325-55-3Relevant articles and documents
Use of kinetic isotope effects in mechanism studies. Isotope effects and element effects associated with Hydron-Transfer steps during alkoxide-promoted dehydrohalogenations
Koch, Heinz F.,Lodder, Gerrit,Koch, Judith G.,Bogdan, David J.,Brown, Geoffrey H.,Carlson, Carrie A.,Dean, Amy B.,Hage, Ronald,Han, Patrick,Hopman, Johan C. P.,James, Lisa A.,Knape, Petra M.,Roos, Eric C.,Sardina, Melissa L.,Sawyer, Rachael A.,Scott, Barbara O.,Testa III, Charles A.,Wickham, Steven D.
, p. 9965 - 9974 (2007/10/03)
The Arrhenius behavior of the primary kinetic isotope effect, (k(H)/k(D))(Obs) and (k(H)/k(T))(Obs), associated with the methanolic sodium methoxide-promoted dehydrohalogenations of m-ClC6H4C(i)HClCH2Cl (I), m-CF3C6H4C(i)-HClCH2Cl (II) and p-CF3C6H4C(i)HClCH2F (III) has been used to calculate the internal-return parameters, a = k(-1)/K(Elim)(X), 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, K(Elm)(X). Isotope effects at 25°C for I, (k(H)/k(D))(Obs) = 3.40 and (k(H)/ k(T))(Obs) = 6.20, and II, (k(H)/k(D))(Obs) = 3.49 and (k(H)/k(T))(Obs) = 6.55, result in similar values for a: a(H) = 0.59, a(D) = 0.13-0.14 and a(T) = 0.07. Smaller values of (k(H)/k(D))(Obs) = 2.19 and (k(H)/k(T))(Obs) = 3.56 for III are due to more internal return [a(H) = 1.9, a(D) = 0.50, and a(T) = 0.28] associated with the dehydrofluorination reaction. Calculation of k1 ( k(Obs) [a + 1]) results in similar isotope effects for hydron transfer in these reactions: k1(H)/k1(D) = 4.74 and k1(H)/K1(T) = 9.20; II, k1(H)/k1(D) = 4.91 and k1(H)/k1(T) = 9.75; III, k1(H)/k1(D) = 4.75 and k1(H)/k1(T) = 9.17. Reactions of m-ClC6H4C(i)HBrCH2Br and m-ClC6H4C(i)HClCH2Br have very small amounts of internal return, a(H) = 0.05 and a(D) = 0.01, and (k(H)/k(D))(Obs) = 4.95 results in k1(H)/k1(D) = 5.11 The measured isotope effects are therefore due to differences in the amount of internal return and not in the symmetry of transition state structures for the hydron transfer, and the element effect, (k(HBr)/ k(HCl)) = 29, for m-ClC6H4CHClCH2X is mainly due to the hydron-transfer step, k1(HBr)/k1(HCl) = 19, and not the breaking of the C-X bend. The kinetic solvent isotope effects, k(MeOD)/k(MeOH) ~ 2.5, are consistent with three methanols of solvation lost prior to the hydron-transfer step. The energetics associated with desolvation of methoxide ion are part of the measured reaction energetics of these systems.
MECHANISMS OF FREE-RADICAL REACTIONS. XIII. MECHANISM AND SELECTIVITY OF THE FREE-RADICAL HALOGENATION OF ALKYL AROMATIC HYDROCARBONS WITH FLUOROALKYL SUBSTITUENTS
Dneprovskii, A. S.,Eliseenkov, E. V.,Mil'tsov, S. A.
, p. 317 - 324 (2007/10/02)
The free-radical chlorination and bromination of 1-fluoro-2-arylethanes and 1,1,1-trifluoro-2-arylethanes was studied by the method of competing reactions.In all cases a good correalation between log krel and the Brown ?+ constants was observed.The variation of the selectivity in the transition from one reaction series to the other indicates that two independent factors which determine the reactivity (the change in the dissociation energy of the C-H bond and the polar effect of the substituents) have a simultaneous effect.
