Alkyl chlorides kinetic isotope effects

The competition between nucleophilic substitution and base-induced elimination in the gas phase has been studied using deuterium kinetic isotope effects (KIE).6 The overall reaction rate constants and KIE have been measured for the reactions of RC1 + CIO- (R = Me, Et, t -Pr, and r-Bu). As the extent of substitution in the alkyl chloride increases, the KIE effects become increasingly more normal. These results indicated that the E2 pathway becomes the dominant channel as the alkyl group becomes more sterically hindered. 

That alkenes are formed from those alkyl groups containing a jS-hydrogen atom strongly implies that the mechanism of alkene formation involves a /3-hydride abstraction step. There is a very pronounced kinetic isotope effect when C6D5CD2CH2COCI is decarbonylated, which indicates that not only does a jS-deuteride abstraction take place but that it is also rate determining. Further evidence for the participation of a /3-hydride abstraction comes from the decarbonylation of erythro- or t/ reo-2,3-diphenylbutanoyl chloride, where the former yields the (ii)-alkene and the latter the (Z)-isomer. 

Cerfontain and co-workers suggested that the last step of the reaction, namely proton removal from the complex, was not rate-contributory, but this argument may not be valid since a more recent publication reported a higher value of the kinetic isotope effect (kn kxi) of 1.9-2.5. In sulfonylation using alkylsulfonyl chlorides, there is competing alkylation the latter is enhanced by increasing stability of the alkyl cation. Thus reaction of isopropyl, tertiary butyl and benzylsulfonyl chlorides caused facile alkylation of aromatic substrates in the presence of aluminium chloride in nitromethane at 25 °C. ... 

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. 

Penton and Zollinger 233.238) reported recently that this could indeed be the case. The coupling reaction of m-toluidine and N,N-dimethylaniline with 4-methoxy-benzenediazonium tetrafluoroborate in dry acetonitrile showed a number of unusual characteristics, in particular an increase in kinetic deuterium isotope effect with temperature. Predominant C-coupling occurs ( 86% for m-toluidine). but on addition of a tert.-alkyl ammonium chloride the rate became much faster, and predominantly diazoamino compounds (with loss of a methyl group from N,N-dimethylaniline) were formed. Therefore, initial attack of the diazonium ion is probably at the amine N-atom and aminoazo formation occurs via rearrangement. 

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