THE KINETICS OF REACTION OF 2,2-DIPHENYL-1-PICRYLHYDRAZYL WITH PHENOLS

Abstract

The kinetics of reaction between 2,2-diphenyl-1-picrylhydrazyl (DPPH) and a wide variety of phenols have been studied. The rate of disappearance of DPPH is of first order with respect to both the DPPH and the reacting phenol. The rates of reaction can be roughly correlated with the Hammett σ value of the phenol substituent in the range −0.4 < σ < 0.2, a ρ value of −6 being obtained. t-Butyl groups in both ortho positions of the phenol give rise to steric hindrance, the reduction in rate being largely due to a reduction in the A factor. Hydrogen abstraction from the less reactive phenols is strongly retarded by the product, 2,2-diphenyl-1-picrylhydrazine.The rate-determining step probably involves the abstraction of a hydrogen atom from the phenol by the DPPH to give diphenylpicrylhydrazine and a phenoxy radical. The retardation by diphenylpicrylhydrazine is readily explained if this primary step is reversible.