Basic Kinetic Principles

The simplest concept of chemical and physical reaction is the case of a drug D reacting to form a product P. This process is described by the following scheme:

The extent to which D rearranges to P will depend on the free-energy differences between D and P. If P is of much lower free energy than D, then the reaction is better defined by

Most drugs degrade by reactions that involve a so-called bimolecular reaction in which drug D collides with a reactant A to produce one or more products. This is illustrated in its simplest form by the following equation:

P will be formed if D and A collide with sufficient energy (and an appropriate orientation) to result in a molecular rearrangement to form P. In this simple case, the rate of loss of D, -d[D]/dt, is said to be proportional to the activity (or, more simply, the concentration) of both D and A, as indicated by Eq. (2.1).

When the proportionality constant is included, the following equation is obtained:

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