IB Chemistry Topic R3.3.3 Substitution Reactions with Alkanes Revision Notes (SL/HL)

IB Chemistry Topic R3.3.3 Substitution Reactions with Alkanes Revision Notes (SL/HL) | RevisionDojo

Radical Substitution Reactions: Mechanism and Stability of Alkanes

Radical substitution is a reaction in which halogen radicals replace hydrogen atoms in an alkane, one at a time, through a chain mechanism.

The Mechanism of Radical Substitution

Radical substitution reactions proceed through three key stages: initiation, propagation, and termination.
Let’s break these down with the example of methane ($CH_4$) reacting with chlorine ($Cl_2$) under UV light.

1. Initiation: Formation of Radicals

The initiation step generates radicals, which are essential to start the chain reaction.
For $Cl_2$, UV light provides the energy needed to break the chlorine-chlorine bond via homolytic fission: $$
Cl_2 \xrightarrow{\text{UV light}} Cl• + Cl•
$$
Each chlorine radical now has one unpaired electron, making it highly reactive.

Tip

Always include the energy source (e.g., UV light) in equations for initiation steps.
Without it, the reaction cannot proceed.

2. Propagation: The Chain Reaction

In the propagation stage, radicals react with stable molecules to form new radicals, perpetuating the chain reaction.
For methane and chlorine, this involves two key steps:

Step 1:

A chlorine radical reacts with a methane molecule, abstracting a hydrogen atom and forming hydrogen chloride ($HCl$) and a methyl radical ($CH_3•$):

$$
Cl• + CH_4 \rightarrow CH_3• + HCl
$$

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Radical Substitution Reactions: Mechanism and Stability of Alkanes

Radical substitution is a reaction in which halogen radicals replace hydrogen atoms in an alkane, one at a time, through a chain mechanism.

The Mechanism of Radical Substitution

Radical substitution reactions proceed through three key stages: initiation, propagation, and termination.
Let’s break these down with the example of methane ($CH_4$) reacting with chlorine ($Cl_2$) under UV light.

1. Initiation: Formation of Radicals

The initiation step generates radicals, which are essential to start the chain reaction.
For $Cl_2$, UV light provides the energy needed to break the chlorine-chlorine bond via homolytic fission: $$
Cl_2 \xrightarrow{\text{UV light}} Cl• + Cl•
$$
Each chlorine radical now has one unpaired electron, making it highly reactive.

Tip

Always include the energy source (e.g., UV light) in equations for initiation steps.
Without it, the reaction cannot proceed.

2. Propagation: The Chain Reaction

In the propagation stage, radicals react with stable molecules to form new radicals, perpetuating the chain reaction.
For methane and chlorine, this involves two key steps:

Step 1:

A chlorine radical reacts with a methane molecule, abstracting a hydrogen atom and forming hydrogen chloride ($HCl$) and a methyl radical ($CH_3•$):

$$
Cl• + CH_4 \rightarrow CH_3• + HCl
$$

Definition

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Structure 1. Models of the particulate nature of matter5 subtopics

Structure 2. Models of bonding and structure4 subtopics

Structure 3. Classification of matter2 subtopics

Reactivity 1. What drives chemical reactions?4 subtopics

Reactivity 2. How much, how fast and how far?3 subtopics

Reactivity 3. What are the mechanisms of chemical change?4 subtopics

IB Chemistry R3.3.3 Substitution Reactions with Alkanes Notes

Structure 1. Models of the particulate nature of matter5 subtopics

Structure 2. Models of bonding and structure4 subtopics

Structure 3. Classification of matter2 subtopics

Reactivity 1. What drives chemical reactions?4 subtopics

Reactivity 2. How much, how fast and how far?3 subtopics

Reactivity 3. What are the mechanisms of chemical change?4 subtopics

Radical Substitution Reactions: Mechanism and Stability of Alkanes

The Mechanism of Radical Substitution

1. Initiation: Formation of Radicals

2. Propagation: The Chain Reaction

Step 1:

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Duis aute irure dolor in reprehenderit

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Radical Substitution Reactions: Mechanism and Stability of Alkanes

The Mechanism of Radical Substitution

1. Initiation: Formation of Radicals

2. Propagation: The Chain Reaction

Step 1:

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Duis aute irure dolor in reprehenderit

Excepteur sint occaecat cupidatat non proident

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