Exchange of Alkanes

Isotopic exchange is unaccompanied by skeletal isomerization. In particular, even when Do is reduced to 10%, 3,3-dimethylpentane is unisomerized (11). This makes it unlikely that carbonium ion reactions are involved. 

There is considerable variation in the ratio of the two processes from activation to activation because of minor differences which we do not understand in the details of activation. 

The many-atom-exchange process appears only in molecules which have the grouping 

it is absent in methane, ethane, and 3,3-dimethylpentane but present, for example, in hexane, cyclohexane, and 2,3-dimethylpentane. 

Rates of many-atom and single-atom exchange increase rapidly as one activates above 300° but, at all activation temperatures between 300 and 470°, the distribution patterns are similar and the activation energy for isotopic exchange is the same, about 16 kcal. 

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Figure 4. Transition state for H/H exchange of alkanes with zeolites.

We have presented a mechanism for the hydrogenation of olefins and exchange of alkanes based upon heterolytic dissociative adsorption and its reverse, associative desorption (21). Ignoring the question of whether the acidic sites are Cr or Cr2+, the mechanism is represented... 

Steps (b), which allow the ring to be ruptured at different positions from where it was closed, are probably the ones which account for the multiple hydrogen-deuterium exchange of alkanes. The interconversions in Steps (b) are very fast reactions, while Steps (a) and (c) are rate determining. 

Direct evidence for a common stage in the oxidation and exchange of alkanes in the presence of platinum(II) complexes has been obtained [19], where both the reactions were studied jointly at different concentrations of acid and... 

It will not have escaped notice that adsorbed species implicated in the exchange of alkanes with deuterium are formally the same as those invoked in the hydrogenation of alkenes indeed the reiteration of the alkyl-alkene transformation (process 6.J) held responsible for multiple exchange in linear and branched alkanes, and designated the afi exchange mechanism, is on the face it of identical with the old and well-tried Horiuti-Polanyi mechanism for alkene hydrogenation. This will be discussed further in the next chapter (sections 7.1 and 7.21), but briefly it supposes the sequential addition of two hydrogen atoms to some adsorbed form of the alkene, e.g. 

To illustrate the propensity of these complexes to react by associative processes, consider the exchange of alkanes and arenes in complexes of the t5rpe [PtL2(R)(R-H)] by associative or dissociative mechanisms. Although it is difficult to conceive of a more weakly bound ligand than the alkane in a a-complex, the rate of associative substitution of acetonitrile for the bound alkanes is faster than simple dissociation of the alkane (Equation 5.16). ... 

Hydrogen-deuterium exchange of alkanes catalysed by platinum(n) in AcOD and DgO is thought to involve hydrido(alkyl)platinum intermediates which in hydrogenation reactions may revert to the parent alkene. However, attempts to function-... 

An interesting type of reactions including the activation of chemically inert C—bonds is based on oxidative addition. This process appears, in particular, in isotope exchange of alkanes with molecular hydrogen. 

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