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Carboxylic acids

What is here?

Carboxylic acids are organic compounds that:

  • contain the carboxyl functional group, —COOH
  • have molecules are based on alkane molecules 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 here show members of this series with 1 – 4 carbon atoms. You can also see their structural and displayed formulae.

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chemdoodle.png 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 group

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 formula

A general formula is a chemical formula used to represent the members of a homologous series. The general formula for the 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

Name of alkane
Structural formula
Displayed formula
methanoic acid HCOOH displayed formula of methanoic acid
ethanoic acid CH3COOH displayed formula of ethanoic acid
propanoic acid CH3CH2COOH displayed formula of propanoic acid
butanoic acid CH3CH2CH2COOH displayed formula of butanoic acid

Do not show the functional group as —CO2H.
There is no need to colour the functional group in displayed formulae. This is done here just to make it clearer.