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Elucidation of Reaction Mechanisms - Crossover Experiments and Kinetics

Investigation of the Reaction Kinetics

The investigation of the reaction kinetics plays an important role in the elucidation of reaction mechanisms.

Initially, the rate law of the reaction is usually experimentally determined by measuring the reaction rate at numerous concentrations of the individual reactants and, possibly, the catalysts that are applied. Among other things, the rate law yields the reaction order of the overall reaction, as well as the reaction order with regard to each reactant. However, the reaction mechanism cannot be unequivocally inferred from the rate law, as the rate law can usually be explained by several mechanisms.

How can the rate law help in the elucidation of the reaction mechanism?

The experimentally determined rate law is usually not pertinent until the number of possible mechansims has been reduced by other investigative methods. The accompanying rate law can be theoretically derived for a suggested reaction mechanism. If this rate law is not identical with the experimentally determined rate law, the suggested reaction mechanism cannot be the actual mechanism of the reaction. In comparing the experimentally determined rate law with the theoretically derived rate laws of the mechanisms that remain as a result of the preceding investigations, the number of possible mechanisms can be further restricted. Ideally, if all remaining, theoretically derived mechanisms depict different rate laws, the number of possible mechanisms can be reduced to just one!

Exemplary illustration of the role of investigating the reaction kinetics in the elucidation of reaction mechanims by the SN1 and SN2 mechanisms:

How can the distinction between a SN1 and a SN2 mechanism be made?

Fig.1
SN2 and SN1 mechanism.

Nu = entering nucleophile, NuF = leaving nucleofuge, R1 to R3 = any substituents.

It is assumed, in the following, that the rates of the back reactions of the individual reaction steps are virtually zero and that they therefore do not have to be considered. In addition, it is assumed that the first step of the SN1 mechanism is the slowest, rate-determining step. Under these preconditions, the rate law of the SN2 mechanism is

- d[R1R2R3C(NuF)] / dt = k1 [R1R2R3C(NuF)] [Nu¯].

In contrast, according to the rate law

- d[R1R2R3C(NuF)] / dt = k1 [R1R2R3C(NuF)],

the reaction rate of the SN1 reaction only depends on the concentration of the starting product R1R2R3C(NuF). If there is experimantal evidence that the reaction rate is not altered with the concentration of the nucleophile Nu, the SN2 mechanism can be excluded.

In practice, the rate laws of organic reactions are often considerably more complex, while the differentiation between an SN1 and an SN2 mechanism is often not as simple and unequivocal as it appears, since the preconditions that have been taken into consideration, among other things, (no back reaction) do not completely take place.

However, different possible reaction mechanisms can, in many cases, be distinguished by investigating the reaction kinetics.

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