Addition to unsymmetrical alkenes

An unsymmetrical alkene is one like propene where the groups at either end of the carbon-carbon double bond are different.The reaction happens under the same conditions as with a symmetrical alkene, but there is a complication because the hydrogen and the bromine can add in two different ways. Which way they add depends on whether there are organic peroxides (or oxygen) present or not.

Normally, when a molecule HX adds to a carbon-carbon double bond, the hydrogen becomes attached to the carbon with the more hydrogens on already. This is known as Markovnikov’s Rule.

Because the HBr adds on the “wrong way around ” in the presence of organic peroxides, this is often known as the peroxide effect or anti-Markovnikov addition.

In the absence of peroxides, hydrogen bromide adds to propene via an electrophilic addition mechanism. That gives the product predicted by Markovnikov’s Rule.

The free radical mechanism