Using Voltammetry to Measure the Relative Hydrogen-Bonding Strengths of Pyridine and Its Derivatives in Acetonitrile

The voltammetric behavior of 2,3,5,6-tetramethyl-1,4-phenylenediamine was found to be able to differentiate the hydrogen acceptor abilities of electroinactive pyridine compounds in acetonitrile. Weak and strong hydrogen acceptors were distinguished through the onset of a third oxidation process that came about at sub-stoichiometric amounts of strong hydrogen acceptors, but not in the presence of weak hydrogen acceptors. This additional oxidation reaction occurred at a potential between the two 1 e⁻-oxidation reactions that phenylenediamines typically undergo (i.e. E_P ^ox(1)<E_P ^ox(3)<E_P ^ox(2), with E_P ^ox(1) and E_P ^ox(2) representing the electrochemical conversion of the neutral phenylenediamine into the radical cation and thereafter to the quinonediimine dication) as well as at the expense of the second electrochemical reaction. E_P ^ox(2) and E_P ^ox(3) were observed to shift towards less positive potentials with increasing concentrations of weak or strong hydrogen acceptors, respectively, whereas E_P ^ox(1) remained virtually unaffected. This allows the electrochemical parameters ΔE_P ^{ox(1, 2)}=|E_P ^ox(1)−E_P ^ox(2)| and ΔE_P ^{ox(1, 3)}=|E_P ^ox(1)−E_P ^ox(3)| to be employed as measures of the hydrogen-bonding strengths within each category, to which they were found to be highly reproducible and responsive to steric, electronic, inductive, and mesomeric effects. The electrochemical findings concur with available aqueous pK_a data of the protonated pyridine compounds but were, however, in poor agreement with results obtained by density functional theory calculations.