Kinetics of the acid-catalysed hydration of prop-2-en-1-ol. Evidence for a free, ion-dipole intermediate in the heterolysis of propane-1,2-diol

Abstract

Prop-2-en-1-ol in aqueous acid solution gives propane-1,2-diol and propanal. At 100ºC the amount of diol in this product ratio is found to decrease from 65% in 1 M HClO₄ to 50% in 4 M HClO₄. Pseudo-first-order kinetics are found to apply. The values obtained for the activation parameters, E_a and ΔS^‡ and for a Hammett acidity plot, are similar to results reported for other alkenes.

The relative rates of partitioning between regeneration of the substrate and rearrangement for the pinacol rearrangement of optically active propane-1,2-diol have been calculated previously. When these are compared with product ratios obtained from the hydration of prop-2-en-1-ol, good agreement is found between the respective values if it is assumed that the regenerated substrate has undergone complete racemization of configuration. This is interpreted to be evidence for a short-lived, but free ion-dipole as the intermediate in the heterolysis of propane-1,2-diol.