When an alkene reacts with bromine water, the orange colour of the bromine water disappears, turning the solution colourless. This distinct change serves as a fundamental test for the presence of carbon-carbon double bonds in organic chemistry.
The Bromine Water Test: A Visual Indicator
Bromine water is an orange solution formed by dissolving bromine (Br₂) in water. This solution is widely used as a chemical reagent to detect the presence of unsaturation, specifically carbon-carbon double or triple bonds, in organic compounds like alkenes and alkynes. The decolourisation of bromine water is a hallmark reaction indicating that an addition reaction has taken place.
How the Reaction Occurs
Alkenes are characterized by their carbon-carbon double bond, which is a region of high electron density. This makes them susceptible to electrophilic addition reactions. When bromine water is added to an alkene, the bromine molecule (Br₂) acts as an electrophile, attacking the double bond.
The double bond breaks, and a bromine atom adds to each carbon atom that was part of the double bond, forming a new, colourless compound known as a dibromoalkane.
For example, with ethene (a simple alkene):
C₂H₄ (ethene) + Br₂ (bromine in water) → C₂H₄Br₂ (1,2-dibromoethane)
This reaction consumes the coloured bromine molecules, leading to the observed colour change.
Why Alkenes Decolourise Bromine Water
The key to the colour change lies in the consumption of the bromine (Br₂) itself. Bromine water is an orange solution of bromine. When it is shaken with an alkene, the bromine molecules are chemically incorporated into the alkene structure, forming a new product that is colourless. Since the coloured bromine is removed from the solution, the solution appears colourless. This makes the bromine water test an excellent visual indicator for the presence of alkenes.
Contrasting with Alkanes
It is important to note that alkenes can decolourise bromine water, but alkanes cannot. Alkanes are saturated hydrocarbons, meaning they contain only carbon-carbon single bonds. These single bonds are much more stable and less reactive than double bonds. Therefore, alkanes do not readily undergo addition reactions with bromine under normal conditions.
If bromine water is added to an alkane, the orange colour persists, indicating the absence of carbon-carbon double or triple bonds. This difference in reactivity forms the basis for distinguishing between saturated and unsaturated hydrocarbons.
Here's a quick comparison:
| Feature | Alkene (e.g., Ethene) | Alkane (e.g., Ethane) |
|---|---|---|
| Bond Type | Contains C=C double bond | Contains only C-C single bonds |
| Reactivity | Highly reactive (due to double bond) | Less reactive (due to single bonds) |
| Bromine Water | Decolourises (orange to colourless) | No reaction (remains orange) |
| Reaction Type | Electrophilic Addition | No reaction under standard conditions |
Practical Applications and Safety
The bromine water test is a fundamental qualitative test performed in organic chemistry laboratories to:
- Identify unsaturated compounds: Quickly determine if a sample contains double or triple bonds.
- Distinguish between alkenes/alkynes and alkanes: As detailed above, the colour change is a clear differentiator.
When performing the test:
- Add a few drops of bromine water to a small sample of the unknown organic compound.
- Shake the mixture gently.
- Observe the colour change. If the orange colour disappears instantly, an alkene (or alkyne) is present. If it remains orange, it's likely an alkane or another saturated compound.
Safety Note: Bromine water is corrosive and an irritant. Always handle it in a well-ventilated area, preferably under a fume hood, and wear appropriate personal protective equipment, including gloves and eye protection.
