Chirality Retention in Aqueous Propylene Oxide Hydration: Chirality of the Transition State

The hydration kinetics of enantiomerically pure propylene oxide (PO, CH3*CHCH2O) to chiral propylene glycol (PG, CH3*CH(OH)CH2OH) in aqueous solution have been studied using FTIR while simultaneously monitoring the net chirality of the reaction mixture. The hydration reaction appears to be first-order in the PO concentration with a rate constant of k approximately equal to 0.05 hr(-1). More importantly, the reaction is enantioselective; the product PG retains the chirality of the 2C carbon in PO with similar to 2 : 1 selectivity. The fact that there is some inversion of the chirality suggests that the dominant transition state is one in which the 2C-O bond in PO is cleaved, resulting in a close to planar transition state capable of inversion during hydration. If the transition state involved 1C-O cleavage it would retain the rigid chiral center of the PO reactant, preventing significant inversion.

Automated summary

The study investigates the hydration kinetics of enantiomerically pure propylene oxide to chiral propylene glycol in aqueous solution using FTIR, with a focus on the enantioselectivity of the reaction. The results show that the reaction is first-order with a rate constant of approximately 0.05 hr(-1), and the product retains the chirality of the 2C carbon in propylene oxide with a 2:1 selectivity. Additionally, the mechanism suggests that the dominant transition state involves cleavage of the 2C-O bond, allowing for significant inversion during hydration.