The Dual Roles of Phenylenediamines: Using their Voltammetric Behavior to Measure the Hydrogen Donor and Acceptor Abilities of Alcohols in Acetonitrile

Cyclic voltammetry experiments on 2,3,5,6-tetramethyl-1,4-phenylenediamine (P) in acetonitrile in the presence of varying concentrations of alcohols indicate that the oxidized forms of the compound (P^.+ and P²⁺) interact with the alcohols through a hydrogen-bonding mechanism where P^.+ and P²⁺ act as the hydrogen donor and the alcohols act as acceptors. However, the neutral form (P) largely acts as a hydrogen acceptor but only for strong hydrogen donors that do not undergo proton-transfer reactions with the phenylenediamine. These results were ascertained based on measuring the difference in potential of the two one-electron transfer reactions (ΔE_P ^{ox(1, 2)}=|E_P ^ox(1)−E_P ^ox(2)|) in the oxidative electrochemistry of P, which thereby allows a simple measure of relative hydrogen bonding strengths.