Hydration, entropy, and energy relationships for periodate oxidations

Periodate oxidations of ethanediol and pinacol each occur in two phases; these are (1) formation and (2) decomposition of the intermediate complex. In phase (1), an increase in acidity gives IO4- -> H5IO6 -> H6IO6+. The rate of oxidation of ethanediol decreases with increasing acidity, whereas the rate of oxidation of pinacol maximizes with H5IO6. For both glycols, the activation energy increases and Delta S-act decreases with increasing acidity. In phase (2), the energy of activation is essentially constant with pH, whereas the rate decreases, and the entropy of activation decreases modestly as pH decreases. The latter correlates with the nonhomogeniuty of product formation. Rates for 3-chloro-1,2-propanediol are also listed. Pentaerythritol forms an inactive complex with IO4- or H5IO6 indicating the importance of chelation in the formation of the intermediate complex. Copyright (c) 2007 John Wiley & Sons, Ltd.