Versatile carbon: catenation, tetravalency and hydrocarbons
Carbon forms millions of compounds because of catenation (bonding with other carbon atoms) and tetravalency (four bonds). Its chains can be straight, branched or ring-shaped.
Why carbon forms so many compounds
- Catenation: carbon links with other carbon atoms to form long chains, branches and rings. The carbon-carbon bond is very strong, so these compounds are stable.
- Tetravalency: carbon has a valency of 4, so it can bond with four other atoms, such as carbon, hydrogen, oxygen, nitrogen, sulphur or chlorine.
- Carbon is a small atom, so its nucleus holds the shared electrons tightly. This makes its bonds strong.
- Silicon also links to itself, but only in short chains of seven or eight atoms, and these compounds are very reactive.
Saturated and unsaturated hydrocarbons
| Type | Bonds | General formula | Examples |
|---|---|---|---|
| Alkane (saturated) | Only single C-C bonds | CₙH₂ₙ₊₂ | Methane CH₄, ethane C₂H₆, propane C₃H₈, butane C₄H₁₀ |
| Alkene (unsaturated) | One C=C double bond | CₙH₂ₙ | Ethene C₂H₄, propene C₃H₆ |
| Alkyne (unsaturated) | One C≡C triple bond | CₙH₂ₙ₋₂ | Ethyne C₂H₂, propyne C₃H₄ |
Chains, branches and rings
| Shape | Example | Formula |
|---|---|---|
| Straight chain | Butane: four carbon atoms in a row | C₄H₁₀ |
| Branched chain | Isobutane: three carbon atoms in a row, with one carbon branching off the middle one | C₄H₁₀ |
| Ring with single bonds | Cyclohexane: six carbon atoms in a ring | C₆H₁₂ |
| Ring with double bonds | Benzene: six carbon atoms in a ring, with single and double bonds taking turns | C₆H₆ |
Structural isomers
Butane and isobutane have the same molecular formula, C₄H₁₀, but different structures. Compounds like these are called structural isomers. So a molecular formula alone does not always tell you which compound you have.
How to draw the structures
- Ethane: two C atoms joined by a single bond, with three H on each C. Write H₃C-CH₃.
- Ethene: two C atoms joined by a double bond (two lines), with two H on each C. Write H₂C=CH₂.
- Ethyne: two C atoms joined by a triple bond (three lines), with one H on each C. Write HC≡CH.
- Check: every carbon atom must have exactly four lines (bonds) coming out of it.
- For electron dot structures, show 1, 2 or 3 shared pairs between the two carbon atoms.
Almost everything you eat, from rice starch and dal protein to ghee and vitamins, is a carbon compound. The fuel LPG is mainly butane, C₄H₁₀, a saturated hydrocarbon.
2 marks: “Both carbon and silicon can link to atoms of their own kind, yet carbon forms millions of compounds and silicon very few. Give two reasons.” Catenation: carbon forms long, stable chains, branches and rings, because the C-C bond is strong; silicon chains stop at seven or eight atoms and are very reactive. Tetravalency: each carbon forms four strong bonds with C, H, O, N, S or Cl.
2 marks: “Differentiate between saturated and unsaturated hydrocarbons, with one example each.” Saturated: only single bonds, e.g. ethane C₂H₆. Unsaturated: at least one double or triple bond, e.g. ethene C₂H₄ or ethyne C₂H₂.
3 marks: “What are structural isomers? Draw the structures of two isomers of butane.” Compounds with the same molecular formula but different structures. Straight-chain butane, CH₃CH₂CH₂CH₃, and branched isobutane: a three-carbon chain with a CH₃ branch on the middle carbon.
-ane is single, -ene is double, -yne is triple. Carbon "cat"-enates (links to itself) and has "tetra" (four) hands.