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A Complete Guide To Radical Reactions

Radicals contain at least one unpaired electron. The arrows that we draw for radical reaction mechanisms are different from regular arrows.


Radical arrow: breaking and making a bond
Radical arrow: breaking and making a bond

Radical reactions generally have 3 steps in their mechanism:

  1. Initiation (making a radical)

  2. Propagation (making the desired product)

  3. 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


Radical bromination of an alkane
Radical bromination of an 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.

mechanism of radical bromination of an alkane
mechanism of radical bromination of an alkane

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

allylic halogenation with NBS
allylic halogenation with NBS

Mechanism of Allylic Halogenation

Mechanism of Allylic Halogenation
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.


Resonance in Radical Allylic Intermediates
Resonance in Radical Allylic Intermediates

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).


 Radical Addition of HBr to Alkenes
 Radical Addition of HBr to Alkenes

Mechanism Of Radical Addition of HBr to Alkenes


Mechanism Of Radical Addition of HBr to Alkenes
Mechanism Of Radical Addition of HBr to Alkenes

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