Transition state theory addition-elimination

The connection between statistical mechanics and transition state theory is made by eliminating, from the partition function of the activated complex, the translational component due to the motion along the reaction coordinate (see (Glasstone et al. 1941, p. 189)). This step, however, is captured through the addition of a special law on statistical mechanics, and moreover, this special law characterises the transition state precisely in terms of its degrees of freedom. 

The slopes of the lines obtained from the Taft correlations of aliphatic primary, secondary and tertiary chlorides obtained at different temperatures by the extrapolation p%2lP%i T IT2 indicate that the positive nature at the carbon reaction center of the C—Cl bond in the transition state increases from a primary to a tertiary carbon atom (Table 8)70. An additional fact is that for each type of alkyl halide, the degree of positive charge at the carbon reaction center tends to decrease as the temperature increases. This means that the pyrolytic eliminations tend to be more concerted and less polar at very high temperatures. These data support Maccoll s theory on the heterolytic character of the alkyl halides pyrol-yses in the gas phase1. 

It would appear from the foregoing that there is a class of gas-phase reactions for which the transition state is best represented as having an essentially carbonium-ion pair character. In this way the effect of substitution at or near the centre of reaction can be interpreted, and the vast body of theory in the literature of physical organic chemistry used for the purpose of predicting rates of gas-phase reactions. In addition, the known properties of carbonium ions, as determined by the mass-spectrometer, can be invoked—as indeed they were in discussions of the SN1 and El reactions in polar solvents (Evans, 1946)—to correlate the effects of substituents in gas-phase eliminations. The advantage of studies in the gas-phase lies in the fact that the behaviour of a single molecule can be observed, without the added complication of the cooperative effect of the solvent. But gas-phase studies may, in turn,... 

Subsequently, Green investigated the oxidahve addition of aryl chlorides to palladium complexes derived from NHCs and their role in catalytic aminations [206], by using density functional theory (DFT) methods. Here, the most likely transition-state model for oxidative addition is the monocarbene-palladium complex (NHC)Pd(ArCl). For the amination of chlorobenzene, coordination of the T-shaped oxidative addition product by the amine occurs with a rearrangement, placing the amine in a ds-position to the aryl group, a prerequisite for reductive elimination [2]. 

The unimolecular gas-phase elimination kinetics of 2-methoxy-l-chloroethane, 3-methoxy-l-chloropropane, and 4-methoxy-l-chlorobutane has been studied using density functional theory (DFT) methods. Results calculated for 2-methoxy-l-chloroethane and 3-methoxy-l-chloropropane suggest that the corresponding olefin forms by dehydrochlorination through a concerted nonsynchronous four-centered cyclic transition state. In the case of 4-methoxy-l-chlorobutane, in addition to the 1,2-elimination mechanism, anchimeric assistance by the methoxy group, through a polar five-centered cyclic transition state, provides 4-methoxybutene, tetrahydrofuran, and chloromethane. Polarization of the C-Cl bond is rate limiting in these elimination reactions. 

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