1. Use of Kinetic Isotope Effects in Mechanism Studies. 5.1 Isotope Effects and Element Effects Associated with Hydron-Transfer Steps during Alkoxide-Promoted Dehydrohalogenations
- Author
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Amy B. Dean, Barbara O. Scott, Ronald Hage, Gerrit Lodder, Judith G. Koch, David J. Bogdan, Lisa A. James, Eric C. Roos, Carrie A. Carlson, Geoffrey H. Brown, Melissa L. Sardina, Petra M. Knape, Patrick Han, Steven D. Wickham, Rachael A. Sawyer, Heinz F. Koch, Johan C. P. Hopman, and Charles A. Testa
- Subjects
Hydron ,Arrhenius behavior ,Sodium ,Halide ,chemistry.chemical_element ,General Chemistry ,Kinetic energy ,Biochemistry ,Catalysis ,chemistry.chemical_compound ,Colloid and Surface Chemistry ,chemistry ,Kinetic isotope effect ,Alkoxide ,Physical chemistry ,Nuclear chemistry ,Carbanion - Abstract
The Arrhenius behavior of the primary kinetic isotope effect, (kH/kD)Obs and (kH/kT)Obs, associated with the methanolic sodium methoxide-promoted dehydrohalogenations of m-ClC6H4CiHClCH2Cl (I), m-CF3C6H4CiHClCH2Cl (II) and p-CF3C6H4CiHClCH2F (III) has been used to calculate the internal-return parameters, a = k-1/ , in a two-step mechanism featuring a hydrogen-bonded carbanion. This carbanion partitions between returning the hydron to carbon, k-1, and the loss of halide, . Isotope effects at 25 °C for I, (kH/kD)Obs = 3.40 and (kH/kT)Obs = 6.20, and II, (kH/kD)Obs = 3.49 and (kH/kT)Obs = 6.55, result in similar values for a: aH = 0.59, aD = 0.13−0.14 and aT = 0.07. Smaller values of (kH/kD)Obs = 2.19 and (kH/kT)Obs = 3.56 for III are due to more internal return [aH = 1.9, aD = 0.50, and aT = 0.28] associated with the dehydrofluorination reaction. Calculation of k1 (= kObs [a + 1]) results in similar isotope effects for hydron transfer in these reactions: I, / = 4.74 and / = 9.20; II, /kD1 = 4.91 and / = ...
- Published
- 1997
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