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 C_nH_2n+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.

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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 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

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
methanoic acid	HCOOH
ethanoic acid	CH₃COOH
propanoic acid	CH₃CH₂COOH
butanoic acid	CH₃CH₂CH₂COOH

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.

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Carboxylic acids

What is here?

methanoic acid, CH2O2

ethanoic acid, C2H4O2

propanoic acid, C3H6O2

butanoic acid, C4H8O2