Chemistry

6.14 - Carboxylic Acids

7 min

The carboxylic acids

Weak acids

Carboxylate ions & salts

Made from alcohols

note 1 of 2

The carboxylic acids

Carboxylic acids are a homologous series - a family built on the same pattern: they share a general formula, each member differs from the next by a CH₂ unit, and they all have similar chemical properties.
They all share the functional group -COOH, the carboxyl group:
- A carbon atom joined to two oxygen atoms - one by a double bond, the other by a single bond.
- That second oxygen carries a hydrogen atom on a single bond.
- The carbon uses its fourth bond to join to the rest of the chain.
It's the hydrogen atom on that oxygen that makes the compound an acid.

Naming carboxylic acids

All names end in -oic acid, and the prefix says how many carbon atoms the molecule has: meth- (1), eth- (2), prop- (3), but- (4).
The carbon atom in the carboxyl group is counted in - Ethanoic acid has two carbon atoms in all, one of them the carbon of the COOH.

The first four members

Written on one line, the four atoms of the group are kept together at the end as COOH, so the group can be seen at a glance.
The molecular formula gives the totals and hides the group. For example, C₂H₅COOH and C₃H₆O₂ are the same molecule written two different ways.
All four acids are liquids at room temperature and pressure.

Four carboxylic acid structures in a row with every atom and bond drawn, named beneath with their shortened and molecular formulae: methanoic, ethanoic, propanoic and butanoic acid; a box on the first is labelled carboxyl group.

Where they turn up

Vinegar is a dilute solution of ethanoic acid in water, which gives it its sharp taste and smell.
Carboxylic acids are used in foods (as flavours and preservatives) and in personal care products such as soaps.

note 2 of 2

What carboxylic acids do

Carboxylic acids are a family of organic acids. A solution of one behaves the way any other acid solution behaves.
When dissolved in water, a carboxylic acid forms an acidic solution.
It's acidic because the acid releases hydrogen ions, H⁺, into the water.
Because it contains hydrogen ions, the solution has a pH below 7 and turns universal indicator red, orange or yellow.
When a carboxylic acid reacts, it reacts as an acid: its acidic hydrogen is replaced by a metal (or other positive ion) to form a salt.

key terms

TermDefinition

Homologous seriesA family of compounds with the same general formula and similar chemical properties. Each member differs from the next by a CH₂ unit.

Carboxyl groupThe -COOH functional group that all carboxylic acids share.

end of notes

Key points to remember

The carboxylic acids

Carboxylic acids are a homologous series - a family built on the same pattern: they share a general formula, each member differs from the next by a CH₂ unit, and they all have similar chemical properties.

They all share the functional group -COOH, the carboxyl group:

A carbon atom joined to two oxygen atoms - one by a double bond, the other by a single bond.
That second oxygen carries a hydrogen atom on a single bond.
The carbon uses its fourth bond to join to the rest of the chain.

It's the hydrogen atom on that oxygen that makes the compound an acid.

Naming carboxylic acids

All names end in -oic acid, and the prefix says how many carbon atoms the molecule has: meth- (1), eth- (2), prop- (3), but- (4).

The carbon atom in the carboxyl group is counted in - Ethanoic acid has two carbon atoms in all, one of them the carbon of the COOH.

The first four members

Written on one line, the four atoms of the group are kept together at the end as COOH, so the group can be seen at a glance.

The molecular formula gives the totals and hides the group. For example, C₂H₅COOH and C₃H₆O₂ are the same molecule written two different ways.

All four acids are liquids at room temperature and pressure.

What carboxylic acids do

Carboxylic acids are a family of organic acids. A solution of one behaves the way any other acid solution behaves.

When dissolved in water, a carboxylic acid forms an acidic solution.

It's acidic because the acid releases hydrogen ions, H⁺, into the water.

Because it contains hydrogen ions, the solution has a pH below 7 and turns universal indicator red, orange or yellow.

When a carboxylic acid reacts, it reacts as an acid: its acidic hydrogen is replaced by a metal (or other positive ion) to form a salt.