Updated: Dec 25, 2022
Radicals contain at least one unpaired electron. The arrows that we draw for radical reaction mechanisms are different from regular arrows.
Radical reactions generally have 3 steps in their mechanism:
Initiation (making a radical)
Propagation (making the desired product)
Termination (getting rid of all the radicals)
1. Radical Halogenation Of Alkanes
Reagent: Br2, light.
Product: Adds Br to the most substituted carbon on the alkane
In the mechanism, the first step is initiation. The bond between Br-Br breaks to give us two Br radicals.
Propagation is made up of 2 steps. First, we draw the alkane starting material. Bond between hydrogen and carbon breaks (on the carbon we want to attach Br to) and bromine radical forms a bond with hydrogen, resulting in HBr and alkane radical. In the second step, alkane radical reacts with Br-Br. The bond between Br-Br breaks and the alkane radical forms a bond with the bromine to give the desired product.
Termination is used to get rid of any radical created in the reaction. During termination any radicals created in the reaction come together and make a bond.
Please note that this reaction is also possible with Cl2 and light. However, chlorine is not selective like bromine and can attach to any carbon, allowing for many different products to form.
2. Allylic Halogenation
Allylic carbon is a carbon that is next to a C=C. Allylic halogenation is a reaction that adds bromine to an allylic carbon.
Reagent: NBS, light
Product: Attach Br to the allylic carbon
Mechanism of Allylic Halogenation
A couple of notes:
Br2 with light will also result in an allylic halogenation since allylic radical is resonance stabilized and would like to happen.
Allylic radical is resonance stabilized and therefore this reaction may result in multiple products.
3. Radical Addition Of HBr to Alkenes
Reagent: HBr, ROOR (peroxide)
Product: Turn double bond into single and add Br to the least substituted carbon from the double bond
In this reaction, we must start with an alkene (C=C). The double bond is converted to a single bond and Br is added to the LEAST substituted carbon (Anti Markovnikov).
Mechanism Of Radical Addition of HBr to Alkenes