Activation energies analogous systems

Table 2. Activation energies for the reaction of protonated benzyl methyl ketoximes and their sulfur analogous systems calculated by MP2/6-31G. ... 

Oae (251,252) as well as Darwish and Datta (253) investigated the process of thermal racemization of chiral alkylarylsulfimides and diarylsulfimides. It was found to proceed at temperatures as low as 65 to 100°C with a rate constant of the order 1 to 10 X 10" sec" , which corresponds to an activation energy of about 23 to 30 kcal/mol. These data indicate that the thermal racemization of sulfimides is much faster than that of analogous sulfoxide systems. The racemization of sulfimides is a unimolecular reaction practically independent of the polarity of the solvent this property, coupled with the absence of decomposition products, supports the view that racemization of sulfimides occurs by pyramidal inversion. 

If the isomeric transition is taking place under conditions where the molecules cannot interact with one another or with the walls of the vessel in which they are contained other than in sueh a way as to transfer energy or momentum, the activation energy Qa and Qb will be determined entirely by the internal characteristics of the molecule and will be what might be termed natural constants of the system, analogous to the heat of formation. Thus under these conditions the reaction rates for the isomeric transitions will be fixed by the internal constitution of the molecules. At a given temperature the reaction rate can be changed... 

The activation energy of this process is only 26 keal., which implies considerable stability for CuH relative to Cu and a hydrogen atom. Thus, it seems that the same species may form in other systems that can transfer hydride ions, and there is indeed evidence in related systems for the formation of analogous hydrido complexes by such hydride transfer—e.g.,... 

When the halide is bonded to an allylic system (CH CH-CH -X) an alkoxide ion will react analogously to the previously described S 2 displacement on an alkyl halide. The most significant difference is the rate enhancing effect of the alkene moiety which has been attributed to a decrease in the activation energy of the reaction (9). A second possible mode of reaction is available with allylic halides. This mode of displacement is usually called S 2 and, in general, will be promoted relative to the normal displacement when there are substituents on the alpha carbon which tend to inhibit the normal SN2 pathway by inductive or steric effects (Reaction VII). 

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