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Mechanism of epoxyether hydrolysis
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|Title:||Mechanism of epoxyether hydrolysis|
|Authors:||Porzio, Michael Anthony|
|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.|
Thesis (Ph. D.)--University of Hawaii, 1969.
Bibliography: leaves -116.
ix, 116 l
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|Appears in Collections:||Ph.D. - Chemistry|
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