9.4 Distinguishing Saturated and Unsaturated Hydrocarbons

Introduction

In the previous sections, we learned about saturated hydrocarbons (alkanes) and unsaturated hydrocarbons (alkenes and alkynes). But how can we tell them apart in the laboratory?

There are two simple chemical tests that can distinguish saturated hydrocarbons from unsaturated hydrocarbons:

• Bromine water test
• Baeyer's test (KMnO₄ test)

This section explains both tests, their procedures, observations, and the chemical equations involved.

Definition

Distinguishing Saturated and Unsaturated Hydrocarbons: The process of using chemical tests to identify whether a hydrocarbon is saturated (alkane) or unsaturated (alkene or alkyne).

In simple words: These are tests that tell us whether a hydrocarbon has only single bonds or has double/triple bonds.

Main Idea

To distinguish between saturated and unsaturated hydrocarbons, you can perform the bromine water test. This test involves adding bromine water (Br₂ in water, which is an orange-brown solution) to the hydrocarbon and observing the colour change.

Key observations:

• Saturated hydrocarbons (alkanes) do not react with bromine water. The solution remains orange-brown.
• Unsaturated hydrocarbons (alkenes and alkynes) react with bromine water. The solution decolourises (turns colourless) because the bromine reacts with the double or triple bonds.

A second test is the Baeyer's test using KMnO₄:

• Saturated hydrocarbons (alkanes) do not react with KMnO₄. The solution remains purple.
• Unsaturated hydrocarbons (alkenes) react with KMnO₄, causing the solution to become colourless as the permanganate ion is reduced to manganese dioxide (MnO₂), which precipitates as a brown solid.

Core Concept

Key point: Both tests work because unsaturated hydrocarbons have double or triple bonds that react with the reagents, while saturated hydrocarbons do not.

Detailed Explanation

Bromine Water Test

What it is: The bromine water test involves adding bromine water (Br₂ in water) to the hydrocarbon and observing the colour change.

Procedure (from the source):

• Add bromine water to the hydrocarbon sample.
• Observe the colour change.

Observations (from the source):

• Saturated hydrocarbons (alkanes) will not react with bromine water. The solution will remain orange-brown.
• Unsaturated hydrocarbons (alkenes and alkynes) will react with bromine water. The solution will decolourize (turn colourless) because the bromine reacts with the double or triple bonds.

Symbol Equations (from the source):

For an alkene (unsaturated hydrocarbon): Ethene reacts with bromine to form dibromoethane.

CH₂ = CH₂ + Br₂ → CH₂Br - CH₂Br

Ethene (colourless) + Bromine (reddish-brown) → 1,2-Dibromoethane (colourless)

For an alkyne (unsaturated hydrocarbon): Ethyne reacts with bromine to form tetrabromoethane.

H - C ≡ C - H + 2Br₂ → CHBr₂ - CHBr₂

Ethyne (colourless) + Bromine (reddish-brown) → 1,1,2,2-Tetrabromoethane (colourless)

For an alkane (saturated hydrocarbon):

C₂H₆ + Br₂ → No reaction

Ethane does not react with bromine water, so the colour remains unchanged.

Why this test works: This test is effective because the presence of a double or triple bond in unsaturated hydrocarbons allows them to react with bromine, breaking the bond and forming a dibromo compound, which is colourless.

Key point: Bromine water test: Unsaturated hydrocarbons decolourize bromine water; saturated hydrocarbons do not.

Baeyer's Test (KMnO₄ Test)

What it is: The Baeyer's test involves adding a few drops of cold, dilute alkaline, aqueous KMnO₄ solution to the hydrocarbon sample and observing the colour change.

Procedure (from the source):

• Add a few drops of cold, dilute alkaline, aqueous KMnO₄ solution to the hydrocarbon sample.
• Observe the colour change.

Observations (from the source):

• Saturated hydrocarbons (alkanes) do not react with KMnO₄, and the solution remains purple.
• Unsaturated hydrocarbons (alkenes) react with KMnO₄, causing the solution to become colourless as the permanganate ion is reduced to manganese dioxide (MnO₂), which precipitates as a brown solid.

Symbol Equations (from the source):

For an alkene (unsaturated hydrocarbon):

3C₂H₄ + 2KMnO₄ + 4H₂O → 3C₂H₄(OH)₂ + 2MnO₂ + 2KOH

Ethene reacts with potassium permanganate, resulting in the formation of ethane-1,2-diol, manganese dioxide, and potassium hydroxide. The solution turns brown due to MnO₂ and loses its purple colour.

For an alkyne:

Alkynes do not react with cold 1% alkaline aqueous solution of KMnO₄ (Baeyer's agent).

Key point: Baeyer's test: Alkenes decolourize KMnO₄ solution; alkanes do not. Alkynes do not react with cold 1% alkaline KMnO₄.

Activity 9.1: Distinguishing Saturated and Unsaturated Hydrocarbons

Objective: To distinguish between saturated (butter representing saturated hydrocarbon) and unsaturated (vegetable oil representing an unsaturated hydrocarbon) hydrocarbons based on the reaction with bromine.

Materials Required:

• Vegetable oil (e.g., olive oil or sunflower oil)
• Butter or lard
• Bromine water (a dilute solution of bromine in water. CAUTION: Bromine is toxic and corrosive; handle with gloves and under supervision)
• Two small glass containers or test tubes
• Droppers or pipettes
• Safety goggles and gloves
• Lab apron
• White paper or white tiles (to better observe colour change)
• Tongs or test tube holders
• Fume hood or well-ventilated area for safety

Safety Precautions:

• Ensure all students wear safety goggles, gloves, and lab aprons.
• Conduct the experiment in a well-ventilated area or fume hood.
• Explain the hazards associated with bromine water and the importance of not directly inhaling or contacting the solution.

Procedure (from the source):

• Pour a small amount (about 5 ml) of vegetable oil into one glass container or test tube.
• Place a small piece of butter in another glass container or test tube.
• Label each container appropriately (e.g., "Vegetable Oil" and "Butter").
• Using a dropper, add a few drops of bromine water to each container.
• Observe and note the initial colour of the bromine water (orange-brown).

Observations (from the source):

Gently swirl each container to mix the contents:

• Vegetable Oil (Unsaturated Hydrocarbon): The bromine water will react with the double bonds in the unsaturated hydrocarbons, causing the orange-brown colour to disappear or lighten. This reaction occurs because the bromine adds across the double bonds, forming a colourless dibromo compound.
• Butter (Saturated Hydrocarbon): The bromine water will remain orange-brown in colour, indicating no reaction. Saturated hydrocarbons do not have double bonds, so they do not react with bromine under these conditions.

Discussion (from the source):

• Ask the students to discuss their observations:
• Why did the colour of bromine water change with vegetable oil but not with butter?
• What does this indicate about the types of bonds present in vegetable oil and butter?

Key point: This activity demonstrates the difference between saturated and unsaturated hydrocarbons using the bromine water test.

Summary Table

Intermediate Level

Bromine Water Test:

• Reagent: Bromine water (orange-brown).
• Saturated: No reaction; remains orange-brown.
• Unsaturated: Decolourises; forms dibromo (alkene) or tetrabromo (alkyne) compound.

Equations:

• Ethene: CH₂ = CH₂ + Br₂ → CH₂Br - CH₂Br
• Ethyne: H - C ≡ C - H + 2Br₂ → CHBr₂ - CHBr₂
• Ethane: C₂H₆ + Br₂ → No reaction

Baeyer's Test (KMnO₄ Test):

• Reagent: Dilute alkaline KMnO₄ (purple).
• Saturated: No reaction; remains purple.
• Unsaturated (alkene): Decolourises; brown precipitate of MnO₂.
• Unsaturated (alkyne): Does not react with cold 1% alkaline KMnO₄.

Equation:

- Ethene: 3C₂H₄ + 2KMnO₄ + 4H₂O → 3C₂H₄(OH)₂ + 2MnO₂ + 2KOH

Key point: Both tests rely on the reactivity of the double or triple bond in unsaturated hydrocarbons.

Solved Example

Example: Distinguish Between Hexane and Hexene

Question: In a laboratory, you need to distinguish between a sample of hexane and hexene. Describe the test to distinguish between them.

Solution:

Test: Bromine Water Test

• Add a few drops of bromine water to each sample.
• Observe the colour change.

Observations:

• Hexane (saturated hydrocarbon): The bromine water remains orange-brown. No reaction occurs.
• Hexene (unsaturated hydrocarbon): The bromine water decolourises (turns colourless). The bromine reacts with the double bond in hexene.

Conclusion: Hexane is saturated (no double bond); hexene is unsaturated (has a double bond).

Final Answer: The bromine water test can distinguish between hexane and hexene. Hexane does not decolourize bromine water; hexene does.

Important Points

• - Bromine Water Test:
• - Reagent: Bromine water (orange-brown).
• - Saturated: No reaction; remains orange-brown.
• - Unsaturated: Decolourises (turns colourless).
• - Equations:
• - Ethene: CH₂ = CH₂ + Br₂ → CH₂Br - CH₂Br
• - Ethyne: H - C ≡ C - H + 2Br₂ → CHBr₂ - CHBr₂
• - Ethane: C₂H₆ + Br₂ → No reaction
• - Baeyer's Test (KMnO₄ Test):
• - Reagent: Dilute alkaline KMnO₄ (purple).
• - Saturated: No reaction; remains purple.
• - Unsaturated (alkene): Decolourises; brown precipitate of MnO₂.
• - Unsaturated (alkyne): Does not react with cold 1% alkaline KMnO₄.
• - Equation:
• - Ethene: 3C₂H₄ + 2KMnO₄ + 4H₂O → 3C₂H₄(OH)₂ + 2MnO₂ + 2KOH
• - Activity 9.1: Vegetable oil (unsaturated) decolourizes bromine water; butter (saturated) does not.
• - Safety: Bromine water is toxic and corrosive; handle with care.

Remember

• - Bromine water test: Unsaturated decolourizes; saturated does not.
• - Baeyer's test: Alkenes decolourize KMnO₄; alkanes do not.
• - Alkynes do not react with cold 1% alkaline KMnO₄.
• - Ethene + Br₂ → 1,2-Dibromoethane.
• - Ethyne + 2Br₂ → 1,1,2,2-Tetrabromoethane.
• - Ethane + Br₂ → No reaction.
• - Ethene + KMnO₄ + H₂O → Ethylene glycol + MnO₂ + KOH.
• - Vegetable oil (unsaturated) decolourizes bromine water.
• - Butter (saturated) does not decolourize bromine water.
• - Bromine water is toxic and corrosive.

Common Mistakes

Quick Revision

Distinguishing Saturated and Unsaturated Hydrocarbons:

To distinguish between saturated and unsaturated hydrocarbons, you can perform the bromine water test. This test involves adding bromine water (Br₂ in water, which is an orange-brown solution) to the hydrocarbon and observing the colour change.

• Add bromine water to the hydrocarbon sample.
• Observe the colour change.

Observations: Saturated hydrocarbons (alkanes) will not react with bromine water. The solution will remain orange-brown. Unsaturated hydrocarbons (alkenes and alkynes) will react with bromine water. The solution will decolourize (turn colourless) because the bromine reacts with the double or triple bonds.

Symbol Equations:

For an alkene (unsaturated hydrocarbon) Ethene reacts with bromine to form dibromoethane.

• CH₂ = CH₂ + Br₂ → CH₂Br - CH₂Br

For an alkyne (unsaturated hydrocarbon) Ethyne reacts with bromine to form tetrabromoethane.

• H - C ≡ C - H + 2Br₂ → CHBr₂ - CHBr₂

For an alkane (saturated hydrocarbon):

• C₂H₆ + Br₂ → No reaction

Baeyer's Test (KMnO₄ Test):

Add a few drops of cold, dilute alkaline, aqueous KMnO₄ solution to the hydrocarbon sample.

Observations: Saturated hydrocarbons (alkanes) do not react with KMnO₄, and the solution remains purple. Unsaturated hydrocarbons (alkenes) react with KMnO₄, causing the solution to become colourless as the permanganate ion is reduced to manganese dioxide (MnO₂), which precipitates as a brown solid.

Symbol Equations:

For an alkene (unsaturated hydrocarbon):

• 3C₂H₄ + 2KMnO₄ + 4H₂O → 3C₂H₄(OH)₂ + 2MnO₂ + 2KOH

Alkynes do not react with cold 1% alkaline aqueous solution of KMnO₄ (Baeyer's agent).

Activity 9.1: To distinguish between saturated (butter representing saturated hydrocarbon) and unsaturated (vegetable oil representing an unsaturated hydrocarbon) hydrocarbons based on the reaction with bromine.

60-Second Revision

• Bromine water test: Unsaturated decolourizes; saturated does not.
• Baeyer's test: Alkenes decolourize KMnO₄; alkanes do not.
• Alkynes: Do not react with cold 1% alkaline KMnO₄.
• Ethene + Br₂ → 1,2-Dibromoethane.
• Ethyne + 2Br₂ → 1,1,2,2-Tetrabromoethane.
• Ethane + Br₂ → No reaction.
• Ethene + KMnO₄ + H₂O → Ethylene glycol + MnO₂ + KOH.
• Vegetable oil (unsaturated): Decolourizes bromine water.
• Butter (saturated): Does not decolourize bromine water.
• Bromine water is toxic and corrosive.
• ⚠️ Warning: bromine water is toxic and corrosive — handle with gloves and under supervision.