Kinetic studies of a phenoxy radical intermediate using optical and electron spin resonance spectroscopy

Abstract

The reaction between 2,2-diphenyl-1-picrylhydrazyl (DPPH) and 4-isopropyl-2,6-di-t-butylphenol has been studied by optical and electron spin resonance methods. The concentration of the intermediate 4-isopropyl-2,6-di-t-butylphenoxy radical, determined by electron spin resonance spectroscopy, rises fairly rapidly to a maximum value and then decays relatively slowly. Addition of 2,2-diphenyl-1-picrylhydrazine (DPPH-H) reduces the maximum concentration of phenoxyl but does not significantly change the initial rate of production of the intermediate.The kinetic results are interpreted in terms of a reversible primary reaction between DPPH and the phenol to give DPPH-H and the phenoxy radical. The phenoxyl can disappear by reaction with itself or with a second molecule of DPPH. The rate constant for the primary step at 20 °C is 0.18 ± 0.02 l/(mole s) and for the reverse reaction is 1.34 ± 0.2 l/(mole s); the rate constant for the reaction between 4-isopropyl-2,6-di-t-butylphenoxyl and DPPH is 0.68 ± 0.2 l/(mole s) at 20 °C. The activation energies for the respective reactions are 6.8 ± 0.6, 6.0 ± 1.0, and 6.4 ± 1.5 kcal/mole.