Entropy-Driven Proton-Transfer Reactions

Article abstract of DOI:10.1021/j100170a039

The reation between kinetics and thermochemistry in fat reactions is examined, including reactions with substantial entropy changes.Rate constants for such reactions, in the range of (0.02-3.0) * 10^-9 cm² s^-1, were measured by pulsed high-pressure mass spectrometry.The following relations were observed: (1) The reaction efficiency in either direction is controlled uniquely and completely by the overall reaction free energy change.Specifically, the efficieny r is determined by the equilibrium constant according to r= K/(1+K). (2) The sum of reaction efficiencies in the forward (exergonic) and reverse (endergonic) directions is near unity (r_f + r_r = ca. 1).These relations are obseved in anionic and cationic systems, in reactions with ΔH⁰ up to 12 kcal/mol and with ΔS⁰ up to 15 cal/(mol K).Consistent with (1), reactions that are endothermic up to 7 kcal/mol can nevertheless proceed near the collision rate, when positive entropy changes make the reactions exergonic.The entropy canges are effective regardless of their stuctural origin.Relations analogous to (1) and (2) are also derived for reactions with multiple channels that proceed without significant barriers through a common intermediate.