Experiment 87

Objective

To prepare ethyl bromide in the laboratory using ethyl alcohol, concentrated sulfuric acid, and potassium bromide.

Introduction

Ethyl bromide (bromoethane) can be synthesized by reacting ethanol with concentrated sulfuric acid and potassium bromide. The reaction involves two key steps: the formation of ethene from ethanol, followed by an electrophilic addition reaction with hydrogen bromide (HBr), generated in situ from KBr and sulfuric acid.

Reaction Steps

1.    Ethanol to Ethene

CH3 ​– CH2 ​– OH + H2​SO4 → CH3 – CH3​-HSO4​ + H2​O

CH3 – CH2 – HSO4 → CH2 = CH2 + H2SO4

2.    Formation of HBr

KBr + H2SO4​ → HBr + KHSO4

3.    Formation of Ethyl Bromide

CH2 ​= CH2 ​+ HBr → CH3 ​– CH2 -Br

Materials

Chemicals

·       Ethanol (100% pure)

·       Concentrated sulfuric acid (H₂SO₄)

·       Potassium bromide (KBr)

Apparatus

·       Flask

·       Bunsen burner

·       Stand

·       Beaker (for ethyl bromide collection)

·       Thermometer

Reagents

·       Use 10 mL of 100% pure ethanol, 2-3 mL of concentrated sulfuric acid, and 1.657 g of potassium bromide.

Procedure

1.    Preparation of Reaction Mixture

o   In a flask, mix ethyl alcohol, concentrated sulfuric acid, and potassium bromide.

2.    Setting Up Apparatus

o   Attach a thermometer and a glass tube for collecting ethyl bromide at the mouth of the flask.

3.    Heating the Mixture

o   Heat the mixture to 165°C, ensuring the temperature does not exceed this limit.

4.    Collection of Ethyl Bromide

o   Collect the ethyl bromide through the glass tube into a beaker, which should be placed in an ice bath for condensation.

Observation

·       The final reaction mixture contains ethyl bromide, potassium bisulfate, and sulfuric acid.

Questions

1.    What would happen if a large amount of concentrated H₂SO₄ were used?

2.    How is ethene produced from H₂SO₄ and ethanol?

3.    Why is KBr included in the mixture?