14.3.3 Substitution Reaction

This topic explains substitution reactions — reactions in which one atom or group of atoms is replaced by another atom or group of atoms.

Definition

In a substitution reaction, one atom or group of atoms is replaced by another atom or group of atoms.

Simple explanation: A substitution reaction is when one part of a molecule is swapped for another.

Main Idea

Alkanes undergo substitution reactions with halogens in the presence of sunlight.

• A hydrogen atom in the alkane is replaced by a halogen atom.
• This is called halogenation.
• The reaction is a photochemical reaction (needs sunlight).

General reaction:

R–H + X₂ sunlight→ R–X + HX

Simple explanation: In a substitution reaction, a hydrogen atom is replaced by a halogen atom.

Core Concept

Key Features of Substitution Reactions

Example from the Book

CH₄ + Cl₂ sunlight→ CH₃Cl + HCl

• One H in methane is replaced by Cl.
• Products: chloromethane + hydrogen chloride.

Simple explanation: Methane's hydrogen is replaced by chlorine in sunlight.

Detailed Explanation

What Is a Substitution Reaction?

• One atom or group of atoms is replaced by another atom or group of atoms.
• The basic structure of the molecule remains the same.
• Only a part of the molecule changes.

• The H atom is replaced by a halogen atom (X).
• The reaction is called halogenation.

Simple explanation: Substitution means swapping one part of a molecule for another.

Substitution vs Addition

Simple explanation: Substitution = swap; addition = add.

Monosubstitution

When one hydrogen atom is replaced by a halogen:

CH₄ + Cl₂ sunlight→ CH₃Cl + HCl

Product: Chloromethane (CH₃Cl)

Simple explanation: Monosubstitution means only one H is replaced.

Polysubstitution

When more than one hydrogen atom is replaced by halogens:

CH₄ Cl₂→ CH₃Cl Cl₂→ CH₂Cl₂ Cl₂→ CHCl₃ Cl₂→ CCl₄

Simple explanation: With excess halogen, more than one H can be replaced.

Photochemical Nature of Halogenation

• The reaction is a photochemical reaction — it needs sunlight.
• Sunlight provides energy to break the halogen–halogen bond (Cl–Cl).
• This produces free radicals (Cl•), which start the reaction.

Without sunlight: - The reaction does not occur or occurs very slowly.

Simple explanation: Sunlight is needed to start the substitution reaction.

Mechanism of Substitution (Free Radical)

Step 1 — Initiation:

Cl₂ sunlight→ 2Cl^•

Step 2 — Propagation:

CH₄ + Cl^• → CH₃^• + HCl

CH₃^• + Cl₂ → CH₃Cl + Cl^•

Step 3 — Termination:

Cl^• + Cl^• → Cl₂

CH₃^• + Cl^• → CH₃Cl

Simple explanation: Sunlight breaks Cl₂ into Cl• radicals that attack methane, replacing H with Cl.

Reactivity of Halogens in Substitution

Simple explanation: Chlorine and bromine are used for substitution; fluorine is too reactive; iodine is too slow.

Intermediate Level

Substitution in Alkanes — In Detail

General reaction:

R–H + X₂ sunlight→ R–X + HX

Example:

CH₄ + Cl₂ sunlight→ CH₃Cl + HCl

• The H atom is replaced by Cl.
• The product is a haloalkane.

Monosubstitution vs Polysubstitution

Mechanism of Free Radical Substitution

Initiation: Cl₂ light→ 2Cl^•

Propagation: CH₄ + Cl^• → CH₃^• + HCl CH₃^• + Cl₂ → CH₃Cl + Cl^•

Termination: Cl^• + Cl^• → Cl₂ CH₃^• + Cl^• → CH₃Cl

Simple explanation: The reaction proceeds through free radicals — atoms with unpaired electrons.

Why Alkanes Undergo Substitution (Not Addition)

• Alkanes have only single bonds (saturated).
• They cannot undergo addition (no multiple bonds to add across).
• Instead, they undergo substitution — one atom replaced by another.

Simple explanation: Alkanes are saturated, so they substitute rather than add.

Importance of Substitution Reactions

• Used to make haloalkanes.
• Haloalkanes are used as solvents, refrigerants, anesthetics.
• The reaction is important in organic synthesis.

Important Points

• - Substitution reaction: one atom or group of atoms is replaced by another atom or group of atoms.
• - Alkanes undergo substitution with halogens in sunlight.
• - General reaction: R–H + X₂ → R–X + HX.
• - Example: CH₄ + Cl₂ → CH₃Cl + HCl.
• - Type: photochemical reaction (needs sunlight).
• - Mechanism: free radical (initiation, propagation, termination).
• - Monosubstitution: one H replaced (CH₃Cl).
• - Polysubstitution: multiple H replaced (CH₂Cl₂, CHCl₃, CCl₄).
• - Reactivity of halogens: F₂ > Cl₂ > Br₂ > I₂.

Remember

• - Substitution = swap one atom/group for another.
• - Alkanes → substitution with halogens.
• - Condition: sunlight (photochemical).
• - General reaction: R–H + X₂ → R–X + HX.
• - Example: CH₄ + Cl₂ → CH₃Cl + HCl.
• - Products: haloalkane + hydrogen halide.
• - Monosubstitution: CH₃Cl; polysubstitution: CH₂Cl₂, CHCl₃, CCl₄.
• - Reactivity: F₂ > Cl₂ > Br₂ > I₂.

Common Mistakes

• 1. Confusing substitution with addition. Substitution = swap; addition = add.
• 2. Thinking alkanes undergo addition. Alkanes are saturated — they undergo substitution.
• 3. Forgetting the role of sunlight. Halogenation is a photochemical reaction.
• 4. Thinking only one H can be replaced. With excess halogen, multiple substitutions can occur.
• 5. Confusing the products. Substitution gives a haloalkane + HX.
• 6. Thinking fluorine is used for halogenation. Fluorine is too reactive (explosive); Cl₂ and Br₂ are used.

Quick Revision

• Substitution reaction: one atom or group of atoms is replaced by another atom or group of atoms.
• In alkanes: the H atom is replaced by a halogen atom (halogenation).
• General reaction: R–H + X₂ → R–X + HX.
• Example: CH₄ + Cl₂ → CH₃Cl + HCl (in sunlight).
• Type: photochemical reaction (needs sunlight).
• Mechanism: free radical substitution (initiation, propagation, termination).
• Monosubstitution: one H replaced (CH₃Cl).
• Polysubstitution: multiple H replaced (CH₂Cl₂, CHCl₃, CCl₄).
• Reactivity of halogens: F₂ > Cl₂ > Br₂ > I₂.

60-Second Revision

• Substitution = swap one atom/group for another.
• Alkanes → substitution with halogens.
• Condition: sunlight.
• Example: CH₄ + Cl₂ → CH₃Cl + HCl.
• Warning: alkanes undergo substitution, not addition — they are saturated.