What is Bromine Water Test? Definition, Preparation, and Features

Bromine Water Test: A Key Method for Detecting Unsaturation in Compounds

Introduction

The bromine water test is a fundamental qualitative analysis technique used to identify the presence of unsaturated bonds (carbon-carbon double or triple bonds) in organic compounds. This test is widely applied in organic chemistry to differentiate between alkanes, alkenes, and other functional groups that exhibit unsaturation.

What Is Bromine Water?

Bromine water is a reddish-brown solution composed of bromine (Br₂) dissolved in water. It is known for its strong oxidizing properties and is used to determine the presence of unsaturated compounds through color change reactions.

Properties of Bromine Water

• Molecular Formula: Br₂
• Molecular Mass: 159.81 g/mol
• Density: 1.307 g/mL
• Color: Reddish-brown
• Reaction Mechanism: Halogenation or electrophilic addition

The characteristic color of bromine water fades or disappears when it reacts with unsaturated compounds, indicating the presence of double or triple bonds.

Preparation of Bromine Water

Traditionally, bromine water is prepared by dissolving elemental bromine in water. However, due to safety concerns, an alternative laboratory method involves the reaction of sodium bromide (NaBr) with hydrochloric acid (HCl) and bleach (sodium hypochlorite, NaClO):

2NaBr + NaClO + 2HCl → Br₂ + 3NaCl + H₂O

Features of the Bromine Water Test

The bromine water test operates on the following key principles:

1. Unsaturation Detection

Bromine readily reacts with compounds containing carbon-carbon double (C=C) or triple (C≡C) bonds, leading to the disappearance of the reddish-brown color. This change serves as an indication of the presence of unsaturation.

2. Electrophilic Addition

Bromine acts as an electrophile and reacts with alkenes or alkynes in an electrophilic addition reaction, forming dibromo-compounds. The reaction mechanism involves the formation of a bromonium ion intermediate.

3. Selectivity

• Alkanes do not react with bromine water, retaining the reddish-brown color.
• Alkenes and alkynes undergo addition reactions, resulting in decolorization.
• Aromatic compounds (benzene and derivatives) do not react due to resonance stability.

4. Qualitative Nature

The test provides a simple yes/no result to determine whether a compound is saturated or unsaturated but does not quantify the number of double or triple bonds.

Bromine Water Test for Various Functional Groups

1. Alkanes (No Reaction)

Alkanes do not undergo an electrophilic addition reaction with bromine water, meaning the solution retains its reddish-brown color.

CH₄ (Methane) + Br₂ → No reaction

C₂H6 (Ethane) + Br₂ → No reaction

C₃H₈( Propane ) + Br₂ → No reaction

2. Alkenes (Reaction: Decolorization)

Alkenes react with bromine water via an addition reaction, leading to the formation of dibromo-compounds and decolorization of the solution.

Br₂+ C₂H₄ → Br – CH₂ – CH₂  – Br

3. Alkynes (Reaction: Decolorization)

Alkynes, like alkenes, undergo electrophilic addition with bromine water, leading to a similar color change.

4. Phenols (Substitution Reaction)

Phenols react with bromine water in a substitution reaction, forming brominated phenols and leading to the formation of a white precipitate.

Chemical Reaction:

C₆H₅OH+3Br₂(aq) →  C₆H₂Br₃OH+3HBr

Structural Formula:

• Reactant:Phenol (C₆H₅OH)
  • A benzene ring with a hydroxyl (-OH) group.
• Reagent: Bromine water (Br₂​).
• Product:2,4,6-Tribromophenol (C₆H₂Br₃OH)
  • A benzene ring with an -OH group and three bromine (Br) atoms at positions 2, 4, and 6.
• Byproduct: Hydrobromic acid (HBr).

Observation:

• The reddish-brown color of bromine water disappears, indicating a positive test for phenol.
• A white precipitate of 2,4,6-tribromophenol forms.

5. Aniline (Reaction: White Precipitate Formation)

Aniline reacts with bromine water to form tribromoaniline, resulting in a white precipitate.

Reaction Details:

C₆H₅NH+3Br₂→ C₆H₂Br₃NH+3HBr

Explanation:

• Reactant:Aniline (C₆H₅NH₂​)
  • A benzene ring with an amino (-NH₂) group.
• Reagent: Bromine (Br₂​).
• Product:2,4,6-Tribromoaniline (C₆H₅NH₂​)
  • A benzene ring with an -NH₂ group and three bromine (Br) atoms at positions 2, 4, and 6.
• Byproduct: Hydrobromic acid (HBr).

Observation:

• Decolorization of bromine water (reddish-brown to colorless), indicating a positive test for aniline.
• Formation of a white precipitate of 2,4,6-Tribromoaniline.

6. Aldehydes (Oxidation Reaction)

Aldehydes undergo oxidation with bromine water, forming carboxylic acids. The disappearance of the bromine color signifies the completion of the reaction.

CH₃​CHO + Br ​+ H₂​O → CH₃​COOH + 2HBr

7. Ketones (Alpha-Substitution Reaction)

Ketones undergo an alpha-bromination reaction, forming brominated products.

8. Enols (Reaction: Decolorization)

Enols react with bromine water, forming brominated ketones.

9. Glucose and Fructose (Differentiation Test)

Glucose, containing an aldehyde functional group, reacts with bromine water and decolorizes. Fructose does not react as it lacks an aldehyde group.

Reaction:

C₆H₁₂O₆+Br₂+H₂O→C₆H₁₂O₇+2HBr

Explanation:

• Glucose (C₆H₁₂O₆ ​) has an aldehyde (-CHO) group, which gets oxidized to a carboxylic acid (-COOH) group.
• Bromine water (Br₂/H₂O) acts as an oxidizing agent.
• The aldehyde is converted into gluconic acid (C₆H₁₂O₇​).
• Hydrobromic acid (HBr) is produced as a byproduct.
• The decolorization of bromine water confirms the presence of glucose.

10. Benzene (No Reaction)

Despite being unsaturated, benzene does not react with bromine water because of its stable aromatic nature.

C₆H₆+Br₂→No Reaction

Bromine Water Test for Lipids

The bromine water test can also assess the degree of unsaturation in vegetable oils and fats. Unsaturated fats (containing double bonds) react with bromine water, causing decolorization, whereas saturated fats do not.

Summary Table

Conclusion

The bromine water test is an essential tool in organic chemistry for detecting unsaturation and distinguishing between different functional groups. Its qualitative nature makes it a quick and effective method for laboratory analysis.

Frequently Asked Questions (FAQs)

1. What is the purpose of the bromine water test?

The bromine water test is used to detect unsaturation in organic compounds by observing a color change from reddish-brown to colorless.

2. Why does benzene not react with bromine water?

Benzene does not react because of its aromatic stability, which resists simple addition reactions.

3. How is bromine water prepared safely in the laboratory?

Bromine water can be prepared using sodium bromide, bleach, and hydrochloric acid to generate bromine in situ, avoiding direct handling of pure bromine.

4. What does it mean if a compound does not react with bromine water?

If a compound does not react, it indicates that the compound is saturated (contains only single bonds) or has aromatic stability.

5. Can bromine water differentiate between glucose and fructose? Yes, glucose reacts due to its aldehyde group, while fructose does not, making it a distinguishing test for these sugars.

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