Chlorine kinetic isotope effect

Kinetic isotope effects also show a dependence upon the reactivity of the electrophile. Thus some reactions, e.g. positive chlorination, show no isotope effect whereas others, e.g. sulphonation, do show an isotope effect. There are two ways of visualising the reasons for this and they are closely related. Very... 

Westaway KC, T Koerner, Y-R Fang, J Rudzinski, P Paneth (1998) A new method of determining chlorine kinetic isotope effects. A a/ Chem 70 3548-3552. 

Valnes of have been measnred for a number of polychlorinated biphenyl (PCB) congeners and applied to a nnmber of commercial PCB mixtures. Both the number and the position of the chlorine snbstitnents affected the depletion of C, and this reflected the mannfacturing procednres that involved kinetic isotope effects as well as the source of the biphenyl starting material (Jarman et al. 1998). It was snggested that this could be applied to determine the source of PCBs in the environment. 

Isotope ratios for and Cl were measured for the aerobic degradation of dichlorometh-ane by a methanotroph MC8b (Heraty et al. 1999). Values of the fractionation factor (a) were 0.9586 for carbon and 0.9962 for chlorine, and kinetic isotope effects were 1.0424 for carbon and 1.0038 for chlorine. 

Kinetic isotope effects have not been observed for chlorination, and only rarely for bromination, i.e. the reactions normally follow pathway [2a] like nitration. In iodination, which only takes place with iodine itself on activated species, kinetic isotope effects are the rule. This presumably arises because the reaction is readily reversible (unlike other halogenations), loss of I occurring more often from the a complex (14) than loss of H, i.e. k, > k2 ... 

For heavy atom isotope effects tunneling is relatively unimportant and the TST model suffices. As an example the dehalogenation of 1,2-dichloroethane (DCE) to 2-chloroethanol catalyzed by haloalkane dehalogenase DhlA is discussed below. This example has been chosen because the chlorine kinetic isotope effect for this reaction has been computed using three different schemes, and this system is among the most thoroughly studied examples of heavy atom isotope effects in enzymatic reactions. 

The haloalkane dehalogenase DhlA mechanism takes place in two consecutive Sn2 steps. In the first, the carboxylate moiety of the aspartate Aspl24, acting as a nucleophile on the carbon atom of DCE, displaces chloride anion which leads to formation of the enzyme-substrate intermediate (Equation 11.86). That intermediate is hydrolyzed by water in the subsequent step. The experimentally determined chlorine kinetic isotope effect for 1-chlorobutane, the slow substrate, is k(35Cl)/k(37Cl) = 1.0066 0.0004 and should correspond to the intrinsic isotope effect for the dehalogenation step. While the reported experimental value for DCE hydrolysis is smaller, it becomes practically the same when corrected for the intramolecular chlorine kinetic isotope effect (a consequence of the two identical chlorine labels in DCE). 

Another approach to modeling the chlorine kinetic isotope effect of this reaction has been carried out using a true QM/MM scheme. 

O Neil JR (1986) Theoretical and experimental aspects of isotopic fractionation. Rev Mineral 16 1-40 Oi T (2000) Calculations of reduced partition function ratios of monomeric and dimeric boric acids and borates by the ab initio molecular orbital theory. J Nuclear Sci Tech 37 166-172 Oi T, Nomura M, Musashi M, Ossaka T, Okamoto M, Kakihana H (1989) Boron isotopic composition of some boron minerals. Geochim Cosmochim Acta 53 3189-3195 Oi T, Yanase S (2001) Calculations of reduced partition function ratios of hydrated monoborate anion by the ab initio molecular orbital theory. J Nuclear Sci Tech 38 429-432 Paneth P (2003) Chlorine kinetic isotope effects on enzymatic dehalogenations. Accounts Chem Res 36 120-126... 

Kaiser, E. W and T. J. Wallington, Comment on Inverse Kinetic Isotope Effect in the Reaction of Atomic Chlorine with C2H4 and C2D4, J. Phys. Chem. A, 102, 6054-6055 (1998). 

Stutz, J., M. J. Ezell, A. A. Ezell, and B. J. Finlayson-Pitts, Rate Constants and Kinetic Isotope Effects in the Reactions of Atomic Chlorine with n-Butane and Simple Alkenes at Room Temperature, J. Phys. Chem., 102, 8510-8519 (1998). 

Kinetic data, interpretation, 40, 44, 78 Kinetic isotope effects, 46 carbon, 47 chlorine, 47... 

Lewandowicz A, Rudzinski J, Tronstad L, Widersten M, Ryberg P, Matsson O, Paneth P (2001) Chlorine Kinetic Isotope Effects on the Haloalkane Dehalogenase Reaction. J Am Chem Soc 123 4550... 

Paneth P (2003) Chlorine Kinetic Isotope Effects on Enzymatic Dehalogenations. Acc Chem Res 36 120... 

The use of the kinetic isotope effect as a selectivity parameter also indicates that the chlorination reaction is solvent dependent (Brown and Russell, 1952). Thus the chlorination of toluene, in toluene instead of carbon tetrachloride, increases the isotope effect kn/kD, indicating that the reaction in toluene is more selective and hence that the attacking species is less reactive in this solvent. 

This effect, called a kinetic isotope effect, is clearly seen in the chlorination of methane. Methane undergoes free-radical chlorination 12 times as fast as tetradeuteriomethane (CD4). 

Jones has studied the kinetic isotope effect in the chlorination of H2 and HT inducing the reaction with visible radiation and also with tritium p-particles. He found the same isotope effect with either method. The reactions and quantity of interest are... 

Bigeleisen et al. measured the relative rates of chlorine atom attack on H2 and HD to evaluate kinetic isotope effects. Experimentally, mixtures of H2, HD and CI2 were photolyzed with wavelengths longer than 3900 A and at temperatures ranging from 243 to 250 °K. Measuring mass spectrometrically the isotopic composition of the hydrogen gas both before and after photolysis permitted calculation of the rate coefficient ratio... 

Thorough kinetics studies of the chlorination of aliphatic, alicyclic, and arylalkyl ketones with CBT were carried out by Indian workers (82PIA921). Kinetic measurements were performed using aqueous acetic acid and the addition of HC104 and NaCl. In the presence of mineral acid the reaction is first order in ketone and acid and zero order in CBT. A large kinetic isotopic effect was observed (for acetone kHlkD = 6.6). Addition of chloride ion causes some changes in the reaction order they become first order in CBT, 0.6 in ketone, and 0.2 in chloride ion. The rate constant for chlorination of substituted acetophenones correlate with a constants for substituents in the aryl ring (p is -0.57). On the basis of these data the mechanism in the absence and in the presence of chloride ion was developed. 

Recent applications of VTST/MT to polyatomic gas-phase reactions that illustrate the power of the theory include the reactions of chlorine atoms with hydrogen molecules [56] and the reactions of hydrogen atoms with ethylene [57,58] and methane [59], including kinetic isotope effects. 

Isotope effect and relative rate studies also suggest an early TS for benzylic chlorination and bromination. The benzylic position is only moderately activated toward uncomplexed chlorine atoms. Relative to ethane, toluene reactivity is increased only by a factor of 3.3. The kinetic isotope effect observed for bromination and chlorination of toluene suggest little rehybridization at the TS. 

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