Substituent Effects in the Benzofuran System. II. Electrophilic Bromination

Abstract Bromination of benzofuran (BF) and its derivatives (5-CH3, 6-CH3, 5-CH3O, and 5-Cl) has been studied kinetically in acetic acid and by product analysis under various conditions. Product analysis revealed that bromine adds to BF in trans fashion, electrophilic bromine attacking at the 2-position. Analysis of substituent effects on the rate of bromination showed that the electronic effects are transmitted through both the bond 8–3 and the bonds 9–1–2 by a comparable magnitude. The transition state of the reaction was concluded to closely resemble a cyclic bromonium ion intermediate.