Measuring the Relative Hydrogen‐Bonding Strengths of Alcohols in Aprotic Organic Solvents

Abstract

Voltammetric experiments with 9,10-anthraquinone and 1,4-benzoquinone performed under controlled moisture conditions indicate that the hydrogen-bond strengths of alcohols in aprotic organic solvents can be differentiated by the electrochemical parameter ΔEp (red) =|Ep (red(1)) -Ep (red(2)) |, which is the potential separation between the two one-electron reduction processes. This electrochemical parameter is inversely related to the strength of the interactions and can be used to differentiate between primary, secondary, tertiary alcohols, and even diols, as it is sensitive to both their steric and electronic properties. The results are highly reproducible across two solvents with substantially different hydrogen-bonding properties (CH3 CN and CH2 Cl2 ) and are supported by density functional theory calculations. This indicates that the numerous solvent-alcohol interactions are less significant than the quinone-alcohol hydrogen-bonding interactions. The utility of ΔEp (red) was illustrated by comparisons between 1) 3,3,3-trifluoro-n-propanol and 1,3-difluoroisopropanol and 2) ethylene glycol and 2,2,2-trifluoroethanol.

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

“…[6,12] The neutral compound Pl argely behaves as aw eak base and so preferentially functions as ah ydrogen acceptort hrough the lone pairs of electrons on its nitrogen atoms, but can also serve as ah ydrogen donor via the hydrogen atoms of the amino groups. We have previously applied the electrochemical reduction of quinones for measuring the hydrogen donor abilities of alcohols andd iols in aprotic organic solvents [13] and now report The voltammetric behavior of 2,3,5,6-tetramethyl-1,4-phenylenediamine wasf ound to be able to differentiate the hydrogen acceptorabilities of electroinactivepyridine compounds in acetonitrile. We have previously applied the electrochemical reduction of quinones for measuring the hydrogen donor abilities of alcohols andd iols in aprotic organic solvents [13] and now report The voltammetric behavior of 2,3,5,6-tetramethyl-1,4-phenylenediamine wasf ound to be able to differentiate the hydrogen acceptorabilities of electroinactivepyridine compounds in acetonitrile.…”

“…This additional oxidation reactiono ccurred at ap otential between the two 1e À -oxidation reactions that phenylenediamines typically undergo (i.e. [13] However,s ystems that experience slow heterogeneous electron transfer,u ndergo adsorption,o ra re studied under highly resistantc onditions that result in distorted voltammograms are not amendable for correlating DE with hydrogen bonding effects. E P ox(2) and E P ox(3) were observed to shift towards less positive potentials with increasing concentrations of weak or strongh ydrogen acceptors, respectively,w hereas E P ox(1) remained virtually unaffected.…”

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

“…Primary hydrogen bonding between the phenylenediamine species and alcohols typically occurred through either one or two alcohol molecules, to which, despite the existence of other sites for primary interactions, the subsequent alcohol molecules were found to preferentially interact with these hydrogen‐bonded alcohol molecules rather than directly with the phenylenediamine. The propensity for forming such secondary interactions might be linked to the cooperativity and anti‐cooperativity effects of hydrogen bonding, which was also observed in calculations on reduced quinones in the presence of alcohols …”

“…The best‐fit line Δ E P ox(1, 2) plots for the eight simple alcohols are shown in Figure to which their hydrogen acceptor strengths (when compared at ≈5 m ) follows the order primary < secondary < tertiary, specifically MeOH < i BuOH < n PrOH < EtOH < n BuOH < s BuOH < i PrOH < t BuOH (see inset), so indicating that the electronic influence has outweighed the steric effects in governing the hydrogen‐bonding interactions. This sequence of the alcohols according to their hydrogen acceptor strengths was 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 …”

Impurities in Nitrile Solvents Commonly Used for Electrochemistry, and their Effects on Voltammetric Data

“…It is also not possible for trace water to be behind the large deviations of the electrochemical responses of BQ and PD from their EE behavior, nor for the occurrence of additional electrochemical processes. This is because water's hydrogen‐bonding effects are known to only cause a shift in the oxidation and reduction potentials of PD and BQ, respectively, with a concomitant decrease in Δ E P and Δ E Q , but leave their general electrochemical behavior unchanged . It was previously determined that approximately 12 m of water was required for the voltammograms of BQ to show a single electrochemical process that corresponded to the direct two‐electron reduction of the neutral species into the dianion .…”