What is Free Radical Substitution Reaction - Mechanism, Types, and Examples
The rate of reactivity in free radical substitution reactions is a key concept in organic chemistry, essential for understanding how different alkyl radicals form. This concept plays a vital role in predicting the outcome of halogenation reactions like chlorination and bromination, which are frequently tested in exams like JEE Main 2025. This article aims to explain the factors influencing reactivity, provide insights into reaction mechanisms, and offer tips for effective preparation. By mastering this topic, students can confidently solve related questions and enhance their overall chemistry performance.
What is Free Radical Substitution Reaction?
A substitution reaction is defined as a reaction in which one chemical substance's functional group is replaced by another group or a reaction in which one atom or molecule of a compound is replaced by another atom or molecule.
π΄βπ΅+πβπ΄βπ+π΅
These reactions can be divided into three categories:
Free Radicals Substitution
Electrophilic Substitution
Nucleophilic Substitution
We will discuss here the free radical substitution and its reactions and mechanism as well as the substitution of alkanes and the free radical halogenation process.
Free Radical Substitution
A free radical substitution is a substitution reaction in which the reactive intermediate is free radicals.
At least two phases, and perhaps a third, are always involved in the reaction.
The interaction of methane and chlorine in the presence of UV light (or sunlight) is a simple example of free radical substitution. One of the hydrogen atoms has been replaced with a chlorine atom in the methane.
Another example is the free radical substitution of Cl2 on benzene to form chlorobenzene.
\[\text{C}_6\text{H}_6 + \text{Cl}_2 \xrightarrow{\text{hv}} \text{C}_6\text{H}_5\text{Cl} + \text{HCl}\]
Free Radical Substitution Reactions
\[\text{C}_6\text{H}_6 + \text{Cl}_2 \xrightarrow{\text{hv}} \text{C}_6\text{H}_5\text{Cl} + \text{HCl}\]
Free Radical Substitution Mechanism
In the mechanism of free radical substitution, three stages are involved:
Initiation
Propagation
Termination
Initiation: The initiation phase is the first step in the creation of a radical species. Because of the enormous energy barriers involved, this is usually a homolytic cleavage event that occurs only infrequently. To overcome the energy barrier, heat, UV light, or a metal-containing catalyst are frequently used.
Propagation: The 'chain' aspect of chain reactions is described by the propagation phase. Once a reactive free radical has been produced, it can combine with stable molecules to produce more reactive free radicals. These new free radicals produce even more free radicals, and so on. Hydrogen abstraction or radical addition to double bonds are frequently used in propagation phases.
Termination:When two free radical species react to generate a stable, non-radical adduct, chain termination occurs. Because of the low concentration of radical species and the minimal chance of two radicals interacting, this is a highly rare thermodynamically downward occurrence.
Mechanism of Free Radical Substitution Reaction
1. Chain initiation- In the presence of heat or light, the chlorine molecule undergoes homolytic cleavage, releasing a chlorine-free radical, which commences the reaction.
As a result, the C-C and C-H bonds are easier to break.
\[\textbf{Chain Initiation:}\]
In the presence of heat or light, the chlorine molecule undergoes homolytic cleavage:
\[\text{Cl}_2 \xrightarrow{\text{heat/light}} 2\ \text{Cl}^\bullet\]
This generates chlorine-free radicals, which initiate the reaction. As a result, the C-C and C-H bonds in the substrate become easier to break.
2. Chain Propagation- The chlorine free radical attacks the methane molecule and drives the reaction ahead by breaking the C-H bond, resulting in the creation of methyl free radical and H-Cl.
\[\textbf{Chain Propagation:}\]
The chlorine-free radical \[(\text{Cl}^\bullet)\] attacks the methane molecule \[(\text{CH}_4)\], breaking the C-H bond and forming a methyl free radical \[(\text{CH}_3^\bullet)\] and HCl:
\[\text{CH}_4 + \text{Cl}^\bullet \rightarrow \text{CH}_3^\bullet + \text{HCl}\]
This step drives the reaction forward by generating reactive intermediates.
With the liberation of another chlorine-free radical through homolytic fission of chlorine molecules, the methyl radical attacks the second molecule of chlorine to create CH3-Cl.
With the liberation of another chlorine-free radical through the homolytic fission of chlorine molecules, the methyl radical reacts with a chlorine molecule to form chloromethane \[(\text{CH}_3\text{Cl})\]:
\[\text{CH}_3^\bullet + \text{Cl}_2 \rightarrow \text{CH}_3\text{Cl} + \text{Cl}^\bullet\]
This step regenerates the chlorine-free radical, allowing the chain reaction to continue.
A series of reactions was started by the chlorine and methyl free radicals produced in the reaction. The propagation steps (a) and (b) directly give primary products, although numerous other propagation phases can and will occur. The creation of more halogenated compounds from the principal product, chloromethane, can be explained in two ways.
3. Chain Termination- The reaction comes to a halt after some time due to the consumption of reactants and/or the occurrence of the following side reactions:
The following are examples of probable chain-ending steps:
\[\textbf{Chain Termination:}\]
The reaction gradually halts as the reactants are consumed or due to the occurrence of side reactions. Probable chain-ending steps include:
1. **Combination of Two Chlorine Free Radicals:**
\[\text{Cl}^\bullet + \text{Cl}^\bullet \rightarrow \text{Cl}_2\]
2. **Combination of a Chlorine Free Radical and a Methyl Radical:**
\[\text{Cl}^\bullet + \text{CH}_3^\bullet \rightarrow \text{CH}_3\text{Cl}\]
3. **Combination of Two Methyl Free Radicals:**
\[\text{CH}_3^\bullet + \text{CH}_3^\bullet \rightarrow \text{C}_2\text{H}_6\]
These termination steps result in the formation of stable molecules, effectively ending the chain reaction.
Substitution Reaction of Alkanes
In the presence of light, alkanes undergo a substitution reaction with halogens.
Methane, for example, interacts with halogen molecules like chlorine and bromine when exposed to ultraviolet light.
Consider the following scenario:
Methyl bromide + hydrogen bromide β methane + bromine
CH3Br + HBr β CH4 + Br2
Rate of Reactivity:
The relative rates of formation of alkyl radicals by chlorine radicals at room temperature are typically based on the type of hydrogen atom being abstracted (primary, secondary, or tertiary) due to differences in bond dissociation energies and radical stability.
Here's the general trend:
Because one of the hydrogen atoms in methane is replaced by a bromine atom, this is a substitution reaction.
Free Radical Halogenation
Free-radical halogenation is such that under UV light, this chemical reaction is typical of alkanes and alkyl-substituted aromatics. Chloroform (CHCl3), dichloromethane (CH2Cl2), and hexachlorobutadiene are all manufactured via this procedure. A free-radical chain mechanism is used to carry it out.
General mechanism
Using the chlorination of methane as an example, the chain mechanism is as follows:
1. Initiation: Ultraviolet radiation causes a chlorine molecule to split or homolyze into two chlorine atoms. A chlorine atom is a free radical because it has an unpaired electron.
\[\text{Cl}_2 \xrightarrow{\text{heat/light}} 2\ \text{Cl}^\bullet\]
2. Propagation: A hydrogen atom is extracted from methane, leaving a primary methyl radical in the process of chain propagation (two processes). After that, the methyl radical draws a Clβ’ from Cl2.
Step 1: A chlorine-free radical attacks methane, breaking the C-H bond to form a methyl radical:
\[\text{CH}_4 + \text{Cl}^\bullet \rightarrow \text{CH}_3^\bullet + \text{HCl}\]
Step 2: The methyl radical reacts with a chlorine molecule to produce chloromethane and regenerate a chlorine-free radical:
\[\text{CH}_3^\bullet + \text{Cl}_2 \rightarrow \text{CH}_3\text{Cl} + \text{Cl}^\bullet\]
This produces the required product as well as a second chlorine radical. This radical will then participate in a second propagation reaction, resulting in a chain reaction. Other products, such as CH2Cl2, may occur if there is enough chlorine.
3. Termination: The reaction comes to a halt after some time due to the consumption of reactants-
The final option in the termination step will produce an impurity in the final combination; specifically, an organic molecule with a longer carbon chain than the reactants.
The overall reaction is as follows:
Combination of two chlorine free radicals:
\[\text{Cl}^\bullet + \text{Cl}^\bullet \rightarrow \text{Cl}_2\]
Combination of a chlorine-free radical and a methyl radical:
\[\text{Cl}^\bullet + \text{CH}_3^\bullet \rightarrow \text{CH}_3\text{Cl}\]
Combination of two methyl
Significance of Free Radical Substitution Mechanism for JEE Main 2025
Forms the foundation for understanding reactions involving free radicals, which are commonly tested in JEE Main.
Questions related to free radical substitution, especially in halogenation reactions, appear 1-2 times in JEE Main each year.
Explains industrial and lab-scale processes, such as the chlorination of methane, a vital reaction in chemical industries.
Serves as a stepping stone for understanding complex organic mechanisms like polymerisation and radical halogenation.
Frequently involves reaction pathways, stability of free radicals, and selectivity in substitution.
Conceptual questions on initiation, propagation, and termination are straightforward, offering high-scoring opportunities.
Connects to hybridisation, bond dissociation energy, and hyperconjugation concepts, reinforcing organic chemistry fundamentals.
Includes the chlorination and bromination of alkanes, making it a commonly tested reaction type.
Tips to Prepare Relative Reactivity in Free Radical Substitution
Learn why tertiary radicals are more stable than secondary and primary radicals.
Remember the rate ratios: Tertiary > Secondary > Primary.
Study how factors like temperature and light influence substitution rates.
Solve questions on chlorination and bromination reactions to understand selectivity.
Relate hydrogen abstraction rates to bond strengths for deeper clarity.
Focus on examples like methane chlorination to apply the reactivity trend.
Practice previous year's questions to build confidence on this high-scoring topic.
Conclusion
Understanding the rate of reactivity in free radical substitution is crucial for mastering organic chemistry concepts. The reactivity orderβtertiary > secondary > primaryβis determined by the stability of the resulting alkyl radicals. This knowledge helps predict product formation in reactions like chlorination and bromination, which frequently appear in exams like JEE Main. A solid grasp of this topic ensures better problem-solving skills and boosts overall confidence in tackling reaction mechanism questions.
JEE Main 2025 Subject-Wise Important Chapters
The JEE Main 2025 subject-wise important chapters provide a focused strategy for Chemistry, Physics, and Maths. These chapters help students prioritise their preparation, ensuring they cover high-weightage topics for better performance in the exam.
Important Related Links for JEE Main 2025
FAQs on Free Radical Substitution Mechanism of Alkanes for JEE Main 2025
1. What is the free radical substitution mechanism?
The free radical substitution mechanism involves initiation, propagation, and termination steps where free radicals are formed and drive the reaction forward.
2. What is a free radical substitution reaction?
Free radical substitution reaction is a process where a hydrogen atom in a molecule is replaced by a halogen atom through radical formation.
3. What is the free radical substitution reaction of alkanes?
Free radical substitution reaction of alkanes involves halogenation, where halogen atoms replace hydrogen atoms in alkanes, commonly seen in chlorination and bromination.
4. What is the free radical substitution reaction mechanism?
The mechanism includes initiation (radical formation), propagation (chain reaction), and termination (radicals combine to form stable products). Vedantu explains this in detail with examples for JEE Main 2025.
5. What is a free radical substitution reaction example?
An example is the chlorination of methane:
$C{{H}{4}}+C{{l}{2}}\xrightarrow{hv}C{{H}_{3}}Cl+HCl$
This reaction is commonly tested in exams like JEE Main.
6. What are the conditions for free radical substitution?
Conditions for free radical substitution include the presence of heat or light to break halogen molecules into radicals. Vedantu explains how these conditions affect the reaction to JEE Main 2025 preparation.
7. How to do free radical substitution?
Follow three steps: Initiation (radical formation), Propagation (reaction progression), and Termination (radical neutralisation). Practice examples like halogenation reactions for better clarity.
8. Why is free radical substitution important?
It explains the mechanism behind halogenation, which is crucial for understanding organic reactions and frequently appears in exams like JEE Main.
9. What role does light play in free radical substitution?
Light provides energy for the homolytic cleavage of halogen molecules, initiating the reaction by forming free radicals.
10. How does free radical substitution differ from electrophilic substitution?
Free radical substitution involves radicals, while electrophilic substitution involves positively charged electrophiles. Understanding these differences is crucial for mastering reaction mechanisms.
