Evidence and tests for the existence of intermediates

Prior knowledge of the behaviour of a proposed intermediate under a particular set of reaction conditions is often available and facilitates experimental design. For example, species which are transient under one set of conditions (solvent, temperature) may be stable under others, and then observable by conventional methods. Similar considerations apply to structural variation, which may stabilise charge or unusual valence states. Systematic studies of the effects of variation of conditions, or of structural variation on reactivity, often permit useful extrapolation to behaviour of a proposed intermediate under the conditions in question. Importantly, if extrapolations of this kind indicate that a proposed intermediate would have a lifetime of less than 10 13 s under a particular set of reaction conditions, then that proposal must be re-evaluated. Either the mechanism involving the proposed intermediate is fundamentally flawed, or the bonding changes involved in its formation and destruction are actually concerted. 

More directly, a proposed intermediate may be generated from alternative precursors, in non-equilibrium concentrations, under conditions quite close to those of the reaction in question. Fast kinetic methods may then yield rates for decay of the intermediate, and products may be determined for comparison with observations on the reaction in question. The reader is directed to the recent review edited by Moss, Platz and Jones Jr. [10], for a modern comprehensive survey of progress in reactive intermediate chemistry. The chapters deal in some detail with the methods touched on as outlined above and the results of their application to carbocations, carbanions, carbenes, radicals and strained species. 

In the rest of this chapter, we attempt to extract and illustrate some general principles in the design of experiments to test for the existence of intermediates in particular reactions. 

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