04 Molecular formula, structural formula and isomers
- The molecular formula shows only the kind and number of atoms in the molecule, not how they are linked. Examples: CH₄, C₂H₆, C₂H₆O.
- The structural (displayed) formula shows the kind and number of atoms and how they are joined by covalent bonds. Each covalent bond represents one valency, and valency is fixed in organic compounds: carbon = 4, hydrogen = 1, oxygen = 2, nitrogen = 3, halogen = 1.
- Condensed structural formula of ethanol: CH₃–CH₂–OH.
Isomerism
Isomerism is the existence of several compounds that share one molecular formula but differ in their structural formula and therefore in their physical and chemical properties; such compounds are called isomers.
The example C₂H₆O
- Structural formula — ethyl alcohol: CH₃–CH₂–OH · dimethyl ether: CH₃–O–CH₃.
- Melting point — −114 °C · −138 °C.
- Boiling point — 78 °C · −24 °C.
- Reaction with sodium — the alcohol reacts and sodium replaces the hydroxyl hydrogen; the ether does not react.
A structural formula drawn on paper makes the molecule look flat, while in reality it is three-dimensional; molecular models (coloured balls, one colour and size per element) are used to show the true shape. Methane, for instance, is a tetrahedral molecule with bond angles of 109.5°, the four bonds arranged symmetrically to minimise electron-pair repulsion.