Enthalpies of formation and solvation of some organic anions

Abstract

Procedures are described for measuring ΔH_D, the enthalpy of deprotonation for organic Brönsted acids in DMSO. Evidence from a number of sources is presented to demonstrate that the deprotonation process is clean and complete.

Correlation with Bordwell's pKa data shows that ΔH^°_D=ΔG^°_D in DMSO for the compounds considered here. Good correlations are also found between ΔH_D(DMSO) and ΔH_D(gas) for different classes of compounds.

Solvation enthalpies are calculated by combining heats of deprotonation in the gas phase and in DMSO with heats of solution of the precursor acids. Comparison is made between resonancedelocalized carbanions, enolate anions, halide ions and alkoxide ions. The results are rationalized in terms of ionic size, charge density and steric hindrance to solvation.