Deductions from kinetic behaviour

When this is incorporated into expressions for rates of disappearance of reactant or formation of product, it is clear that the reaction is expected to display first-order behaviour so that, kinetically, the reaction is indistinguishable from a single-step conversion of R into R The rate constant, k0bs, is composite, containing contributions from all the elementary steps, with simplification becoming possible when k2 k-1 or /c i k2. Kinetics alone cannot provide evidence for the existence of a transient intermediate species if all reactions in the sequence are first order. 

In case (a), R and I interconvert by unimolecular processes, and then I is converted into product in a bimolecular reaction with reagent X. Such reactions are expected to be first order in [R], but order with respect to [X] will varybetweenO and 1 depending on the relative magnitudes of k2[X] and k Since X is consumed during the reaction, the variation may be detectable as downward drift in the rate constant when monitoring disappearance of R. 

More often, the extreme situations occur with k- k2[X] ork i A 2 [X]. In the former 

The alternative situation where k- y k2 [X] is also common. Reactions are then zero order in [X], and the clear disparitybetween stoichiometric coefficients and reaction orders demands the existence of an intermediate. The situation occurs in acid- and base-catalysed reactions of ketones with reactive electrophiles (e.g. X = CI2, Br2 or I2), which are usually zero order in the electrophilic reagent, unless concentrations of the electrophiles are extremely low [19]. The intermediate reacting with the electrophile may be the enol tautomer of the ketone or its enolate anion, formed catalytically from the ketone. 

With the buffer ratio held constant ([B-]/[BH] = 0.5), and the ionic strength maintained at 1 mol dm 3 with potassium chloride, initial rates show saturation kinetics over the range 0.01 [B ] 0.25 mol dm 3. This behaviour is compatible with the intermediacy of the carbanion 13, and the rate-determining step changing from its rate-limiting formation at low buffer concentrations to rate-limiting expulsion of quinuclidine from the carbanion, formed rapidly and reversibly, at high buffer concentrations [21]. 

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In short, we may say that there is evidence that the deviations from the relations predicted by the characteristic equations may be due to chemical changes in the substances, which are not taken into consideration in the kinetic deduction of the equations. Weinstein loc. cit.) considers that it is possible to deduce an equation which takes account of these chemical changes as well. It will be sufficient here to re-einphasise the fact that there is at present no characteristic equation known which agrees accurately with the behaviour of a single substance, let alone various substances, over a wide range of temperature. 

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