ABSTRACT
The classical reaction involving nucleophilic displacement at aliphatic carbon has been a test-bed of mechanistic theories since they were first studied almost a century ago. The perceived mechanism for this reaction developed in step with the development of mechanistic ideas. The carbenium ion is often a very reactive species in the presence of nucleophiles and consequently few nucleophilic substitution reactions have revealed stable carbenium ions. The reaction of a strong nucleophile with an unstable carbenium ion is expected to be very fast and indeed if this reaction is diffusion controlled it can be used as a “clock” to determine the rate constant for the decomposition of the intermediate in water. This technique has produced results for the stability of carbenium ions in reactions with mechanisms of the DN +AN type enabling estimates to be made of the lifetimes of putative carbenium ion intermediates.
