🔬 Organic Chemistry – Alkenes
🧪 Test for Saturated vs Unsaturated Hydrocarbons
When we mix a hydrocarbon with aqueous bromine (a bright orange‑brown liquid), we can see whether the hydrocarbon is saturated (alkane) or unsaturated (alkene). Think of bromine as a colorful detective that changes its colour when it finds a “bumpy” bond!
⚙️ How the Test Works
- Alkanes have only single bonds: \$CnH{2n+2}\$. They are smooth and don’t react with bromine.
- Alkenes contain at least one double bond: \$CnH{2n}\$. The double bond is like a traffic intersection that bromine can “drive through” and add itself.
- When bromine reacts with an alkene, it adds across the double bond, forming a dibromide and turning the solution colorless.
- If the solution stays orange‑brown, the hydrocarbon is an alkane.
📝 Step‑by‑Step Procedure
- Take a clean test tube and add a few drops of the hydrocarbon sample.
- Add a small amount of aqueous bromine (about 2–3 drops).
- Observe the colour change:
- 💡 Colorless → Unsaturated (alkene)
- ⚫ Orange‑brown remains → Saturated (alkane)
- Record your observation in the table below.
📊 Observation Table
| Sample | Initial Colour | After Bromine | Result |
|---|
| Ethane (\$C2H6\$) | Clear | Orange‑brown | Saturated (alkane) |
| Ethene (\$C2H4\$) | Clear | Colorless | Unsaturated (alkene) |
🔍 Why It Works – A Simple Analogy
Imagine the hydrocarbon as a road. An alkane is a straight, smooth road with no intersections – it’s hard for bromine to “enter” it. An alkene has a traffic intersection (the double bond). Bromine is like a delivery truck that can only park at intersections. When it parks, it changes colour, showing that the intersection (double bond) was there.
⚠️ Safety Tips
- Wear safety goggles and gloves.
- Work in a well‑ventilated area.
- Dispose of bromine waste according to your school’s guidelines.
By following this simple test, you can quickly tell if a hydrocarbon is an alkane or an alkene – a handy trick for any budding chemist! 🚀