An in-situ radiolysis EPR study of spin trapping by 2-methyl-2-nitrosopropane: steric and electronic effects influencing the trapping of hydroxyalkyl radicals derived from pentanols and substituted pentanols

Abstract

The spin adducts formed by reaction of bulky hydroxyalkyl radicals with the nitroso spin trap 2-methyl-2-nitrosopropane (MNP) were studied using in-situ radiolysis EPR. Parent hydroxyalkyl radicals were produced in aqueous solution either by hydroxyl-radical reaction with unsubstituted and methyl-substituted alcohols (propanols, pentanols and cyclohexanols) or by reaction of the corresponding ketone with the hydrated electron. The parent radicals included 1-hydroxypentyl, 1-hydroxy-1-methylbutyl, 1-ethyl-1-hydroxypropyl, 1-hydroxy-1-isopropyl-2-methylpropyl, 1-hydroxy-2,2-dimethylpropyl, 1,3-dihydroxy-2,2-dimethylpropyl, 1-hydroxycyclohexyl, 1-hydroxy-2-methylcyclohexyl, and 1-tert-butyl-1-hydroxy-2, 2-dimethylpropyl radicals. All but the bulkiest radicals reacted with MNP by addition at the nitroso nitrogen site to form the MNP–C(OH)RR′ spin adduct. In contrast with previous MNP spin-trapping studies using hydroxymethyl, hydroxyethyl, and 1-hydroxy-1-methylethyl parent radicals, steric interactions strongly modulated the yields of the spin adducts produced. Strongly reducing hydroxyalkyl radicals also reacted with MNP to produce the MNP–H adduct by direct reduction of MNP. Steric hindrance between the parent radical and MNP was sufficient in the most extreme case to shut off MNP–R production with concomitant production of MNP–H. Spin-adduct persistence was measured for the MNP–hydroxyalkyl and MNP–alkyl spin adducts. Hydroxyalkyl spin adduct lifetimes varied from seconds (MNP–1-hydroxy-1-methylbutyl) to one year (MNP-1-hydroxycyclohexyl), correlating with the level of aminoxyl function shielding afforded by its substituent groups. MNP spin adducts formed from other non-hydroxyalkyl alcohol radicals had short lifetimes of less than 18 hours.