The key difference between free radical substitution and free radical addition is that free radical substitution involves the replacement of a functional group with another functional group, whereas free radical addition involves the addition of a new functional group to a molecule.
A free radical can be an atom, molecule, or ion that consists of an unpaired valence electron. There are two major types of radical reactions: free radical substitution and free radical addition reactions.
CONTENTS
1. Overview and Key Difference
2. What is a Free Radical
3. What is Free Radical Substitution
4. What is Free Radical Addition
5. Free Radical Substitution vs Free Radical Addition in Tabular Form
6. Summary – Free Radical Substitution vs Free Radical Addition
What is a Free Radical?
A free radical can be an atom, molecule, or ion consisting of an unpaired valence electron. Usually, these unpaired electrons can make the free radicals highly chemically reactive; however, there can be some exceptions. Due to their high reactivity, most of the free radicals tend to dimerize spontaneously. Therefore, they have a very short lifetime.
What is Free Radical Substitution?
Free radical substitution is a type of substitution reaction that involves free radicals as the reactive intermediate. Reactive intermediates are short-lived, high energy, and high reactive molecules. These molecules form during a chemical reaction that tends to convert quickly into more stable molecules. Moreover, a substitution reaction is a type of chemical reaction where one functional group in a chemical compound tends to be replaced by another functional group.
Figure 01: Different Steps in Free Radical Reactions
The above image indicates the steps of free radical reactions in general; step 2 and 3 are named initiation reactions where free radical forms through homolysis. Homolysis can be achieved using heat or UV light and using radical initiators, e.g. organic peroxides, azo compounds, etc. The final steps 6 and 7 are collectively named as termination; here, the radical tends to recombine with another radical species. However, the radical sometimes reacts further where propagation occurs. The propagation is given from steps 4 and 5 in the above image.
Some examples of radical substitution reactions include Barton-McCombie deoxygenation, Wohl-Ziegler reaction, Dowd-Beckwith reaction, etc.
What is Free Radical Addition?
Free radical addition is a type of addition reaction where a functional group is added to a compound through a free radical reactive intermediate. This type of addition can occur between a radical and a non-radical species or between two radical species. The basic steps of free radical addition include initiation, chain propagation, and chain termination.
Figure 02: Radical Addition of HBr on Alkenes
During the initiation process, a radical initiator is used for the initiation, where a radical species forms from a non-radical precursor. During the chain propagation process, a free radical tends to react with a non-radical species in order to produce a new radical species. The final step is chain termination, where the two radicals react with each other, creating a non-radical species. A common example of this type of reaction includes Meerwein arylation.
Typically, free radcal addition reactions are based on the reagents having weak bonds so that they can undergo homolysis forming radical species. When there are strong bonds, the reaction mechanism becomes different from usual free radical addition reactions.
What is the Difference Between Free Radical Substitution and Free Radical Addition?
A free radical can be an atom, molecule, or ion consisting of an unpaired valence electron. The key difference between free radical substitution and free radical addition is that free radical substitution involves the replacement of a functional group with another functional group, whereas free radical addition involves the addition of a new functional group to a molecule.
The following figure lists the differences between free radical substitution and free radical addition in tabular form for side by side comparison.
Summary – Free Radical Substitution vs Free Radical Addition
A free radical can be an atom, molecule or ion consisting of an unpaired valence electron. The key difference between free radical substitution and free radical addition is that free radical substitution involves the replacement of a functional group with another functional group, whereas free radical addition involves the addition of a new functional group to a molecule.
Reference:
1. “What Is Free Radical Substitution?” Chemistry LibreTexts, Libretexts, 13 Sept. 2020, Available here.
Image Courtesy:
1. “FreeRadicalSubstitutionGeneralDescr (1)” By V8rik at English Wikipedia (CC BY-SA 3.0) via Commons Wikimedia
2. “Peroxide Free-radical-addition” By Bryn C – Transferred from en (Public Domain) via Commons Wikimedia