How is Ethyl Bromide Prepared from Ethyl Alcohol, Ethane & Ethene?

What is Ethyl Bromide?

Ethyl bromide is an organic compound. Its IUPAC name is bromoethane.

Basic Information:

• Chemical Formula: C₂H₅Br
• IUPAC Name: Bromoethane
• Class: Haloalkane / Alkyl halide
• Type: Primary alkyl halide

Ethyl bromide is formed when one hydrogen atom of ethane is replaced by a bromine atom. It is a colorless liquid and is used in organic synthesis and laboratory reactions.

Preparation of Ethyl Bromide

Ethyl bromide (C₂H₅Br) can be prepared in three main ways:

1. From Ethyl Alcohol (Ethanol)
2. From Ethane
3. From Ethene

Let us understand each method in detail.

1. Preparation of Ethyl Bromide from Ethyl Alcohol (Ethanol)

This is the most common laboratory method.

Reaction Involved:

When ethanol reacts with hydrobromic acid (HBr), ethyl bromide is formed.

Chemical Equation:

C₂H₅OH + HBr → C₂H₅Br + H₂O

Other Reagents Used:

• Phosphorus tribromide (PBr₃)
• Red phosphorus + Bromine

Type of Reaction

This reaction is a nucleophilic substitution reaction (SN2 reaction).

In this reaction:

• The OH group of alcohol is replaced by Br.
• Water is formed as a by-product.

Mechanism

• The –OH group of ethanol is not a good leaving group.
• HBr converts –OH into water (H₂O).
• Bromide ion (Br⁻) attacks the carbon.
• Water leaves.
• Ethyl bromide is formed.

Since ethanol is a primary alcohol, it follows the SN2 mechanism.

Important Exam Points

• Best method for laboratory preparation.
• Gives better yield.
• Commonly asked in board exams.
• Example of substitution reaction of alcohol.

2. Preparation of Ethyl Bromide from Ethane

Ethyl bromide can also be prepared from ethane by bromination.

Reaction Involved:

C₂H₆ + Br₂ → C₂H₅Br + HBr

This reaction takes place in the presence of UV light or sunlight.

Type of Reaction

This is a free radical substitution reaction.

It is also called:

• Halogenation of alkane
• Photochemical reaction

Mechanism of Free Radical Reaction

The reaction occurs in three steps:

1. Initiation

Br₂ breaks into two bromine radicals under UV light.

Br₂ → 2Br•

2. Propagation

Bromine radical reacts with ethane.

Br• + C₂H₆ → C₂H₅• + HBr

C₂H₅• + Br₂ → C₂H₅Br + Br•

3. Termination

Two radicals combine to stop the reaction.

Important Points

• Requires UV light.
• Not very selective.
• Can form side products.
• Used mainly for industrial purposes.

3. Preparation of Ethyl Bromide from Ethene

Ethyl bromide can be prepared from ethene by adding hydrobromic acid.

Reaction Involved:

C₂H₄ + HBr → C₂H₅Br

Type of Reaction

This is an electrophilic addition reaction.

It is also called:

• Hydrohalogenation of alkene
• Addition reaction of ethene

Mechanism

• Ethene contains a double bond.
• The double bond breaks.
• H⁺ attaches first.
• Br⁻ attaches next.
• Ethyl bromide is formed.

Role of Markovnikov’s Rule

In unsymmetrical alkenes, hydrogen attaches to the carbon with more hydrogen atoms. But ethene is symmetrical, so only one product is formed.

Comparison of All Three Methods

Starting Compound	Type of Reaction	Mechanism	Condition Required	Best Use
Ethanol	Substitution	SN2	HBr / PBr₃	Laboratory
Ethane	Substitution	Free Radical	UV Light	Industrial
Ethene	Addition	Electrophilic	Normal temperature	Simple preparation

Difference Between Substitution and Addition Reaction

Substitution Reaction

• One atom replaces another.
• Example: Ethanol → Ethyl bromide
• Occurs in alkanes and alcohols.

Addition Reaction

• Atoms are added across double bond.
• Example: Ethene + HBr
• Occurs in alkenes.

Mechanism Summary (For Quick Revision)

SN2 Reaction

• One-step reaction.
• Backside attack.
• No intermediate.
• Seen in primary alcohol.

Free Radical Reaction

• Involves radicals.
• Needs UV light.
• Occurs in alkanes.

Electrophilic Addition

• Double bond breaks.
• Carbocation intermediate formed.
• Occurs in alkenes.

Which Method is Best?

• For laboratory preparation → Ethanol method
• For industrial production → Ethane method
• For simple textbook explanation → Ethene method

Important Board Exam Questions

1. How is ethyl bromide prepared from ethanol?
2. Write the mechanism of bromination of ethane.
3. Explain addition of HBr to ethene.
4. What type of reaction converts alcohol to alkyl halide?
5. Why does ethanol follow SN2 mechanism?

Uses of Ethyl Bromide

• Used in organic synthesis.
• Used to prepare Grignard reagent.
• Used in laboratory experiments.
• Intermediate in chemical industry.

Conclusion

Ethyl bromide (C₂H₅Br), also known as bromoethane, is an important organic compound. It can be prepared in three main ways:

• From ethanol by nucleophilic substitution (SN2 reaction)
• From ethane by free radical substitution
• From ethene by electrophilic addition reaction

Each method involves a different type of organic reaction such as substitution, addition, or free radical mechanism. Understanding these reactions helps students score better in board exams and competitive exams.

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 Ethyl Bromide from Ethyl Alcohol, Ethane & Ethene related FAQs

1. How is ethyl bromide prepared in the laboratory?

It is prepared by reacting ethanol with hydrobromic acid (HBr).

2. What type of reaction is bromination of ethane?

It is a free radical substitution reaction.

3. Is ethyl bromide formed by addition or substitution?

It can be formed by both, depending on starting compound.

4. Why does ethanol follow SN2 reaction?

Because it is a primary alcohol.

5. What is the formula of ethyl bromide?

The formula is C₂H₅Br.