4.2.7 Complex reactions

There are many examples of drugs in which decomposition occurs simultaneously by two or more pathways, or involves a sequence of decomposition steps or a reversible reaction. Indeed, the degradative pathways of some drugs include examples of each of these types of complex reactions. Modification of the rate equations is necessary whenever such reactions are encountered.

Reversible reactions

Treatment of the kinetics of a reversible reaction involves two rate constants; one, kf, to describe the rate of the forward reaction and the other, kr, to describe the rate of the reverse reaction. For the simplest example in which both of these reactions are first-order, that is

the rate of decomposition of reactant is

Determine the order of reaction and calculate the rate constant.


Application of equation (4.23) requires values for log C0 and log t05; thus log C0 0.665 0.230 -0.140 -0.540 log f05 1.94 2.38 2.75 3.15

A plot of log t05 against log C0 is linear (Fig. 4.3) with slope (1 - n) = -1.01. Hence n = 2.01, i.e. the reaction is second-order.

The integrated form of the rate equation is t =

0 0

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