Kinetics and equilibria of the ring-opening reactions of epoxides with halide ions

Abstract

A study was made of the reversible ring-opening reactions of epoxides with halide ions in methanol at 25 C. The reactions produce halohydrlns and methoxide ion. The concentration of methoxide ion was determined by a spectrophotometric technique utilizing Its reversible reaction with p-nitrophenol. The rate and equilibrium constants for the reactions of chloride, bromide and Iodide Ions with propylene oxide were evaluated. The equilibrium constants for the reactions of bromide ion with 1-oxasplro[2,5] octane and with l,2-epoxy-2,3-dlmethylbutane were also evaluated. The results showed that alkyl and gem-dialkyl substitution stabilized the epoxide ring toward opening by halide Ions. The extra ring stability obtained by substituting an isopropyl substituent for a hydrogen substituent amounted to at least 3.6 kcal. The results were discussed in terms of possible explanations for the stabilizing effect of alkyl groups.