Voltammetric determination of the rate constant for the hydrolysis of the vitamin E cation.

Abstract

The electrochemical oxidation of x-tocopherol (vitamin E)(x-TOH) occurs in a -2e-/-H+ process to form the phenoxonium cation (x-TO+) which survices for several hours at room temperature in ultra-dry organic solvents such as acetonitrile and dicloromethane. As the water content of the organic solvent is increased, x-TO+ is known to undergo a hydrolysis type reaction with water to form a hemiketal, as hightlighted in Scheme1. Variable scan rate cyclic voltammetry was used to study the hydrolysis reaction of x-TO+ in the presence of varying concentrations of deliberately added water, that was accurately measured using Karl Fisher titrations. It was determined that in order to study the hydrolysis reaction over experimentally convenient scan rate ranges( 0.1- 20 v/s), between 100 to 600 mM of water needed to be added to the acetonitrile solutions. Digital simulations of the cyclic voltammetry data were used to estimate the rate and equilibrium constant for the hydrolysis reaction as well as the relevant electrochemical parameters (Eo-values, diffusion coefficients and heterogeneous electron transfer rate constants).