Section 1 of 5Features of Addition
In an addition reaction, a small molecule joins across the carbon double bond, so the product has no double bond left.
Alkene
- Alkene An unsaturated hydrocarbon with a carbon-carbon double bond and the general formula CₙH₂ₙ.
Addition
- Addition A reaction in which two substances join together to form one product, with no other products formed.
Saturated product
- Saturated product The product contains only single bonds, because the double bond has been converted to single bonds.
Decolourising
- Decolourising Alkenes turn orange bromine water colourless, which is a test for the presence of a C=C double bond.
Section 2 of 5Reagents and Products
Each reagent needs different conditions, and the product type changes with the reagent used.
Reagents and Products
Bromine
- Bromine Adds across the double bond at room temperature, giving a dibromoalkane with two bromine atoms.
Hydrogen
- Hydrogen Adds with a nickel catalyst at about 150 °C, producing an alkane from the alkene.
Steam
- Steam Adds with a phosphoric acid catalyst at high temperature and pressure, producing an alcohol.
| Reagent | Conditions | Product | Equation |
|---|---|---|---|
| Bromine water (Br₂) | Room temperature | 1,2-dibromoethane | C₂H₄(g) + Br₂(aq) → C₂H₄Br₂(l) |
| Hydrogen (H₂) | Nickel catalyst, about 150 °C | Ethane | C₂H₄(g) + H₂(g) → C₂H₆(g) |
| Steam (H₂O) | Phosphoric acid catalyst, about 300 °C, 60 to 70 atm | Ethanol | C₂H₄(g) + H₂O(g) → C₂H₅OH(g) |
Section 3 of 5Testing for a Double Bond
The bromine water test is a quick way to tell an alkene from an alkane in a laboratory.
1
Place a few drops of the hydrocarbon in a test tube and add orange bromine water.
2
Shake the tube gently and observe whether the orange colour disappears.
3
An alkene decolourises the bromine water, turning it colourless, because addition takes place.
4
An alkane does not react with bromine water at room temperature, so the orange colour remains.
Section 4 of 5Worked Example
Question
- Calculate the mass of 1,2-dibromoethane formed when 14 g of ethene reacts completely with bromine.
- (Relative atomic masses: H = 1, C = 12, Br = 80.)
- Relative formula mass of C₂H₄ = (2 × 12) + (4 × 1) = 28.
- Moles of ethene = 14 ÷ 28 = 0.5 mol.
- Mole ratio C₂H₄C₂H₄Br₂ is 1 : 1, so 0.5 mol of product forms.
- Relative formula mass of C₂H₄Br₂ = 24 + 4 + 160 = 188, so mass = 0.5 × 188 = 94 g.
Answer 94 g of 1,2-dibromoethane.
Section 5 of 5Exam Checklist & Tips
7.2.3 – Key Points for the Exam
- Addition reaction A reaction in which atoms or molecules join across a double bond, with no other product.
- Alkene An unsaturated hydrocarbon containing at least one carbon-carbon double bond.
- Saturated A compound containing only single bonds between carbon atoms.
- Hydration Addition of water across a double bond, here forming an alcohol with a phosphoric acid catalyst.
- Decolourise To remove colour, as alkenes do to orange bromine water.
Common Mistakes
- Describing addition as atoms being replaced;
- in addition, atoms join onto the double bond without any being removed.
- Stating that alkanes decolourise bromine water; only unsaturated alkenes do this.
- Forgetting the catalyst and conditions when a question asks for them for hydrogenation or hydration.
- Miscounting the hydrogen atoms in the product, so the formula of the product is written incorrectly.