Electrophilic substitution in pyrroles. Part I. Reaction with 4-dimethylaminobenzaldehyde (Ehrlich's reagent) in acid solution

The kinetics of the reaction between various pyrroles and 4-dimethylaminobenzaldehyde (DMAB) in acid solution has been examined by stopped-flow spectrophotometry. The mechanism is rate-determining attack by O-protonated DMAB on free pyrrole followed by elimination of water. The activating effect of a methyl group on various positions in the pyrrole ring has been determined.