When propene CH2=CHCH3 reacts with HBr in the presence of peroxide as a catalyst, the reaction follows a free radical mechanism, which is also known as an anti-Markovnikov addition. In this case, the bromine atom Br will attach to the less substituted carbon of the double bond, and the hydrogen atom H will attach to the more substituted carbon. The product formed in this reaction is 1-bromopropane CH3CH2CH2Br .