The determination of proton affinities of secondary alcohols from the dissociation of proton-bound molecular trios. A new application of the kinetic method

摘要

The kinetic method for determining proton affinity (PA) values requires that the competing dissociations of proton-bound pairs of bases, AH~+B_n, essentially only lead to AH~+ and B_nH~+ ions. The method fails for secondary alcohols because other reactions, involving rearrangement of the AH~+B_n ion, are dominant. It was found that proton-bound trios of bases, A_2H~+B_n, (which incorporate secondary alcohols B_n with a primary alcohol, A) do not suffer from this disadvantage. The proton-bound trios decomposed cleanly, to yield only A_2H~+ and AH~+B_n 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, [-ΔH], 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~+-B_n] 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 C_3-C_5 rings display lower PA values than their acyclic analogues. This effect is attributed to restricted charge delocalization in the small rings.