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Cambridge IGCSE Chemistry · 0620
Chapter 11: Organic Chemistry — Part 1
Topic 11.1 · Formulae, functional groups and terminology
Organic Formulae
Organic chemistry involves several ways to represent molecules:
- Displayed Formula: Shows all the atoms and all the bonds in a molecule.
- Structural Formula: An unambiguous description of how the atoms in a molecule are arranged (e.g., CH3CH2OH for ethanol).
- Molecular Formula: Represents the actual number and type of atoms in one molecule (e.g., C2H6O).
- General Formula: A formula followed by all compounds in the same homologous series.
| Homologous Series | General Formula | Functional Group | Saturated / Unsaturated |
|---|---|---|---|
| Alkanes | CnH2n+2 | None (only C-C single bonds) | Saturated |
| Alkenes | CnH2n | C=C double bond | Unsaturated |
| Alcohols | CnH2n+1OH | Hydroxyl group (-OH) | Saturated |
| Carboxylic Acids | CnH2n+1COOH | Carboxyl group (-COOH) | Saturated |
Exam Traps
- Do not use CnH2n for alkanes — that is the alkene general formula.
- Do not call alcohols or carboxylic acids unsaturated just because they contain oxygen — unsaturated means C=C double bonds.
- Do not confuse displayed formula (all atoms and bonds shown) with structural formula (arrangement without every bond drawn).
Homologous Series and Terminology
- Homologous Series: A family of similar compounds with the same functional group and same general formula. They show a trend in physical properties and share similar chemical properties. Each member differs from the next by a -CH2- unit.
- Functional Group: An atom or group of atoms that determines the chemical properties of a homologous series.
- Saturated Compound: Molecules in which all carbon–carbon bonds are single bonds (e.g., alkanes).
- Unsaturated Compound: Molecules containing one or more carbon–carbon double bonds (e.g., alkenes).
Exam Traps
- Do not confuse a homologous series with structural isomers — isomers have the same molecular formula but different structures.
Structural Isomers
Structural isomers are defined as compounds with the same molecular formula but different structural formulae.
- Isomers of C4H10: Includes butane (straight chain) and 2-methylpropane (branched chain).
- Isomers of C4H8: These differ in the position of the double bond, such as but-1-ene and but-2-ene.
Exam Traps
- Do not give but-1-ene and but-2-ene as isomers of C4H10 — they are C4H8 alkenes, not C4H10 alkanes.
- Do not reverse the definition — isomers share a molecular formula; they do not share a structural formula.
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