Encounter rate and diffusion control

One facet of kinetic studies which must be considered is the fact that the observed reaction rate coefficients in first- and higher-order reactions are assumed to be related to the electronic structure of the molecule. However, recent work has shown that this assumption can be highly misleading if, in fact, the observed reaction rate is close to the encounter rate, i.e. reaction occurs at almost every collision and is limited only by the speed with which the reacting entities can diffuse through the medium the reaction is then said to be subject to diffusion control (see Volume 2, Chapter 4). It is apparent that substituent effects derived from reaction rates measured under these conditions may or will be meaningless since the rate of substitution is already at or near the maximum possible. 

Diffusion control can be particularly important in reactions in which two aromatic substances of differing reactivity are reacting with a deficiency of reagent. The more reactive aromatic will react first and since diffusion is slow compared with the rate of reaction it becomes impoverished in the reaction zone, and ensuing reaction will occur mainly with the less reactive aromatic which is now in large excess. The observed relative reaction rate then comes out to be less than it would otherwise be. It follows that this may also be true even when the aromatics are reacting at considerably less than the encounter rate. 

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