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Studies on the Mechanism of the Enolization Reaction of Grignard Reagents and Ketones. Part 2. Pseudo-first-order Rate and Deuterium Isotope Effect Studies
Pinkus, George A.,Sabesan, Arunachalam
, p. 473 - 477 (2007/10/02)
Kinetics of enolization reactions of some alkyl mesityl ketones with alkylmagnesium bromides were studied under pseudo-first-order conditions by measuring formation of gaseous alkane.Using a large fixed excess of isopropyl mesityl ketone, the reaction was first order in ethylmagnesium bromide; with a fixed excess of ethylmagnesium bromide, the reaction was first order in ketone.At high excess variable concentrations of ketone, however, k2 values decreased with increasing ketone concentration in a good linear relation between the two variables.When both Grignard and ketone concentrations (at 1:1 mole ratios) were varied, a nonlinear relationship resulted between concentrations and k2 values.Rates did not vary significantly between highly pure and reagent grade magnesium.Reactions in tetrahydrofuran at b.p. 67 deg C took place at significantly slower rates than reactions in ether at b.p. 36 deg C at comparable concentrations whereas reactions in di-n-butyl ether at b.p. 141 deg C occurred at a much faster rate at higher concentrations.Reactions of α-deuterio-substituted methyl, ethyl, and isopropyl mesityl ketones with ethylmagnesium bromide showed isotope effects confirming breaking of the C-H bond as the rate-determining step in accord with the proposed mechanism.Values of kH/kD=2.6-3.1 indicate a moderate degree of C-H bond stretching in the transition state.A two-step mechanism is proposed.
