Test for Unsaturation

The carbon-carbon double bond is a region of high electron density due to the presence of the $\pi$ electron cloud above and below the plane of the molecule.

This electron-rich area attracts electrophiles, which are electron-deficient species seeking to form new chemical bonds.

During the reaction, the $\pi$ bond breaks, and the atoms of the reagent are added across the two carbon atoms, resulting in a saturated product where each carbon has four single bonds.

In the case of non-polar reagents like $Br_2$, the high electron density of the alkene induces a temporary dipole in the reagent molecule, facilitating the attack.

Reagent: Bromine water ($Br_2(aq)$) is an aqueous solution of bromine that typically appears orange, yellow, or reddish-brown depending on its concentration.

Procedure: A few drops of bromine water are added to the unknown organic sample and shaken at room temperature.

Observation: If the compound is unsaturated, the orange color disappears rapidly, leaving a colorless solution. This is due to the formation of a colorless dibromoalkane.

Saturated Response: Alkanes do not react with bromine water in the dark; the solution remains orange unless exposed to UV light, which triggers a much slower substitution reaction.

Reagent: Cold, dilute, alkaline or acidified potassium manganate(VII) ($KMnO_4$), which is a deep purple solution.

Mechanism: This is an oxidation reaction where the $KMnO_4$ acts as an oxidizing agent, adding hydroxyl ($-OH$) groups across the double bond to form a diol (glycol).

Observation: The purple color of the permanganate disappears. In alkaline conditions, a brown precipitate of manganese dioxide ($MnO_2$) may also form.

Sensitivity: This test is highly sensitive but must be performed with cold, dilute reagents to avoid further oxidation and cleavage of the carbon chain.

While both tests detect unsaturation, the Bromine Water Test is generally preferred for simple hydrocarbons because it is less likely to react with other functional groups like aldehydes.

Baeyer's Test is useful for detecting double bonds in molecules where bromine might cause unwanted side reactions, but it can give false positives with other easily oxidizable groups.

Terminology Precision: Always use the word 'decolourised' rather than 'clear' or 'disappears.' A solution can be clear but still colored (like the initial $KMnO_4$).

State Both Colors: When describing the test, always state the starting color and the final color (e.g., 'The orange solution turns colorless').

Conditions Matter: For the $KMnO_4$ test, specify that it must be cold and dilute. Hot, concentrated $KMnO_4$ will break the carbon-carbon skeleton entirely, which is a different reaction.

Identify the Product: Be prepared to draw the product of the addition. For bromine, add one $Br$ to each carbon of the former double bond. For $KMnO_4$, add one $OH$ to each.

Substitution vs. Addition: Students often forget that alkanes can react with bromine, but only via free-radical substitution requiring UV light. In the context of an 'unsaturation test,' we assume room temperature and no UV light.

False Positives: Some compounds like phenols can decolourise bromine water through substitution reactions. However, these often produce a white precipitate, which distinguishes them from the addition reaction of alkenes.

Benzene Exception: Despite having formal double bonds in some representations, benzene does not decolourise bromine water under standard conditions due to its extra stability from delocalized electrons.