Isotope effects with chlorine

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... 

The first step, as we have already seen (12-3), actually consists of two steps. The second step is very similar to the first step in electrophilic addition to double bonds (p. 970). There is a great deal of evidence for this mechanism (1) the rate is first order in substrate (2) bromine does not appear in the rate expression at all, ° a fact consistent with a rate-determining first step (3) the reaction rate is the same for bromination, chlorination, and iodination under the same conditions (4) the reaction shows an isotope effect and (5) the rate of the step 2-step 3 sequence has been independently measured (by starting with the enol) and found to be very fast. With basic catalysts the mechanism may be the same as that given above (since bases also catalyze formation of the enol), or the reaction may go directly through the enolate ion without formation of the enol ... 

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 ... 

The ratio of products (36) and (37) from VNS of hydrogen (Pe) and substimtion of halogen (Px), respectively (Scheme 4), will depend on the strength and concentration of base, provided that the elimination is a kinetically important step in the VNS reaction, namely Pr/Px = kikE[B]/k-ikx. The influence of base will decrease until a constant value Ph/Px = k /kx is reached as kslB] k i. This has been demonstrated for 4-chloronitrobenzene, which undergoes exclusive substimtion of chlorine unless strong base is present to favour the VNS process. The deuterium isotope effect for VNS hydroxylation by Bu OOH, determined as me ratio of H versus D substitution of l-deutero-2,4-dinitrobenzene, varied from 7.0 0.3 to 0.98 0.01 as the base in NH3 was changed from NaOH to Bu OK me former value is consistent with a rate determining E2 process. 

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). 

In order to determine intrinsic isotope effects of benzylic hydroxylations, the metabolism of different deuterated toluenes was investigated in detail with rat liver microsomes and compared with the chemical radical chlorination of... 

Hydroxylation of alkanes preferentially occurs at the more nucleophilic C—H bonds, with a relatively low isotope effect (fcH/fcD = 2.8 for cyclohexane) and a significant amount of epimerization at the hydroxylated carbon atom. Radical carbon intermediates were revealed in this reaction by trapping experiments with chlorine atoms coming from CC14. 

This study on the kinetic chlorine isotope effect in ethyl chloride50 was extended to secondary and tertiary alkyl halides pyrolyses51. The isotope effects on isopropyl chloride and terf-butyl chloride pyrolysis were found to be primary and exhibited a definite dependence on temperature. They increased with increasing methyl substitution on the central carbon atom. The pyrolysis results and model calculations implied that all alkyl chlorides involve the same type of activated complex. The C—Cl bond is not completely broken in the activated complex, yet the chlorine participation involves a combination of bending and stretching modes. 

The systems involving bromine atoms also show the fast drop-off as AH deviates from 0, especially marked in the toluene to cumene series, which implies a small value of the intrinsic barrier. There is a further problem that there is a substantial equilibrium isotope effect of about a factor of 26b accompanied in the aliphatic cases by considerable opportunity for reversibility, thus A H/fcD values in the endothermic attack of Br- on aliphatic RH are unlikely to be less than 2. Correspondingly, isotope effects in the reverse directions can be inverse. This has not been observed, but the trend shown in the last several reactions, in which isotope effects as low as 1.0 are observed suggests this possibility. We attach no meaning to (fcH/ D)max for abstraction from HBr because of the failure of the one-dimensional model. Both chlorine and bromine then fit the scheme of highly variable isotope effects associated with low intrinsic barriers. 

The contribution of polar structures reduces the barrier and also the intrinsic barrier. This results for non thermoneutral reaction in a reduction of isotope effect. This has been a controversial subject for several years it is extensively covered by Russell29. The variation with substituents in the low isotope effects for the reaction of aryl radical with arene thiols were explained using such an effect. We may possibly further account for the lower intrinsic barrier for the R-H-Cl system (3.8 Kcal) than for the R-H—S system (5 Kcal) in terms of the greater electronegativity of chlorine. 

You will note some structure in the Stokes band near 460 which is due to chlorine isotopic frequency shifts. (See Exp. 37 for a discussion of isotope effects in diatomic molecules.) Rescan this region at higher resolution (1 cm or less) with an expansion of the chart display and measure the frequencies of each of the components. Record the ambient temperature near the Raman cell. 

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