The Determination of Proton Affinities of Secondary Alcohols from the Dissociation of Proton-Bound Molecular Trios. A New Application of the Kinetic Method

Abstract

The kinetic method for determining proton affinity ( PA) values requires that the competing dissociations of proton-bound pairs of bases, AH+Bn, essentially only lead to AH+ and BnH+ ions. The method fails for secondary alcohols because other reactions, involving rearrangement of the AH+Bn ion, are dominant. It was found that proton-bound trios of bases, A2H+Bn, (which incorporate secondary alcohols Bn with a primary alcohol, A) do not suffer from this disadvantage. The proton-bound trios decomposed cleanly, to yield only A2H+ and AH+Bn ions. Application of the kinetic method to these competing reactions allowed the PA values for the secondary alcohols to be determined using the appropriate molecular-pair proton affinity [ MPPA] values. The MPPA is the negative of the enthalpy change, [–Δ H], accompanying the formation of a proton-bound pair from the component alkanols and a proton. These were readily estimated using known binding energies, D[AH+–Bn] and established PA values for n-alkanols. In the present brief study, PA values for isopropanol, cyclobutanol and cyclopentanol were measured to be 796, 792 and 798, all ± 6 kJ mol−1. The results for the cyclic alcohols relate well with the available data from similar compounds, where the C3–C5 rings display lower PA values than their acyclic analogues. This effect is attributed to restricted charge delocalization in the small rings.