Experiment 36

Objective

To identify the presence of bromine in a carbonyl compound using the Lassaigne’s test.

Introduction

The Lassaigne’s test is a qualitative analysis used to detect halogens in organic compounds. In this test, bromine in the sample reacts with sodium to form sodium bromide, which then reacts with silver nitrate (AgNO₃) to produce a slightly yellow precipitate of silver bromide (AgBr). This precipitate is only slightly soluble in ammonium hydroxide, confirming the presence of bromine.

Reaction:

NaBr + AgNO3 ​→ AgBr ↓ + NaNO3​

Materials

Chemicals

·       Sodium metal

·       Ammonium hydroxide (NH₄OH)

·       Sample containing bromine

·       Silver nitrate (AgNO₃)

·       Nitric acid (HNO₃)

Apparatus

·       Fusion tube

·       Test tube

·       Mortar

·       Bunsen burner

·       Test tube holder

·       Filter paper

Reagents

·       mg of sample with a piece of sodium metal in a fusion tube

·       250 ml of distilled water in a mortar

Procedure

1.    Add the sample and a piece of sodium metal to a fusion tube.

2.    Heat the fusion tube in the Bunsen burner until the reaction completes.

3.    Break the fusion tube in the mortar and add distilled water to dissolve the reaction product.

4.    Filter the solution using filter paper.

5.    Take the filtrate in a test tube.

6.    Add 1–2 drops of HNO₃ and heat gently.

7.    Add AgNO₃ to the solution; a slightly yellow precipitate of AgBr should form if bromine is present.

8.    Confirm by adding ammonium hydroxide to test the solubility; AgBr is only slightly soluble.

Observation

A slightly yellow precipitate forms upon the addition of AgNO₃, which is only partially soluble in ammonium hydroxide, indicating the presence of bromine.

Questions

1.    What is the Lassaigne’s test?

2.    How is the presence of bromine identified?

3.    Why is the Lassaigne’s test necessary?

4.    What color is observed in the bromine test?

5.    What happens when AgNO₃ is added to a bromide solution?