Formulae, functional groups and terminology
Ways to write a molecule
- molecular formula — how many of each atom, e.g. $\text{C}_2\text{H}_6$.
- displayed formula — every atom and bond drawn out.
- structural formula — arrangement without every bond, e.g. $\text{CH}_3\text{CH}_2\text{OH}$.
- general formula — works for a whole family, e.g. $\text{C}_n\text{H}_{2n+2}$ for alkanes.
Practice
The general formula of the alkanes is:
Alkanes follow CnH2n+2; alkenes are CnH2n.
Homologous series
- A homologous series is a family with the same functional group, the same general formula, differing by $\text{CH}_2$ each step, with similar chemistry.
| Series | Functional group | General formula |
|---|---|---|
| alkanes | C–C only | $\text{C}_n\text{H}_{2n+2}$ |
| alkenes | C=C | $\text{C}_n\text{H}_{2n}$ |
| alcohols | –OH | $\text{C}_n\text{H}_{2n+1}\text{OH}$ |
- Saturated = only single C–C bonds; unsaturated = has a C=C.
- Structural isomers share a molecular formula but differ in structure (e.g. two forms of $\text{C}_4\text{H}_{10}$).
Practice
A saturated compound contains:
Saturated means only single C–C bonds; unsaturated has a C=C (or C≡C).
Practice
Structural isomers have the same molecular formula but different structures.
Isomers share a molecular formula but arrange the atoms differently (e.g. butane vs 2-methylpropane).
You've got it
Key idea
- formulae: molecular, displayed, structural, general ($\text{C}_n\text{H}_{2n+2}$ = alkanes)
- a homologous series shares a functional group + general formula, differing by $\text{CH}_2$
- saturated = single bonds only; isomers = same formula, different structure