Pulse radiolytic investigation of aliphatic amines in aqueous solution

Abstract

The rates of reaction of hydroxyl radicals with various aliphatic amines have been determined as a function of pH. These include methyl, ethyl, isopropyl, t-butyl, trimethyl and triethyl amines. Rate constants of ap 3 × 10 8 dm 3 mol −1s −1 for protonated amines and ap 1 × 10 10 dm 3 mol −1s −1 for unprotonated amines have been obtained. This marked difference is attributed to the electrophilic character of the hydroxyl radical and its unfavorable interaction with the positive center. The transient absorption spectra of the various aminoalkyl and alkylamino radicals produced from the reaction of OH radicals with these amines were derived over the pH range 3–13. The transient species produced from trimethyl and triethyl amines have absorption maxima at ap 260 nm, all the other amines showed no maxima down to ap 230 nm, the limit imposed by experimental condictions. Strong pH dependence of the initial transient absorption is observed for all the amines examined. These changes are caused by the acid-base dissociation of the radicals, and by changes in the site of OH radical attack. Generally, it can be stated that hydrogen atom abstraction by OH radicals from unprotonated amines takes place mainly from an α-position and/or from hydrogen bonded to nitrogen, whereas abstraction from protonated amines occurs mainly at positions β- and/or γ-from the  ▪H group.