Conclusions about Mechanism that can be Drawn from Kinetic Order

2 Conclusions about Mechanism that can be Drawn from Kinetic Order 

Elementary uni- and bimolecular reactions will necessarily show first- and second-order kinetic behaviour, but the reverse is not necessarily true a first-order reaction may not be unimolecular and a second-order reaction may not be bimolecular. For example, we considered the decomposition of dibenzylmercury in Chapter 1, in which the mechanism could either be elementary, giving a mercury atom and a 1,2-diphenylethane molecule directly (reaction 2.13a), or the reaction could be complex, with a slow initial homolysis of a carbon-mercury bond, followed by rapid further reactions to give the products (reaction 2.13b). Similarly for the Cope rearrangement of diene 2 to diene 4, the reaction could be elementary, with a concerted cyclic movement of electrons (reaction 2.14a), or might involve a di-radical intermediate 3 which rapidly reacted further to give the observed product 4 (reaction 2.14b). Both these mechanisms would lead to first-order kinetics, so the establishment of first-order kinetic behaviour for both these reaction schemes does not establish the 

If a reaction has a rate-determining step, only species that are involved in steps up to and including the ratedetermining step can affect the rate of reaction, 

Reactions which show a zero-order dependence on one of the components cannot be elementary reactions. 

A The plot of the Na2S203 titre, which is proportional to [Br ], against time shown below in (a) is linear, whereas a logarithmic plot (b) is curved. This shows that the rate of the bromination reaction does not depend on [Br ]. Thus a bromine molecule is not involved in the rate-determining step, so the initial reaction must involve the other components. A possible scheme, consistent with this evidence, is shown below. There will be a zero-order dependence on [Br2] provided that the rate of reaction c (the only step which involves a bromine molecule) is considerably faster than — b. This mechanism would need to be tested further by other experiments, the first of which would be to confirm the kinetic dependence on the other two components. 

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