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 C_nH_2n+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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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 C_nH_2n+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
ethanoic acid	CH3COOH
propanoic acid	CH3CH2COOH
butanoic acid	CH3CH2CH2COOH

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.