What is here?
methanoic acid, CH2O2
ethanoic acid, C2H4O2
propanoic acid, C3H6O2
butanoic acid, C4H8O2
A homologous series is a series of compounds which:
- have the same functional group
- have the same general formula
- differ by CH2 in molecular formulae from neighbouring compounds
- show a gradual variation in physical properties, such as their boiling points
- have similar chemical properties.
A general formula is a chemical formula used to represent the members of a homologous series. The general formula for carboxylic acids is CnH2n+1COOH.
Note that the functional group contains one of the carbon atoms in the molecule.
Hexanoic acid molecules, for example, contain six carbon atoms. One is in the functional group and the other five are in the remainder of the molecule:
n = (6 – 1) = 5
2n + 1 = (2 × 5) + 1 = 11
So the molecular formula for hexanoic acid is C5H11COOH.
Structural and displayed formulae
A structural formula shows the number of atoms of each element in a molecule, and their positions in the molecule.
A displayed formula shows the same information as a structural formula, but in addition it shows the bonds between the atoms.
|Name of carboxylic acid||Structural formula||Displayed formula|
There is no need to colour the functional group red in displayed formulae. This is done here just to make it clearer.