butyl chloride (BuCl) at room temperature exhibits electron transfer at a bimolecular rate constant of (1.0-2.8) × 10¹⁰ dm³ mol^-1 s^-1. The experiments reveal the direct formation of two types of transients: phenol type radical cations (NpOH^•+, ByOH^•+) and phenoxyl type radicals (NpO^•, ByO^•). This is explained by a mechanism involving two different electron-transfer channels. The solute radical cations exhibit two optical absorption bands in the 570-650 and 360-460 nm regions and undergo electron transfer with triethylamine and proton transfer with ethanol with bimolecular rate constants of (4-12) × 10⁹ and (3-6) × 10⁸ dm³ mol^-1 s^-1, respectively. NpO^• and ByO^• have relatively long lifetimes and show absorption bands in the 340-400 and 470-540 nm regions. By way of comparison, these phenoxyl type radicals are separately generated by pulse radiolysis in aqueous alkaline solution containing sodium azide, i.e., by oxidation of the solutes with N₃^• radicals. Under these conditions, the phenoxyl radicals decay by second-order kinetics with 2k = (1.2-4.5) × 10⁸ dm³ mol^-1 s^-1. The various modes of formation and decay of the phenolic radical cations are analyzed over a wide range of dose rate and solute concentrations. In comparison to radical cations of one-ring phenols, the increased stability of NpOH^•+ and ByOH^•+ is explained by the delocalization of the positive charge over the whole aromatic system, a postulate supported by open-shell quantum chemical calulations.