4.1.3Principles of Chemistry
Classifying Organic Reactions
5 min read 5 parts Lesson 1 of 50 Unit quiz
Section 1 of 5Three Reaction Types
Substitution
- A substitution reaction replaces one atom or group in a molecule with another, and a small molecule such as HBr is also formed.
Addition
- An addition reaction joins two molecules into one larger product, with no other products formed.
Combustion
- Combustion is burning, in which an organic substance reacts with oxygen to form carbon dioxide or carbon monoxide, and water.
Alkanes and alkenes
- Alkanes typically undergo substitution, while alkenes undergo addition because of their C=C double bond.
Conditions
- Substitution of alkanes with halogens needs ultraviolet light, while bromine adds to alkenes at room temperature without light.
Section 2 of 5Substitution in Practice
1
Methane reacts with bromine under ultraviolet light to give bromomethane and hydrogen bromide: CH₄ + Br₂ → CH₃Br + HBr.
2
Ultraviolet light supplies the energy to break the Br–Br bond and start the reaction.
3
One hydrogen atom in methane is replaced by a bromine atom, so one atom is swapped for another.
4
Chlorine reacts with methane in the same way, forming chloromethane and hydrogen chloride.
Section 3 of 5Addition and Combustion
Addition and Combustion
Excess oxygen
- Complete combustion gives carbon dioxide and water as the only products.
Limited oxygen
- Incomplete combustion gives toxic carbon monoxide, together with water.
Soot
- Very limited oxygen can also produce carbon particles, which appear as soot.
Prediction
- You do not need to memorise every equation, but you must predict products from the fuel and the oxygen supply.
| Reaction | Reactants | Products | Equation |
|---|---|---|---|
| Addition | Ethene and bromine | 1,2-dibromoethane | C₂H₄ + Br₂ → C₂H₄Br₂ |
| Complete combustion of methane | Methane and excess oxygen | Carbon dioxide and water | CH₄ + 2O₂ → CO₂ + 2H₂O |
| Complete combustion of propane | Propane and excess oxygen | Carbon dioxide and water | C₃H₈ + 5O₂ → 3CO₂ + 4H₂O |
| Incomplete combustion of methane | Methane and limited oxygen | Carbon monoxide and water | 2CH₄ + 3O₂ → 2CO + 4H₂O |
Section 4 of 5Worked Example
Question Write the balanced equation for the complete combustion of ethanol, C₂H₅OH, and state the type of reaction.
- Ethanol has 2 carbon, 6 hydrogen and 1 oxygen atoms
- Products are 2CO₂ and 3H₂O, which contain 7 oxygen atoms in total
- Ethanol supplies 1 oxygen atom, so the O₂ must supply 6 oxygen atoms, which is 3O₂
Answer
- C₂H₅OH + 3O₂ → 2CO₂ + 3H₂O.
- This is a combustion reaction.
Section 5 of 5Exam Checklist & Tips
4.1.3 – Key Points for the Exam
- Substitution A reaction in which one atom or group is replaced by another.
- Addition A reaction in which two molecules combine to form one larger molecule with no other products.
- Combustion A reaction in which a substance burns in oxygen, releasing energy.
- Complete combustion Burning in excess oxygen, producing carbon dioxide and water.
- Incomplete combustion Burning in limited oxygen, producing carbon monoxide or carbon and water.
Common Mistakes
- Calling the reaction of an alkene with bromine a substitution because bromine appears in the product.
- Stating that incomplete combustion gives carbon dioxide rather than carbon monoxide.
- Forgetting that the reaction of methane with bromine needs ultraviolet light.
- Leaving oxygen out of the reactants when balancing a combustion equation.