Hydrogen bonding association in the electroreduced intermediates of benzoquinones and naphthoquinones

Abstract

Keeping in view the importance of chemical and biological functions of quinone based couples; two different series of quinones, namely benzoquinones and naphthoquinones, are investigated electrochemically. Five compounds of each series are studied systematically in dichloromethane, acetonitrile, and propylene carbonate and from there the half-wave potentials of first and second reductions are obtained through cyclicoltammetry measurements. Four different alcohols are used with increasing concentrations as hydrogen bond donors on the basis of their increasing acidity. The hydrogen-bonding power is analyzed from the positive shifts in both the waves with increasing concentrations of alcohols. The quantitative comparison is made while calculating the thermodynamic association constants and number of alcohol molecules bonded to both anion and dianion of quinones. The qualitative behavior and quantitative data both indicate the quinone-alcohol interaction as hydrogen bonding and the strength of hydrogen bond is dependant upon the nature of species involved in this couple. From the cyclic voltammetric data the relative effects of hydroxylic additives and different substituted quinones on equilibrium constant are also observed. Solvent effect is rationalized in favor of hydrogen bonding in terms of solvent polarity parameters.