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Mechanism of epoxyether hydrolysis

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Title: Mechanism of epoxyether hydrolysis
Epoxyether hydrolysis
Authors: Porzio, Michael Anthony
Keywords: Epoxy compounds
Issue Date: 1969
Publisher: [Honolulu]
Abstract: 2-Ethyl-l-methoxy-l,2-epoxybutane (DMO) was synthesized and characterized. Hydrolysis of DMO in dilute mineral acid solution yields exclusively 2-hydroxy-2-ethylbutanal. Hydrolysis kinetics in 10% (V/V) aqueous dioxane were followed by monitoring formation of product aldehyde by ultraviolet spectroscopy. The hydrolysis kinetics of DMO are second-order, first-order in acid and substrate. The second-order rate constant for catalysis by hydrogen ion at 25° is 7.24 ± .20 M^-l sec^-l. General acid catalysis was observed with acetic acid buffer systems. A Brønsted plot of five acids, acetic, formic, cacodylic, hydrogen ion and water gives an a value of 0.36 ± .03. Addition of strong nucleophiles to acetic acid buffer systems does not affect the observed rate. Replacing solvent water by D2O leads to rate retardation. Kinetic solvent deuterium isotope effects, kH/kD, for catalysis by H3O+ , acetic acid and water are 1.11, 2.27, 1.67, respectively. Mixed solvent isotope studies at 25° with 0.496 mole fraction D2O give kn/kH =1.14 ± 0.05; the calculated value for a concerted proton transfer mechanism from the equation: kn/kH = (1-n+nℓ^(1-α)^2(1-n+nℓ^(1+2α)kD/kH)(1-n+nℓ)^-3 equals 0.98 where a is the Brønsted slope, n is mole fraction D, and R. =0.69. Activation parameters for the hydrogen ion catalyzed hydrolysis obtained from 35°, 30°, 25° and 15° rates give Eact =11.9±0.4 kcal/mole, ΔS* = - 16.5 ± 1.5 e.u., ΔH* = 11.3 ± 0.4 kca1/mo1e and ΔF* = 16.3 ± 0.2 kca1/mo1e. The second-order rate for the hydrogen ion catalyzed hydrolysis of the phenyl substituted epoxyether, 2-methy1-1-pheny1-1-methoxy-1,2-epoxypropane is 27.2 ± 2.1 M-1 sec-1 The hydrolysis mechanism most consistent with the experimental data is the ASE- 2 or rate-determining proton transfer mechanism in which the slow step is the transfer of a proton to the ring-oxygen of the epoxyether with the concerted breaking of the epoxide carbon-oxygen bond. However, there is an alternative interpretation equally consistent with the experimental data which would involve a fast ring-opening step to give an α-hydroxyhemiacetal intermediate, the hydrolysis of which is rate-determining. The determination of which of these two possibilities is actually correct remains a project for further work.
Description: Typescript.
Thesis (Ph. D.)--University of Hawaii, 1969.
Bibliography: leaves [112]-116.
ix, 116 l
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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