Hydrocarbons: alkanes, alkenes, alkynes, homologous series and isomers
Hydrocarbons contain only carbon and hydrogen. Alkanes (CₙH₂ₙ₊₂) have single bonds; alkenes (CₙH₂ₙ) and alkynes (CₙH₂ₙ₋₂) have double or triple bonds. Isomers share a formula but differ in structure.
Kinds of hydrocarbons
| Kind | Meaning | Example |
|---|---|---|
| Aliphatic (open chain) | Carbon atoms in straight or branched chains; the name comes from a Greek word for fat | Methane, ethene, ethyne |
| Saturated: alkanes (paraffins) | Only single bonds; CₙH₂ₙ₊₂ | Ethane, C₂H₆ |
| Unsaturated: alkenes (olefins) | One C=C double bond; CₙH₂ₙ | Ethene, C₂H₄ |
| Unsaturated: alkynes | One C≡C triple bond; CₙH₂ₙ₋₂ | Ethyne (acetylene), C₂H₂ |
| Cyclic (closed chain) | Carbon atoms in a ring: alicyclic rings, or aromatic rings like benzene, which have a smell | Benzene, C₆H₆ (aromatic) |
The first eight alkanes: a homologous series
| Carbons | Name | Formula | Molecular mass (u) | Boiling point (°C) | Isomers |
|---|---|---|---|---|---|
| 1 | Methane | CH₄ | 16 | −162 | 1 |
| 2 | Ethane | C₂H₆ | 30 | −89 | 1 |
| 3 | Propane | C₃H₈ | 44 | −42 | 1 |
| 4 | Butane | C₄H₁₀ | 58 | 0 | 2 |
| 5 | Pentane | C₅H₁₂ | 72 | 36 | 3 |
| 6 | Hexane | C₆H₁₄ | 86 | 68 | 5 |
| 7 | Heptane | C₇H₁₆ | 100 | 98 | 9 |
| 8 | Octane | C₈H₁₈ | 114 | 126 | 18 |
Features of a homologous series
- All members fit one general formula (alkanes: CₙH₂ₙ₊₂).
- Each member differs from the next by CH₂, which adds 14 u to the mass (12 + 2 × 1).
- Physical properties change steadily: boiling points rise as the chain grows.
- Chemical properties are similar. Alkanes are colourless, have no smell and react very little.
- Alkenes and alkynes form their own homologous series.
Alkenes and alkynes
| Carbons | Alkene | Formula | Alkyne | Formula |
|---|---|---|---|---|
| 2 | Ethene | C₂H₄ | Ethyne | C₂H₂ |
| 3 | Propene | C₃H₆ | Propyne | C₃H₄ |
| 4 | Butene | C₄H₈ | Butyne | C₄H₆ |
| 5 | Pentene | C₅H₁₀ | Pentyne | C₅H₈ |
Isomers of butane
Four carbon atoms can be joined in two ways. In butane, all four are in a row: CH₃-CH₂-CH₂-CH₃. In isobutane, three are in a row and the fourth branches off the middle one: CH₃-CH(CH₃)-CH₃. Both are C₄H₁₀, but their structures differ, so they are isomers. The number of isomers grows fast as the chain gets longer: pentane has 3 and decane has 75. Butene has isomers too: in 1-butene and 2-butene the double bond sits in different places.
How to draw the structures
- Ethane: two C atoms joined by one line, with three H atoms on each C.
- Ethene: two C atoms joined by two lines, with two H atoms on each C.
- Ethyne: two C atoms joined by three lines, with one H atom on each C.
- Butane and isobutane: draw the straight four-carbon chain and the branched chain, then add H atoms until every C has four lines.
- Check: every carbon has exactly four bonds, and every hydrogen has one.
The LPG cylinder in your kitchen holds mainly butane and propane. The CNG in autos and buses is mainly methane, the simplest alkane. Ethyne burns with oxygen in the oxy-acetylene torch that welders use.
3 marks: “A hydrocarbon has 4 carbon atoms. Write its formula and name if it is (i) an alkane, (ii) an alkene and (iii) an alkyne.” Use CₙH₂ₙ₊₂, CₙH₂ₙ and CₙH₂ₙ₋₂ with n = 4: (i) C₄H₁₀, butane; (ii) C₄H₈, butene; (iii) C₄H₆, butyne.
2 marks: “What is a homologous series? Write the next two members after ethane in the alkane series.” A family of compounds with the same general formula, each differing from the next by CH₂. After ethane, C₂H₆, come propane, C₃H₈, and butane, C₄H₁₀.
3 marks: “Two gases both have the formula C₄H₁₀ but different boiling points. Explain, with structures, how this is possible.” They are isomers: butane, a straight chain CH₃-CH₂-CH₂-CH₃, and isobutane, a branched chain CH₃-CH(CH₃)-CH₃. Same formula, different structures, so different properties.
Alkane plus 2, alkene even, alkyne minus 2: CₙH₂ₙ₊₂, CₙH₂ₙ, CₙH₂ₙ₋₂.