Bromination with NBS on cyclic structures
As a follow-up to the excellent comments provided thus far, the following thoughts are added. Electrostatic Potential Map of azulene (1a) shows that there is high electron density in the 5-membered ring (red color signifies high electron density). The 5-membered ring is reminiscent of the aromaticity of cyclopentadienide anion and the 7-membered ring bears a positive charge and has aromaticity similar to cycloheptatrienyl cation. This phenomenon is represented in structure 1b. The reaction does not involve bromine atom (a radical) because a bromine atom would be expected to attack the electropositive ring, not the electron-rich ring. The reaction is ionic and likely involves bromine rather than N-bromosuccinimide (NBS, 4) directly. Bromine is liberated from NBS by trace amounts of HBr formed by the slow decomposition of NBS. In the event, bromination of azulene (1c) produces a resonance stabilized cation 2a. Its canonical structure is 2b that is identical to 2b'. Loss of a proton from 2b (or 2b') provides bromoazulene 3.