The test relies on an electrophilic addition reaction, where the electron-rich bond of the alkene attacks the bromine molecule.
As the bromine atoms add across the double bond to form a dibromoalkane, the diatomic molecules are consumed and removed from the solution.
Because the resulting organic product is colorless, the disappearance of the orange-yellow color serves as a direct indicator that a reaction with a multiple bond has occurred.
| Feature | Saturated (e.g., Alkanes) | Unsaturated (e.g., Alkenes) |
|---|---|---|
| Reactivity | Low (requires UV light for substitution) | High (spontaneous addition) |
| Bromine Water Result | Remains Orange/Yellow | Becomes Colorless |
| Reaction Type | No immediate reaction | Electrophilic Addition |
Precise Terminology: Always use the word 'decolourises' to describe the positive result. Avoid saying the solution 'turns clear' or 'disappears'.
State Symbols: Remember that bromine water is aqueous (), while the resulting dibromoalkane is typically a liquid or dissolved in the organic layer.
Equation Patterns: Be prepared to draw the mechanism for the addition of to a specific alkene, showing the formation of the cyclic bromonium ion or the carbocation intermediate.
Substitution vs. Addition: Students often forget that alkanes can react with bromine, but only under UV light via free-radical substitution. The unsaturation test is performed in the dark or under normal light to ensure only addition is observed.
Phase Separation: Since organic compounds are often immiscible with water, the decolourisation might only happen in the top organic layer. Shaking is essential to see the full effect.
