A New Method of Determining Chlorine Kinetic Isotope Effects

Abstract

Two methods have been used to measure the chlorine leaving group kinetic isotope effect for the S(N)2 reduction of benzyl chloride to toluene by sodium borohydride in DMSO at 30.000 °C. The reaction was monitored by titrating the unreacted borohydride ion. One method involved determining the chlorine isotope effect using the classical IRMS method, which requires the conversion of the chloride ions into gaseous methyl chloride that is analyzed in an isotope ratio mass spectrometric analyses (Hill, J. W.; Fry, A. J. Am. Chem. Soc. 1962, 84, 2763. Taylor, J. W.; Grimsrud, E. P. Anal. Chem. 1969, 41, 805.). Two different measurements using this method yielded isotope effects of k(35)/k(37) = 1.007 19 ± 0.000 19 and 1.007 64 ± 0.000 19. The second method was a new technique where the ratio of the chlorine isotopes was obtained by fast atom bombardment mass spectrometry on the silver chloride recovered from the reaction, i.e., from the first step in the classical procedure. Therefore, the new method is much simpler and avoids the time-consuming preparation, purification, and recovery of the gaseous methyl chloride. Although the experimental error is larger (k(35)/k(37) = 1.008 03 ± 0.00 10 and 1.008 02 ± 0.000 65) when the new technique is used to analyze the silver chloride samples from the same set of experiments that were used to measure the isotope effect by the classical method, the chlorine isotope effect found by the two methods is identical within experimental error. This large chlorine kinetic isotope effect indicates there is considerable C(α)-Cl bond rupture in the S(N)2 transition state.

Experimental and Theoretical Multiple Kinetic Isotope Effects for an SN2 Reaction. An Attempt to Determine Transition‐State Structure and the Ability of Theoretical Methods to Predict Experimental Kinetic Isotope Effects

“…After the hydrogen cyanide had been removed (vide supra), the samples were titrated with standard silver nitrate (0.005 m) in a potentiometric titration, [103] and the silver chloride precipitate was filtered and converted into methyl chloride by using the standard procedure. [104] The chlorine KIE was determined from Equation (4):…”

Precise measurement of chlorine stable isotopic ratios by thermal ionization mass spectrometry.

“…Several techniques, involving electron impact ionization mass spectrometry (Boyd et al, 1955;Hoering and Parker, 1961;Taylor and Grimsrud, 1969;Kaufmann et al, 1984;Long et al, 1993), negative thermal ionization mass spectrometry (Shields et al, 1962;Vengosh et al, 1989) and fast atom bombardment-mass spectrometry (Wastaway et al, 1998) have been developed to determine the isotopic composition of Cl. However, the study of Cl isotopes was limited by lack of precision or sensitivity.…”

An unusually large secondary α-deuterium kinetic isotope effect in hydride ion SN2 reactions

“…The description of the kinetic method used to measure the rate constants is described in ref. (3). The details of the theoretical calculations are given in ref.…”

“…[3]), [3] (k H /k D ) vib = (k H /k D ) stretching (k H /k D ) bending (k H /k D ) other where (k H /k D ) stretching is due to the changes that occur in the three C α -H(D)stretching vibrations, (k H /k D ) bending is the contribution due to the changes that occur in the seven C α -H(D) bending (primarily the out-of-plane bending) vibrations, and (k H /k D ) other is due to the changes that occur in the other vibrations, when the reactant is converted into the S N 2 transition state (1). The (k H /k D ) stretching and (k H /k D ) bending contributions to the KIE were calculated to determine how these vibrations affected the magnitude of the secondary α-deuterium KIE and to determine how the (k H /k D ) stretching and (k H /k D ) bending contributions to the KIE were related to the structure of the S N 2 transition state.…”