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

Abstract

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 <E <E , with E and E 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 and E were observed to shift towards less positive potentials with increasing concentrations of weak or strong hydrogen acceptors, respectively, whereas E remained virtually unaffected. This allows the electrochemical parameters ΔE =|E -E | and ΔE =|E -E | 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 data of the protonated pyridine compounds but were, however, in poor agreement with results obtained by density functional theory calculations.

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

“…The fluorinated alcohol also tends to favor secondary over primary hydrogen bonding when interacting with the phenylenediamine and its oxidized forms (see supporting information). Despite there being four molecules of the fluorinated alcohol present, only one primary hydrogen bond was found within P 2+ (TFE) 4 , which was in stark contrast to P 2+ (MeOH) 4 , P 2+ ( t BuOH) 4 and P 2+ (pyridine) 4 hydrogen‐bonded complexes that boasted four direct interactions. The P .+ (TFE) 4 and P(TFE) 4 hydrogen‐bonded complexes contained one and two primary hydrogen bonds, respectively.…”

“…This occurs because Δ E P ox(1,2) , unlike E P ox(1) and E P ox(2) , is unaffected by small changes between experiments in the uncompensated solution resistance, heterogeneous electron‐transfer rates ( k et ), surface condition of the working electrode and drifts in potential of the reference electrode and therefore is an accurate indicator of the strength of the hydrogen‐bonding interactions. The correlation between Δ E P ox(1,2) and hydrogen‐bonding strengths is however, not applicable to electrochemical systems that experience slow heterogeneous electron transfer or undergo adsorption, and for experiments that are performed under highly resistant conditions that result in distorted voltammograms …”

“…that are performed under highly resistantc onditions that result in distorted voltammograms. [9] The best-fit line DE P ox(1, 2) plotsf or the eight simple alcohols are shown in Figure5 to which their hydrogen acceptor strengths( when compared at % 5 m)f ollows the order primary < secondary < tertiary, specifically MeOH < iBuOH < nPrOH < EtOH < nBuOH < sBuOH < iPrOH < tBuOH (seei nset),s oi ndicating that the electronic influenceh as outweighed the steric effects in governing the hydrogen-bonding interactions. This sequence of the alcohols according to their hydrogen acceptor strengthsw as noted to be opposite to the order of their hydrogen donor abilities that was previously obtained when the electrochemical behavior of quinones were used to assess relative hydrogen bonding effects.…”

“…The different oxidized forms of the phenylenediamines can behave as hydrogen donors (with P and P 2+ being the weakest and strongest, respectively). This has led to the voltammetric behavior of 2,3,5,6‐tetramethyl‐1,4‐phenylenediamine, specifically its electrochemical parameter Δ E P ox(1, 2) =| E P ox(1) − E P ox(2) |, to be used for measuring the relative hydrogen acceptor strengths of pyridine compounds in CH 3 CN . E P ox(1) and E P ox(2) were reported to shift towards less positive potentials with increasing concentration or hydrogen‐bonding strength of the hydrogen acceptors because the ionization energies of P and P .+ decrease upon interaction.…”

“…This has led to the voltammetric behavior of 2,3,5,6-tetramethyl-1,4-phenylenediamine, specifically its electrochemical parameter DE P ox(1, 2) = j E P ox(1) ÀE P ox (2) j ,t o be used for measuring the relative hydrogen acceptor strengthso fp yridine compounds in CH 3 CN. [9] E P ox(1) and E P ox (2) were reported to shift towards less positive potentials with increasingc oncentration or hydrogen-bonding strength of the hydrogen acceptors because the ionization energies of Pa nd PC + decrease upon interaction. The shifts of E P ox (2) were, however,m uch larger than the shifts of E P ox(1) because the changes in the ionization energy of PC + were more significant than those of P; DE P ox(1, 2) concomitantly decreased and, consequently,i s inverselyr elatedt ot he strength of the interaction between the phenylenediamine( hydrogen donor) andp yridine compounds( hydrogen acceptor).…”