The Reversible Hydration of Carbonyl Compounds in Aqueous Solution Part II: The Kinetics of the Keto/Gem‐diol Transition

The kinetics of hydration of 17 different carbonyl compounds have been studied in acidic solution. The reactions have been found to be acid‐catalysed. For the aliphatic aldehydes propanal to hexanal the rate constants are k 0 = (3.5 ± 1) · 10 −3 s −1 and k H = (450 ± 30) dm 3 mol −1 s −1 and for α‐keto acids they are in the ranges k 0 = (0.13 to 0.40) s −1 and k H = (1 to 6) dm 3 mol −1 s −1 . For halogenated acetones the rate constants depend strongly on the substituents. The large negative value for the reaction entropy ΔS 0 = −70 J mol −1 K −1 indicates that three water molecules are involved in the hydration process.