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The compleat mechanism of epoxyether hydrolysis

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Item Summary

Title: The compleat mechanism of epoxyether hydrolysis
Epoxyether hydrolysis
Authors: Mori, Arthur Laurance
Keywords: Epoxy compounds
Issue Date: 1971
Publisher: [Honolulu]
Abstract: The hydrolysis of two epoxyethers, l-methoxy-2-ethyl-1,2-epoxybutone (MEE) and l-methoxy-l-phenyl-2-methyl-l,2-epoxybutane (MPME), has been studied in 10% (v/v) aqueous dioxane over the pH range 3-13 at 25°. It proceeds via two consecutive, kinetically observable, pseudo first-order reactions: the first is epoxide ring cleavage above pH 8; the second is the hydrolysis of the hemiacetal intermediate, R2C(OH)CH(OMe)OH, at lower pH values. A rate maximum is observed at a neutral pH. The reaction of MEE in the acidic region is general acid-general base catalyzed, Brønsted α = 0.33 and β = 0.56. Solvent deuterium isotope effects are: kD/kH = 0.90, kD2O/kH2O = 0.60, kOD/kOH = 1.30, kDOAc/kHOAc = 0.44 and kOAc(D2O)/kOAc(H2O) = 0.70; activation entropies: ΔS*(H30+) = -15 eu and ΔS*(H20) = -31 eu. The hydrolysis of MPME proceeds at a slightly higher rate. A cyclic transition state is proposed in which proton removal from the hemiacetal is concerted with proton donation to the rnethoxyl leaving group. The hydroxide ion catalysis may be undergoing a preequilibrium deprotonation step first. The hydrolysis in basic media undergoes H3O+ and pH-independent catalysis. General acid catalysis by H2PO4 is observed for MPME. The hydrolysis of MPME is slower than MEE, as is the pH-independent reaction of MEE in 50% (v/v ) dioxane. Slight hydroxide ion catalysis was detected, but nucleophilic attack was not observed. The solvent deuterium isotope effects for MEE are: kD/kH= 2.02 and kD2O/kH2O = 0.53; for MPME: kD/kH = 1.35 and kD2O/kH2O = 0.35. Activation entropies for MEE are: ΔS*(H3O+) = -11 eu, ΔS*(H2O) = -49 eu, and ΔS*(50% H2O-dioxane) =-26 eu; for MPME: ΔS*(H30+) = -15 eu and ΔS*(H2O) = -36 eu. The proposed transition state for the acid catalyzed reaction involves concerted proton transfer and C-O bond breaking. A possible nucleophilic attack by water is postulated for the pH-independent reaction.
Description: Typescript.
Thesis (Ph. D.)--University of Hawaii, 1971.
Bibliography: leaves 117-120.
xiii, 120 l illus., tables
Rights: All UHM dissertations and theses are protected by copyright. They may be viewed from this source for any purpose, but reproduction or distribution in any format is prohibited without written permission from the copyright owner.
Appears in Collections:Ph.D. - Chemistry

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