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Organic chemists use several different types of formulae to represent the same molecule, each emphasising a different aspect of its structure. The homologous series concept then groups related compounds together so that trends in physical properties can be understood and predicted. This lesson covers OCR A-Level Chemistry A (H432) specification 4.1.1 (b)–(d).
The empirical formula gives the simplest whole-number ratio of atoms of each element in a compound.
Key Definition — Empirical formula: The simplest whole-number ratio of atoms of each element present in a compound.
Example: glucose has a molecular formula C₆H₁₂O₆ but an empirical formula CH₂O.
The molecular formula gives the actual number of atoms of each element present in one molecule.
Key Definition — Molecular formula: The actual number of atoms of each element in a molecule.
Example: butane is C₄H₁₀; the empirical formula is C₂H₅.
The general formula expresses the ratio of atoms for any member of a homologous series using n for the number of carbons.
| Homologous series | General formula |
|---|---|
| Alkane | CₙH₂ₙ₊₂ |
| Alkene (one C=C) | CₙH₂ₙ |
| Cycloalkane | CₙH₂ₙ |
| Alcohol (saturated, one OH) | CₙH₂ₙ₊₁OH |
| Haloalkane (one halogen) | CₙH₂ₙ₊₁X |
| Carboxylic acid | CₙH₂ₙO₂ |
| Ketone / aldehyde | CₙH₂ₙO |
A structural formula shows the minimum detail needed to unambiguously identify the molecule — it shows which atoms are joined to which, without drawing every bond.
Examples:
A displayed formula shows every bond and every atom explicitly. You draw every C–H and every C–C single bond individually.
Ethanol displayed:
H H
| |
H - C - C - O - H
| |
H H
A skeletal formula is the most compact representation. Carbon atoms are implied at every vertex and line end; hydrogen atoms on carbons are implied (each carbon is assumed to have enough H atoms to complete four bonds). Functional groups and heteroatoms are shown explicitly.
Skeletal formula of butan-1-ol:
OH
|
/\ /\/
(Four zig-zag carbons with –OH on the terminal carbon.)
graph LR
A[Molecular formula<br/>C2H6O] --> B[Empirical formula<br/>C2H6O cannot simplify]
A --> C[Structural formula<br/>CH3CH2OH]
A --> D[Displayed formula<br/>all bonds shown]
A --> E[Skeletal formula<br/>vertices imply C]
A homologous series is a family of organic compounds which:
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