Addition Reaction
Summary: An addition reaction is a reaction in which two or more molecules combine to form a single product. At IGCSE, this applies to alkenes where atoms add across the C=C double bond. Only ONE product is formed. Examples include addition of bromine, hydrogen, and steam to alkenes. Tags: igcse chemistry definition organic-chem Created: 2026-07-14 Last Updated: 2026-07-23
An addition reaction is a reaction where two or more molecules combine to form a single, larger molecule. Only one product is formed — no atoms are eliminated.
At IGCSE, addition reactions are studied in the context of alkenes, where the C=C double bond opens up and atoms add across it.
Addition Reactions of Alkenes
Alkenes undergo addition because the C=C double bond can break open, allowing new atoms to attach to the carbon atoms.
1. Addition of Bromine
C₂H₄ + Br₂ → C₂H₄Br₂ (1,2-dibromoethane)
- Bromine water (orange/brown) is decolourised
- This is the test for unsaturation (distinguishes alkenes from alkanes)
- Alkanes do NOT decolourise bromine water (they only undergo substitution with UV light)
2. Addition of Hydrogen (Hydrogenation)
C₂H₄ + H₂ → C₂H₆ (ethane)
- Requires a nickel catalyst
- Used in the manufacture of margarine (hardening vegetable oils)
- Converts unsaturated oils to saturated fats
3. Addition of Steam (Hydration)
C₂H₄ + H₂O(g) → C₂H₅OH (ethanol)
- Requires: 300°C, 6000 kPa (60 atm), phosphoric acid (H₃PO₄) catalyst
- Industrial method for producing ethanol
Why Only One Product?
In an addition reaction, the C=C double bond breaks and the adding atoms simply join onto the carbons. No atoms are removed or replaced — all atoms from both reactants end up in the single product.
Addition vs Substitution
| Feature | Addition | Substitution |
|---|---|---|
| What happens | Atoms ADD across C=C | One atom REPLACES another |
| Substrate | Alkenes (unsaturated, C=C) | Alkanes (saturated, C—C only) |
| Products | ONE product only | TWO products (organic + HX) |
| Conditions | Varies by reaction | UV light required |
| Test | Decolourises bromine water | No reaction without UV |
Key Facts
- Addition reaction = two molecules → one product only
- Alkenes undergo addition at the C=C double bond
- Addition of Br₂: decolourises bromine water → test for unsaturation
- Addition of H₂: requires Ni catalyst → hydrogenation
- Addition of H₂O (steam): requires H₃PO₄, 300°C, 60 atm → ethanol
- Only ONE product formed — all atoms are retained
See Also
This is a key term definition page — part of the IGCSE Chemistry (0620/0971) keyword glossary.