The electrophilic aromatic substitution reaction rate for indole is 10¹³ times greater than that of benzene. It often reacts under mild reaction conditions. Substitution in indole occurs preferably at position 3.
Propose a mechanism for the bromination of indole. You do not have to show steps for generating the Br⁺ ion. You may begin with indole and Br⁺ ion. Draw resonance forms for each sigma complex.

• A.
• B.
• C.
• D.

Answer: (B)