Organic compounds and the special bonding of carbon
Carbon shares its four electrons to form covalent bonds. Catenation, tetravalency, multiple bonds and isomerism let it form millions of compounds.
What are organic compounds?
Organic compounds are compounds of carbon, usually with hydrogen, and often with oxygen, nitrogen or halogens. Long ago, people believed only living things could make them. This idea was called the vital force theory. In 1828, Friedrich Wöhler made urea in the laboratory by heating ammonium cyanate. This ended the vital force theory. Today, organic chemistry simply means the chemistry of carbon compounds. Oxides of carbon, carbonates and cyanides contain carbon too, but they are studied as inorganic compounds.
Characteristics of organic compounds
- Most are covalent compounds, so they do not conduct electricity.
- They have low melting and boiling points compared with ionic (inorganic) compounds.
- Most are insoluble in water but dissolve in organic solvents such as ether and carbon tetrachloride.
- Most burn easily (they are combustible), for example kerosene, petrol and LPG.
- Their reactions are slower than the reactions of ionic compounds.
- Many show isomerism: the same molecular formula but different structures.
Why carbon forms covalent bonds
- 1Carbon has atomic number 6, so its electrons are arranged as 2, 4. It needs 4 more electrons to complete its outer shell (an octet).
- 2Gaining 4 electrons to form C⁴⁻ is hard: 6 protons cannot hold 10 electrons firmly.
- 3Losing 4 electrons to form C⁴⁺ needs too much energy.
- 4★So carbon shares its 4 electrons with other atoms. A bond formed by sharing a pair of electrons is a covalent bond.
- 5Example: in methane (CH₄), carbon shares one pair of electrons with each of 4 hydrogen atoms. This gives 4 single covalent bonds.
★ key step
Four special properties of carbon
| Property | What it means | Example |
|---|---|---|
| Catenation | Carbon atoms join with other carbon atoms to form long chains, branched chains and rings | The chain in butane; the ring in cyclohexane (C₆H₁₂) |
| Tetravalency | Carbon has a valency of 4, so it forms 4 covalent bonds | CH₄: one carbon bonded to 4 hydrogen atoms |
| Multiple bonds | Two carbon atoms can share 1, 2 or 3 pairs of electrons: a single (C-C), double (C=C) or triple (C≡C) bond | Ethane C₂H₆ (single), ethene C₂H₄ (double), ethyne C₂H₂ (triple) |
| Isomerism | Compounds with the same molecular formula but different structural formulas, and so different properties | C₂H₆O: ethanol (a liquid) and methoxymethane (a gas) |
Write the isomers of butane (C₄H₁₀)
Write the structures of two compounds with the formula C₄H₁₀ and name them.
- Count the atoms: 4 carbon atoms and 10 hydrogen atoms.
- Straight chain: CH₃-CH₂-CH₂-CH₃. This is n-butane (butane).
- Branched chain: put one CH₃ on the middle carbon of a 3-carbon chain: CH₃-CH(CH₃)-CH₃. This is isobutane (2-methylpropane).
- Check: each has 4 C and 10 H, so each is C₄H₁₀.
- Same formula, different structures: they are isomers, and they have different boiling points.
n-butane and isobutane (2-methylpropane) are the two isomers of C₄H₁₀.
The LPG in a kitchen cylinder is mainly propane and butane, and the CNG in autorickshaws and buses is mainly methane. All of them are carbon compounds held together by covalent bonds.
1 marks: “Who made urea, an organic compound, in the laboratory in 1828?” Friedrich Wöhler, by heating ammonium cyanate.
2 marks: “What is catenation? Give an example.” The ability of carbon atoms to join with each other to form long chains, branched chains or rings. Example: the chain CH₃-CH₂-CH₂-CH₃ in butane.
4 marks: “Why does carbon form covalent bonds? Explain any two special properties of carbon with examples.” Configuration 2, 4; it cannot easily gain or lose 4 electrons, so it shares them. Then catenation (chain in butane) and isomerism (n-butane and isobutane), with structures.
Carbon's CTMI: Catenation, Tetravalency, Multiple bonds, Isomerism. Say it as 'Carbon Takes Many Identities'.