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Free Radical Reactions - Properties, Mechanisms and Examples
A free radical reaction is one of the chemical reactions that include free radicals. Free radicals are molecules that contain unpaired electrons in the atomic orbit. These radicals have common properties because of unpaired electrons. The free radical reaction is generally found in organic reactions.
Table of Contents:
- What is Free Radical Reaction?
- Reaction of Free Radicals
- Abstraction
- Addition
- Combination and Disproportionation
- Rearrangement
- Characteristics of Free Radical Reactions
- Practice Problems On Free Radical Reactions
What is Free Radical Reaction?
A free radical is a molecular species with one or more unpaired electrons in its orbital. As these electrons are free, they are less stable and react spontaneously with other species. Free radical reactions are part of organic synthesis and chain reactions. Only the reaction inhibitors slow down the reaction process. Generally, it takes place in the gas phase. It is initiated by light and catalysed by acid or base. The free radical reaction is the same in the solution or gas phase.
Reaction of Free Radicals
There are four types of free radical reactions with free radicals. They are as follows:
- Abstraction
- Addition
- Combination and Disproportionation
- Rearrangement
Abstraction
The free radicals start reacting with saturated organic molecules by abstracting the atom from the carbon. The polar effects and the bond dissociation energy determine free radicals' selectivity toward different types of C-H bonds. As the bond dissociation energy decreases, the rate of abstraction increases.
The C-H bonds between allylic and benzylic exhibit high selectivity and reactivity with free radicals. The polar effect in the radical reaction is operative. In the C-H bonds of butyl chloride, the chlorine atom is electronegative and reacts at the high electron density C-H bonds. The alkyl radicals are nucleophilic radicals that react at low electron density C-H bonds.
Addition
The free radicals also experience the addition of carbon-carbon double bonds. This reaction is selective. Let us take an example of an addition to CH2=CHX that occurs at the methyl group and doesn’t depend on the nature of X. Alkynes also show the same reactivity with free radicals.
Combination and Disproportionation
Two free radicals are combined by a disproportionation or dimerisation process. It takes place very fast and has insignificant activation of energies. For example:
- 2Br → Br2
- 2CH3 → H3C-CH3
- CH4 + Br → H3C- Br
- 2Et → EtH + CH2= CH2
Rearrangement
In some cases, the free radicals rearrange themselves. The addition of free radicals can result in the rearrangement of free radicals because of the strain relief in the system.
Characteristics of Free Radical Reactions
The free radicals are classified into two groups. With the combination of two radicals, a new product is formed. For example, 2CH3.→ H3C-CH3. The resultant product formed by the reaction of radicals with molecules is possible by the chain reaction process. For example:
- Initiation: Cl2 → 2 Cl ( with the presence of light)
- Propagation:
Cl + CH4 → HCl + CH3
CH3 + Cl2 → CH3 Cl + Cl - Termination:
2Cl → Cl2
2CH3 → C2 H6
CH3 + Cl → CH3 Cl
Practice Problems On Free Radical Reactions
Q1. Which of the following is involved in free radical reaction?
a. Electrons
b. Paired electrons
c. Unpaired electrons
d. Protons
Ans: c. Unpaired electrons
A free radical reaction is one of the chemical reactions that include free radicals. The free radical is a molecular species with one or more unpaired electrons in its orbital.
Q2. The rate of abstraction increases with ____________ in bond dissociation energy.
a. Increase
b. Decrease
c. Neutral
d. None of the above
Ans: b. Decrease
The polar effects and bond dissociation energy determine free radicals' selectivity toward different types of C-H bonds. As the bond dissociation energy decreases, the rate of abstraction increases.
Q3. Which of the following is involved in initiating free radical reaction?
a. Light
b. Heat
c. Energy
d. None of the above
Ans: a. Light
As a result of molecules absorbing light, free radical reactions take place, releasing electrons and boosting energy levels.
Frequently Asked Questions
Q1. What is meant by free radical substitution?
Answer: A free radical substitution refers to the reaction of substitution where the free radicals are the intermediate reactive.
Q2. What is chain termination?
Answer: Chain termination is a process that takes place when two free radicals react with each other to create a stable and non-radical adduct. This process is very rare because of the low concentration of radicals.
Q3. What are the sources of free radicals?
Answer: Sources such as Mitochondria, Peroxisomes, Inflammation, Phagocytosis and exercises can generate free radicals internally. It can also be generated externally by radiation, pollution, drugs, pesticides, the ozone layer, etc.
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