Electrophilic addition of HBr to propene (Markovnikov)

Alkenes are electron-rich, so they react with electrophiles. With an unsymmetrical alkene like propene, there are two possible carbocation intermediates; the more stable secondary carbocation forms, which decides the major product.

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Electrophilic addition

The C=C π bond attacks the δ+ hydrogen of HBr, giving a carbocation and bromide ion. Bromide then attacks the carbocation. Because the secondary carbocation is more stable than the primary one, 2-bromopropane is the major product.

Overall reaction

1. 1π bond attacks H–Brelectrophilic attack

   propene

   +

   hydrogen bromide

   secondary carbocation

   +

   bromide ion

   Electron movement

   1. The π electrons of the C=C bond attack the δ+ hydrogen of H–Br, forming a C–H bond to the end carbon.
   2. The H–Br bond breaks and both electrons go to bromine.

   The hydrogen adds to the CH₂ end so that the positive charge ends up on the middle carbon, which has two alkyl groups pushing electron density towards it. This secondary carbocation is more stable than the primary alternative.

2. 2Bromide attacks the carbocationnucleophilic attack

   secondary carbocation

   +

   bromide ion

   2-bromopropane

   Electron movement

   1. A lone pair on the bromide ion attacks the positively charged carbon.

   The bromide ion forms a bond to the carbocation, giving the neutral product.