Electrophilic substitution in pyrroles. Part 2. Reaction with diazonium ions in acid solution

The kinetics of the reactions of five substituted benzenediazonium ions with up to eleven different pyrroles have been examined in detail by stopped-flow spectrophotometry. All the evidence favours attack of the unprotonated pyrrole in an SE2 mechanism, with a steady-state intermediate. A consideration of two linear free-energy relationships shows that little can be deduced about the nature of any transition state in these reactions. However, protonated pyrrole is a good model for the steady-state intermediate. The activating effects of a methyl group towards electrophilic attack at various positions on the ring have been calculated. The special reactions of 3,4-dimethyl- and 2,3,4,5-tetra-methyl-pyrrole are discussed.