3.4.4 Test for Unsaturation

Test for Unsaturation with Bromine

What is Unsaturation?

Unsaturated organic compounds contain carbon-carbon double bonds ( $C=C$ ) or triple bonds ( $C≡C$ ), indicating the presence of fewer hydrogen atoms compared to saturated hydrocarbons (like alkanes). These compounds are typically alkenes or alkynes. Testing for unsaturation helps identify whether an organic molecule has these types of carbon-carbon multiple bonds.

Bromine Test for Unsaturation

Bromine is a reactive halogen that can be used to identify unsaturation in organic compounds. The typical test involves bromine water, which is a solution of bromine in water, appearing as an orange-brown liquid.

Procedure for the Bromine Test

Place a small sample of the organic compound to be tested in a test tube.
Add a few drops of bromine water to the sample.
Shake the test tube gently.

Observations

Unsaturated compound (e.g., an alkene): The bromine water will decolourise, changing from orange-brown to colourless. This indicates the presence of a $C=C$ double bond, as bromine reacts with it.
Saturated compound (e.g., an alkane): The bromine water remains orange-brown, showing no reaction because saturated compounds do not have carbon-carbon multiple bonds.

Mechanism of the Bromine Addition Reaction

When an unsaturated compound, such as an alkene, reacts with bromine, it undergoes an electrophilic addition reaction.

Formation of the Bromonium Ion:

The $π$ electrons of the $C=C$ double bond attack the bromine molecule ( $Br₂$ ), which is polarised due to the proximity of the electron-rich alkene.
This results in the formation of a bromonium ion, a three-membered cyclic structure with a positively charged bromine atom.

Nucleophilic Attack:

The remaining bromide ion ( $Br⁻$ ) acts as a nucleophile and attacks the more positively charged carbon in the bromonium ion.
The cyclic bromonium ring opens, leading to the formation of a dibromoalkane, with bromine atoms attached to the two carbons that were originally part of the double bond.

Reaction Example:

infoNote

Example: Testing for Unsaturation with Cyclohexene

Cyclohexene is an unsaturated hydrocarbon containing a carbon-carbon double bond ( $C=C$ ). In contrast, cyclohexane is a similar cyclic compound but fully saturated (only single $C-C$ bonds).

Procedure:

Test Tube A: Add a few drops of cyclohexene to a test tube.
Test Tube B: Add a few drops of cyclohexane to a second test tube (this serves as a comparison).
To both test tubes, add a few drops of bromine water, which appears as an orange-brown solution.
Gently shake each test tube.

Observations:

Test Tube A (Cyclohexene):

The orange-brown bromine water decolourises, turning colourless.
This indicates that cyclohexene contains a $C=C$ double bond, which reacts with bromine. Test Tube B (Cyclohexane):
The orange-brown bromine water remains unchanged.
This shows that cyclohexane does not have a $C=C$ double bond and is therefore a saturated hydrocarbon.

Mechanism for Cyclohexene

The $π$ electrons in the $C=C$ double bond of cyclohexene interact with the bromine molecule ( $Br₂$ ), polarising it.
The $π$ electrons attack one bromine atom, forming a bromonium ion, with the other bromine becoming a $Br⁻$ ion.
The $Br⁻$ ion attacks one of the carbons in the bromonium ion, opening the three-membered ring to form 1,2-dibromocyclohexane.

Reaction Equation:

Cyclohexene + Bromine $→$ 1,2-Dibromocyclohexane

\text{C}6\text{H}{10} + \text{Br}_2 \rightarrow \text{C}6\text{H}{10}\text{Br}_2

Explanation

Cyclohexene reacts because it is unsaturated, confirming the presence of a double bond through decolourisation.
Cyclohexane, being saturated, shows no reaction with bromine water, helping to differentiate between alkenes (or alkynes) and alkanes.