Termolecular kinetics

Where do the termolecular kinetics come from It is, of course, extremely unlikely that three species will collide simultaneously, particularly as two of them are mutually repelling anions. The rate-determining step is actually unimolecular—the spontaneous breakdown of a dianion. But the concentration of the dianion is in the rate expression too and that depends on the reactions before the rate-determining step. With a late rate-determining step, the previous steps are in equilibrium and so we can put in some rate and equilibrium constants for each step and label the intermediates like this. 

The termolecular kinetics result from two equilibria starting with the amide and involving two hydroxide ions followed by a unimolecular rate-determining step, and the termolecular rate constant 3 is actually a product of the two equilibrium constants and a unimolecular rate constant k = kx Ki x K2. 

We have now seen examples of unimolecular and bimolecular reactions and also how termolecular kinetics can arise from unimolecular and bimolecular reactions. 

One of the more unusual examples of hydrocarbon activation was reported by Wayland involving an example of radical homolytic cleavage of the C-H bond of methane (Fig. lb). In this reaction, the methyl group is transferred to one porphyrin metal center and the hydrogen to a second metal center. The reaction follows termolecular kinetics, which suggests a linear transition state for the cleavage (Eq. 21) [88]. In addition to methane, only the benzylic C-H bonds of tolu-... 

From Equation (15) it might be expected that the formation of Mn(VI) would be favored in the presence of another species which readily accepts an electron or combines with an H atom. This may explain the promoting influence of Ag+ on the reaction between H2 and Mn04 (8), where the termolecular kinetics are consistent with a rate-determining step of the form ... 

Another case of homolytic alkane activation was reported by Sherry and Way-land [130], who found that a pair of Rh(II)porphyrin radicals can cleave methane via a binuclear oxidative addition following termolecular kinetics (Eq. 2.47). 

Spears KG, Ferguson EE. Termolecular and saturated termolecular kinetics for Li and F". J Chem Phys. 1973 59 4174-83. 

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