Periodate-Glycol Reaction. 3. Pinacol, a Reevaluation

Article abstract of DOI:10.1021/j100001a012

The rate constants for the reaction of pinacol with periodic acid have been very accurately evaluated over the pH range 0.61-5.60 and at five temperatures 3.2-45 deg C.At 3.2-25 deg C the rate maximizes at pH 1.0-1.5; whereas at 35-45 deg C the maximum rates are at pH 0.61.Both ΔH(excit.) and ΔS(excit.), the energy and entropy of activation, decrease markedly with decreasing pH at a given temperature and decrease modestly with increasing temperature at a given pH.It is concluded that periodic acid and pinacol undergo a direct bimolecular reaction but with the following considerations: (1) In addition to the major reaction of pinacol with periodic acid, there is a minor reaction with the periodate ion. (2) An additional equilibrium, H5IO6+H⁺<*>X⁺, deactivates periodic acid and causes a rate decrease at low pH, 25 deg C or below. (3) The hydration equilibria of periodic acid and periodate ion appear to be largely responsible for the variations in ΔH(excit.) with temperature and with pH. (4) Decreased hydration changes the concentration of the reactive form of the periodate ion or acid and thereby decreases ΔH(excit.) with either decreased pH or increased temperature. (5) ΔS(excit.) is determined from ΔH(excit.) and is also greatly influenced by the extent of hydration. (6) The methyl groups of pinacol, as compared to ethanediol, retard the rate of reaction by steric effects but may enhance it by inductive effects.