A family with a pattern
A homologous series is a group of organic compounds that share the same general formula and the same functional group. Consecutive members differ by –CH2– (a difference in relative molecular mass of 14). Members have similar chemical properties because they share the same functional group, but show a gradual change in physical properties as molecular size grows.
| Series | General formula |
|---|---|
| Alkane | CnH2n+2 |
| Alkene | CnH2n |
| Alcohol | CnH2n+1OH |
| Carboxylic acid | CnH2n+1COOH |
Worked example
Find the molecular formula of the alkane with n = 4. Using CnH2n+2: C4H(2×4+2) = C4H10 (butane). The next member, C5H12, differs by exactly one CH2 unit.
Trends down a series
As the number of carbon atoms increases: boiling point and melting point rise (stronger intermolecular forces), density increases slightly, and solubility in water decreases for hydrocarbons. Volatility falls and the compounds change from gases to liquids to solids.
The members of a series are named with prefixes that show the number of carbon atoms: meth- (1), eth- (2), prop- (3) and but- (4), followed by an ending for the family (–ane, –ene, –anol, –anoic acid). Because the functional group stays the same, one member can be prepared from the next by adding or removing a CH2 unit. The steady change in physical properties lets us predict the properties of a member we have not yet studied.
Remember
- Same general formula + same functional group = one series.
- Consecutive members differ by CH2 (Mr +14).
- Similar chemical properties; gradual physical change.
- Boiling point increases with more carbon atoms.