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The compleat mechanism of epoxyether hydrolysis
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|Title:||The compleat mechanism of epoxyether hydrolysis|
|Authors:||Mori, Arthur Laurance|
|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.|
Thesis (Ph. D.)--University of Hawaii, 1971.
Bibliography: leaves 117-120.
xiii, 120 l illus., tables
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|Appears in Collections:||Ph.D. - Chemistry|
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