14.3.1 Halogenation

This topic explains halogenation — the reaction of alkanes with halogens in the presence of sunlight.

Definition

Halogenation is the reaction in which an alkane reacts with a halogen (Cl₂, Br₂) in the presence of sunlight, producing a haloalkane and a hydrogen halide.

Simple explanation: Halogenation is when a halogen replaces a hydrogen atom in an alkane.

Main Idea

Although alkanes are unreactive towards ionic substances, they readily react with halogens in sunlight.

A chemical reaction that takes place in the presence of sunlight is called a photochemical reaction.

General reaction:

R–H + X₂ sunlight→ R–X + HX

• **R–H** = alkane
• **X₂** = halogen (Cl₂ or Br₂)
• **R–X** = haloalkane
• **HX** = hydrogen halide

Simple explanation: In sunlight, a halogen atom replaces a hydrogen atom in the alkane.

Core Concept

Key Features of Halogenation

Example from the Book

CH₄ + Cl₂ sunlight→ CH₃Cl + HCl

- Methane + Chlorine → Chloromethane + Hydrogen chloride

Simple explanation: Methane reacts with chlorine in sunlight to form chloromethane and HCl.

Detailed Explanation

What Is a Substitution Reaction?

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

• A hydrogen atom in the alkane is replaced by a halogen atom.
• The reaction is a substitution reaction.

Example:

CH₄ + Cl₂ → CH₃Cl + HCl

- One H in CH₄ is replaced by Cl.

Simple explanation: One hydrogen is swapped for a halogen.

What Is a Photochemical Reaction?

Definition: A chemical reaction that takes place in the presence of sunlight is called a photochemical reaction.

• Sunlight provides the energy needed to break the halogen molecule (X₂) into free radicals.
• These free radicals then react with the alkane.
• Without sunlight, the reaction does not occur (or occurs very slowly).

Simple explanation: Sunlight gives the energy needed to start the halogenation reaction.

Step-by-Step Mechanism (Simplified)

Step 1: Initiation (sunlight breaks Cl₂ into radicals):

Cl₂ sunlight→ 2Cl^•

Step 2: Propagation (radicals react with methane):

CH₄ + Cl^• → CH₃^• + HCl

CH₃^• + Cl₂ → CH₃Cl + Cl^•

Step 3: Termination (radicals combine):

Cl^• + Cl^• → Cl₂

CH₃^• + Cl^• → CH₃Cl

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

Multiple Substitutions

If excess chlorine is used, multiple substitutions can occur:

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

• CH₃Cl — chloromethane
• CH₂Cl₂ — dichloromethane
• CHCl₃ — trichloromethane (chloroform)
• CCl₄ — tetrachloromethane (carbon tetrachloride)

Simple explanation: If there's enough chlorine, more than one hydrogen can be replaced.

Reactivity of Halogens

Simple explanation: Chlorine and bromine are the most commonly used halogens for halogenation.

Differences Between Chlorination and Bromination

Intermediate Level

Why Halogenation Needs Sunlight

• Sunlight (UV light) provides energy to break the Cl–Cl bond.
• This produces chlorine free radicals (Cl•).
• Free radicals are highly reactive and start the reaction.
• Without sunlight, the reaction is very slow.

Simple explanation: Sunlight breaks the halogen molecules into reactive fragments.

Substitution Reaction — In Detail

• A hydrogen atom is replaced by a halogen atom.
• This is a substitution reaction.
• The general equation is:

Example:

CH₄ + Cl₂ → CH₃Cl + HCl

Simple explanation: One H is swapped for one Cl.

Photochemical Reaction — In Detail

Definition: A chemical reaction that takes place in the presence of sunlight is called a photochemical reaction.

• The reaction is initiated by light energy.
• Light breaks the halogen molecule into free radicals.
• The reaction proceeds via a free radical mechanism.
• Without light, the reaction does not proceed.

Simple explanation: The reaction needs sunlight to start — it's a photochemical reaction.

Multiple Substitution Products

Simple explanation: Chlorination can replace one, two, three, or all four hydrogens in methane.

Uses of Haloalkanes Produced

Important Points

• - Halogenation: reaction of alkanes with halogens in sunlight.
• - General reaction: R–H + X₂ → R–X + HX.
• - Type: substitution reaction (H replaced by halogen).
• - Condition: sunlight (photochemical reaction).
• - Products: haloalkane + hydrogen halide.
• - Example: CH₄ + Cl₂ → CH₃Cl + HCl.
• - Multiple substitutions: CH₄ → CH₃Cl → CH₂Cl₂ → CHCl₃ → CCl₄.
• - Reactivity of halogens: F₂ (explosive) > Cl₂ > Br₂ > I₂ (very slow).
• - Mechanism: free radical mechanism (initiation, propagation, termination).

Remember

• - Halogenation = alkane + halogen (in sunlight).
• - Type: substitution reaction.
• - Condition: sunlight (photochemical).
• - General reaction: R–H + X₂ → R–X + HX.
• - Example: CH₄ + Cl₂ → CH₃Cl + HCl.
• - Multiple substitutions possible with excess halogen.
• - Reactivity: F₂ > Cl₂ > Br₂ > I₂.
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