Electrophilic addition — HBr

Step 1 of 2

1. 1.The electron-rich π bond attacks the δ+ hydrogen of H–Br — the alkene is the nucleophile here, H is the electrophile.
2. 2.The H–Br bond breaks heterolytically; both electrons go to the more electronegative bromine.

A carbocation forms as H adds to one carbon; Br⁻ is released.

Step 2 of 2

1. 1.A lone pair on the bromide ion forms a bond to the positively charged carbon.

Br⁻ bonds to the carbocation: bromoethane.