Alkene Addition: Breaking Double Bonds
Add hydrogen or bromine to ethene
What this lesson covers
Why it matters
Imagine a double bond as a tightrope walker holding two ropes. When hydrogen or bromine approaches, one rope snaps, letting new partners join the dance.
The idea in plain words
Analyze the equation C₂H₄ + Br₂ → C₂H₄Br₂. Notice how the double bond becomes a single bond as each carbon gains a bromine atom.
Predict first
What happens to the C=C double bond when ethene reacts with bromine?
The pi bond breaks, converting the unsaturated alkene into a saturated compound with single bonds only.
- It breaks, allowing two bromine atoms to attach to the carbon atoms — correct
- It remains intact, and bromine sits on top of the molecule
- It turns into a triple bond to hold the bromine tighter
- The carbon atoms break apart completely
What you do
First, bubble ethene through orange bromine water and watch the colour fade to colourless. Then pass ethene and hydrogen over a heated nickel catalyst (about 300 C): the colourless ethene is converted to colourless ethane, so there is no colour change to see — confirm the product from the equation C2H4 + H2 -> C2H6.
Check yourself
Why does the orange color of bromine water disappear when added to ethene?
The reaction C₂H₄ + Br₂ → C₂H₄Br₂ consumes the colored Br₂, producing a colorless product. This is a standard test for unsaturation.
In the reaction C₂H₄ + H₂ → C₂H₆, what is the role of Nickel (Ni)?
Nickel is a catalyst. It lowers the activation energy for hydrogenation without being consumed in the process.
Which term best describes the change from ethene (C₂H₄) to ethane (C₂H₆)?
Adding hydrogen across a double bond is called hydrogenation, a specific type of addition reaction.
- Ethene absorbs the color like a sponge
- Bromine reacts with ethene to form colorless 1,2-dibromoethane — correct
- The bromine evaporates quickly
- It acts as a reactant and gets consumed
- It acts as a catalyst to speed up the addition of hydrogen — correct
- It provides heat to the reaction
- It prevents the reaction from occurring
- Oxidation
- Hydrogenation (Addition) — correct
- Substitution
- Dehydration