Kinetics, Stoichiometry and Mechanism in the Bromination of Aromatic Heterocycles. VI. Aqueous Bromination of Furan, Pyrrole and Thiophen

Abstract

Furan is oxidized to butenedial by electrogenerated bromine in aqueous solution, while thiophen under the same conditions is converted into 2-bromo-and 2,5-dibromo-thiophen. The reaction of pyrrole appears to approach the encounter-controlled limit. For transfer of Br⁺ from Br₂ (aq., 25ºC) to the 2(5)-positions in these systems, bimolecular rate constants, k^o_bi/dm³ mol^-1 s^-1, are as follows :

furan thiophen 2-bromothiophen 10⁸-10¹⁰ 2x90 x 10⁴ ± 5% 2 x 5.2 x 10⁴ ± 4% 2.65 x 10⁴ ± 2%

Reaction pathways leading to substitution, addition and oxidation are reviewed.