Addition of HBr to an alkene Mechanism: Steps and Conditions

Conditions

HBr, no peroxides (peroxides switch to anti-Markovnikov radical addition).

Red curly arrows show where each pair of electrons moves.

Step 1The π electrons attack H, so H adds to the end carbon and the more stable secondary carbocation forms.

Step 2Bromide attacks the carbocation.

The π electrons of a C=C are exposed and electron-rich, so alkenes react with electrophiles. The two new groups add across the double bond.

Practise this mechanismDraw the curly arrows yourself and Bondline checks each one. Or add it to your notes and watch it in 3D.