Organic Formulae
Section: 11. Organic Chemistry | Syllabus: Cambridge IGCSE Chemistry (0620)
Organic Chemistry and Types of Formulae
Organic chemistry is the study of compounds containing carbon (excluding carbon dioxide, carbon monoxide, carbonates and hydrogencarbonates). The same molecule can be represented in several different ways, each showing a different level of detail.
- Molecular formula: shows the actual number of atoms of each element in one molecule, e.g. C₂H₆O for ethanol
- Displayed formula: shows every atom and every bond in the molecule, drawn out in full
- Structural formula: an unambiguous description of the way the atoms in a molecule are arranged, showing the minimum detail needed - not every individual bond, e.g. CH₂=CH₂, CH₃CH₂OH, CH₃COOCH₃
- General formula: an algebraic formula that represents every member of a homologous series, using "n" for the number of carbon atoms
A functional group is an atom or group of atoms that determines the chemical properties of a compound. Compounds in the same homologous series share the same functional group, so they react in similar ways.
Worked Example
- Question (based on a real exam question): bromomethane (CH₃Br), bromoethane (C₂H₅Br) and 1-bromopropane (C₃H₄Br) have similar chemical properties. Which statement best explains why?
- Answer: they all contain the same functional group (the C-Br bond). Simply sharing the same elements, or all being carbon-based, or all being saturated is not specific enough - alkanes are also saturated but react very differently, so it is the functional group that determines chemical behaviour.
Structural Isomers
Structural isomers are compounds with the same molecular formula, but different structural formulae.
| Molecular formula | Isomer 1 | Isomer 2 |
|---|---|---|
| C₄H₁₀ | CH₃CH₂CH₂CH₃ (butane) | CH₃CH(CH₃)CH₃ (methylpropane) |
| C₄H₈ | CH₃CH₂CH=CH₂ (but-1-ene) | CH₃CH=CHCH₃ (but-2-ene) |
Common Mistakes
- ❌ Thinking that two compounds with the same molecular formula must be the same substance
- ✓ structural isomers share a molecular formula but have their atoms arranged differently, which can give them different physical properties (e.g. different boiling points) even though their molecular formula is identical
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