Carboxylic acids

What is here?

Carboxylic acids are organic compounds that:

  • contain the carbonyl functional group, —COOH
  • have molecules based on alkanes in which one of the H atoms is replaced by a —COOH group
  • form a homologous series which has the general formula CnH2n+1COOH.
The models show members of this series with 1 – 4 carbon atoms. You can also see the structural and displayed formulae for these alcohols.

Use your mouse (or finger on touch devices) to move or scale molecules. Double tap to stop and start each animation.

atom labels
ball and stick model
space-filling model
perspective view

methanoic acid, CH2O2

ethanoic acid, C2H4O2

propanoic acid, C3H6O2

butanoic acid, C4H8O2

Homologous series

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.

Functional groups

A functional group is an atom, group of atoms or a bond responsible for the characteristic chemical reactions of a substance. The functional group in carboxylic acids is shown as —COOH.

General formulae

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 acidStructural formulaDisplayed formula
methanoic acidHCOOH
ethanoic acidCH3COOH
propanoic acidCH3CH2COOH
butanoic acidCH3CH2CH2COOH
Do not show the carboxyl functional group as —CO2H.
There is no need to colour the functional group red in displayed formulae. This is done here just to make it clearer.