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.

Hidden chemistry of substituted aniline radical cations in water: a mechanistic study

“…It is known that by using non‐polar solvents, one can distinguish between radical cations and radicals based on free electron transfer reactions. For example, in DCE, the following reactions occur where S is a solute. This type of reaction termed as free electron transfer, is well studied by pulse radiolysis.…”

“…Later, to explore the chemical reaction paths, Brede et al . have used innovative methods to assign the transient bands formed after oxidation of the anilines and phenols . To enhance the understanding, the experimental data were supported by kinetic computer models.…”

Spectral, kinetics, and redox properties of the transients formed on one-electron oxidation of 4,4?-thiodiphenol: A pulse radiolysis study

“…The lifetime of the radical cations of naphthols and hydroxy biphenols are increased due to the extended aromatic ( ϭ 1 -3 s) moiety and could be observed as the transient intermediates [26]. The lifetime of the transient species formed on reaction of иOH radicals and specific oneelectron oxidants in neutral and acidic solutions was high (ϳ 330 s), and may indicate that the transient species is a phenoxyl radical.…”

“…The radical cation of phenol is highly unstable and phenoxyl radicals are the first ob-( Ͻ 0.2 s) servable transient species [26]. The lifetime of the radical cations of naphthols and hydroxy biphenols are increased due to the extended aromatic ( ϭ 1 -3 s) moiety and could be observed as the transient intermediates [26].…”

General and Theoretical Aspects of Phenols

“…This suggests that the ease with which a deprotonation of phenol radical cations occurs in different solutions 419,423,424 was likely to arise from either a specific participation of the solvent molecules in the supermolecule or a strong continuum effect.…”