Alkanes
Section: 11. Organic Chemistry | Syllabus: Cambridge IGCSE Chemistry (0620)
Alkanes
The bonding in alkanes is single covalent throughout, so alkanes are saturated hydrocarbons. General formula: CnH2n+2.
| Name | Molecular formula | Structural formula |
|---|---|---|
| Methane | CH₄ | CH₄ |
| Ethane | C₂H₆ | CH₃CH₃ |
| Propane | C₃H₈ | CH₃CH₂CH₃ |
| Butane | C₄H₁₀ | CH₃CH₂CH₂CH₃ |
Reactivity of Alkanes
Alkanes are generally unreactive, because the C-C and C-H single bonds are strong. The two reactions they do undergo are combustion and substitution by chlorine.
Combustion
- Complete combustion (plenty of oxygen): alkane + oxygen → carbon dioxide + water
- Incomplete combustion (limited oxygen): produces carbon monoxide and/or carbon (soot) instead of carbon dioxide
Worked Example
- Question (based on a real exam question): (i) write the general formula for alkanes. (ii) Name two substances produced when ethane undergoes complete combustion.
- Answer: (i) CnH2n+2; (ii) carbon dioxide and water. C₂H₆ + 3½O₂ → 2CO₂ + 3H₂O.
Substitution by Chlorine (Supplement)
In a substitution reaction, one atom or group of atoms is replaced by another atom or group of atoms.
Alkanes react with chlorine in a photochemical reaction: ultraviolet light provides the activation energy, E₀, needed to break the Cl-Cl bond and start the reaction. One hydrogen atom of the alkane is replaced by one chlorine atom (limited to monosubstitution).
CH₄ + Cl₂ → CH₃Cl + HCl (in UV light)
The product, chloromethane (CH₃Cl), has one of methane's four hydrogen atoms replaced by a chlorine atom; hydrogen chloride gas is the other product.
Common Mistakes
- ❌ Continuing the substitution to replace a second, third or fourth hydrogen (e.g. writing CH₂Cl₂ or CHCl₃ as the product)
- ✓ this syllabus is limited to monosubstitution only - draw or name just the single-substitution product (one H replaced by one Cl), not the further-substituted products
- ❌ Forgetting that this reaction needs UV light
- ✓ alkanes do not react with chlorine in the dark - UV light provides the activation energy that starts the reaction, which is why it is called a photochemical reaction
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