Question

Complete the reaction $C{H_3} - CH = C{H_2} + HBr$ in presence of organic peroxide?

Answer 1

The given reaction if the addition of a substrate across a double bond. The addition across a double bond occurs mainly according to the Markovnikov’s rule, but as there is a reagent Peroxide being present, it will follow Anti-Markovnikov addition.

Complete Step By Step Answer:
Anti-markovnikov’s mechanisms are for those reactions which do not involve the formation of Carbocation. The anti-markovnikov’s rules state that the negative part of the substrate gets attached to that carbon atom having more no. of hydrogen. This is the correct reverse of Markovnikov addition reaction.
The most common reaction is the free radical addition of HBr in presence of benzoyl peroxide or hydrogen peroxide. Peroxide acts as a catalyst to break down HBr into Br and H radicals. The mechanism for the addition can be given as follows:

The Br radical attacks the alkene in reaction (a). It attacks the less substituted carbon, so that a more substituted carbon radical will be formed, hence it’ll have greater stability. The carbon radical then attacks another HBr molecule and the Hydrogen is attached to the Carbon Radical, and another Bromine radical is released. This radical carries the reaction ahead.
Therefore, the major product formed in the reaction is 1-BromoPropane.

Note:
This radical mechanism occurs only with HBr. It doesn’t occur with HCl and HI. If peroxide wouldn’t have been added then the product that will be formed will be a Markovnikov product, which will be 2-Bromo Propane. 1-Bromopropane will also be formed in this reaction, it will only be in trace quantity.