Organic chemistry is the study of compounds of carbon (excluding oxides and carbonates). Carbon can form long chains and rings because it forms four covalent bonds.
Hydrocarbons
A hydrocarbon is a compound containing only carbon and hydrogen. There are three main families:
| Family | Bonding | General formula | Example |
|---|---|---|---|
| Alkanes | Single bonds (saturated) | CₙH₂ₙ₊₂ | Methane CH₄, ethane C₂H₆ |
| Alkenes | One double bond (unsaturated) | CₙH₂ₙ | Ethene C₂H₄ |
| Alkynes | One triple bond | CₙH₂ₙ₋₂ | Ethyne C₂H₂ |
The First Four Alkanes
- Methane — CH₄
- Ethane — C₂H₆
- Propane — C₃H₈
- Butane — C₄H₁₀
Functional Groups
A functional group is an atom or group of atoms that gives an organic compound its characteristic reactions.
- Alcohols contain –OH (e.g. ethanol, C₂H₅OH).
- Carboxylic acids contain –COOH (e.g. ethanoic acid).
- Alkenes contain C=C.
Reactions of Alkanes and Alkenes
- Combustion: alkanes burn in oxygen to form CO₂ and water (they are fuels).
- Substitution: alkanes react with halogens (e.g. chlorine) in the presence of light.
- Addition: alkenes react with halogens and hydrogen — this is the test for unsaturation (alkenes decolourise bromine water, alkanes do not).
Remember: The test for an alkene (unsaturation) is that it decolourises bromine water. Alkanes (saturated) do not decolourise bromine water.
Summary
Organic compounds are carbon-based. Alkanes (CₙH₂ₙ₊₂) are saturated, alkenes (CₙH₂ₙ) and alkynes are unsaturated. Functional groups like –OH and –COOH determine a compound’s reactions, and alkenes are identified by decolourising bromine water.
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