4.1.3Organic Chemistry
Classifying Organic Reactions — Edexcel International GCSE Science Double Award 4SD0 - Chemistry Revision Notes & Topical Questions
5 min read 12 parts Lesson 4 of 24
Part 1 of 5Three types of reaction
Substitution
- Substitution One atom or group in a molecule is replaced by a different atom or group.
- Two products are formed.
Addition
- Addition Two or more molecules join to make a single larger molecule, with no other product.
- Unsaturated compounds with a C=C bond do this.
Combustion
- Combustion Burning: an organic compound reacts with oxygen to release energy and form oxides, usually carbon dioxide or carbon monoxide, and water.
Part 2 of 5Substitution example
1Reactants
- Reactants Methane, CH4, and bromine, Br2, are mixed.
2Ultraviolet light
- Ultraviolet light UV radiation supplies the energy needed to start the reaction.
3Atom swapped
- Atom swapped A hydrogen atom in methane is replaced by a bromine atom.
4Two products
- Two products CH4 + Br2 → CH3Br + HBr, giving bromomethane and hydrogen bromide.
Part 3 of 5Addition example
1
Ethene, C2H4, is an unsaturated molecule containing a C=C double bond.
2
A bromine molecule approaches and the double bond opens up.
3
One bromine atom attaches to each carbon of the former double bond.
4
The only product is dibromoethane: C2H4 + Br2 → C2H4Br2, a saturated compound.
Part 4 of 5Complete and incomplete combustion
| Type | Oxygen supply | Products | Example equation |
|---|---|---|---|
| Complete combustion | Plenty of oxygen or air | Carbon dioxide and water | CH4 + 2O2 → CO2 + 2H2O |
| Complete combustion | Plenty of oxygen or air | Carbon dioxide and water | C3H8 + 5O2 → 3CO2 + 4H2O |
| Incomplete combustion | Limited oxygen or air | Carbon monoxide and water | CH4 + 1½O2 → CO + 2H2O |
Part 5 of 5How to decide which type
Spot the reaction type
Count the products
- Count the products Addition gives only one product. Substitution gives two. Combustion gives oxides.
Look for oxygen
- Look for oxygen If oxygen is a reactant and carbon dioxide or carbon monoxide and water form, it is combustion.
Check for a double bond
- Check for a double bond A C=C bond that disappears after the reaction signals addition.
Consequence of poor air supply
- Consequence of poor air supply Incomplete combustion makes toxic carbon monoxide and can produce soot (carbon), so appliances need good ventilation.
Key terms
| Term | Meaning |
|---|---|
| Substitution reaction | A reaction in which an atom or group is replaced by another atom or group. |
| Addition reaction | A reaction in which two molecules join to form one larger molecule only. |
| Combustion | A reaction with oxygen that releases energy, commonly called burning. |
| Unsaturated | Containing at least one C=C double bond. |
| Incomplete combustion | Burning in a limited oxygen supply, producing carbon monoxide or carbon instead of carbon dioxide. |
Avoid these mistakes
1
Calling the methane and bromine reaction an addition reaction. Two products form, so it is substitution.
2
Forgetting that substitution with an alkane needs ultraviolet light.
3
Writing carbon dioxide as a product of incomplete combustion. The products there are carbon monoxide and water.
4
Not balancing combustion equationscount carbon first, then hydrogen, then oxygen last.
5
Saying alkanes undergo addition. Only unsaturated compounds such as alkenes add atoms across a double bond.
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