A new method of determining chlorine kinetic isotope effects

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.