The addition reaction initiated by the attack of an electrophile on a double or triple bond is termed the electrophilic addition reaction.

In an electrophilic addition reaction, the pi bond is broken, and two sigma bonds are formed.

Bromine (Br), being more electronegative than hydrogen, pulls the electron from the H-Br bond towards. This reduces the electron density on the hydrogen atom.

The electron-rich pi bond attacks H⁺ and forms a sigma bond with it, thereby generating a carbocation.

${\mathrm{Br}}^{-}$attacks the carbocation and forms a bond with it.

The addition of HBr to the double bond proceeds as shown below;

Addition of to the double-bonded carbon

The carbocation is formed adjacent to the electron-rich ring (ring containing the electron-donating OCH₃ group) as the delocalization of electrons from the ring increases its stability.

Attack of ${\mathrm{Br}}^{-}$on the carbocation