Question

What happens when propene is treated with hydrobromic acid?

Answer 1

When propene (CH₃ – CH = CH₂) is treated with hydrobromic acid (HBr), two possible outcomes occur depending on the reaction conditions:

1. In the absence of Peroxides (Markovnikov Addition):
   Markovnikov’s Rule governs the addition. The H atom from HBr adds to the carbon of the double bond that has the greater number of hydrogen atoms (i.e., the CH₂ group). The Br atom adds to the carbon of the double bond with fewer hydrogen atoms (i.e., the CH group).
   \[\ce{\underset{propene}{CH3 - CH = CH2} + HBr -> \underset{2-bromopropane}{CH3 - CHBr - CH3}}\]
2. In the presence of Peroxides (Peroxide Effect/Anti-Markovnikov Addition): When peroxides like H₂O₂ are present, the addition of HBr follows the anti-Markovnikov rule via a free radical mechanism.
   \[\ce{\underset{propene}{CH3 - CH = CH2} + HBr ->[peroxide][] \underset{1-bromopropane}{CH3 - CH2 - CH3}}\]