Carboxylic acids and esters
Carboxylic acids
are a homologous series whose functional group is –COOH. Their names end in .
| Carboxylic acid | Formula |
|---|---|
| Methanoic acid | HCOOH |
| Ethanoic acid | CH₃COOH |
| Propanoic acid | C₂H₅COOH |
| Butanoic acid | C₃H₇COOH |
Like other acids, they:
- dissolve in water to form acidic solutions (pH below 7)
- react with carbonates to make a salt, water and carbon dioxide (they fizz):
2CH₃COOH + Na₂CO₃ → 2CH₃COONa + H₂O + CO₂
The salt of ethanoic acid is sodium ethanoate.
Key terms
Weak acids (Higher tier only)
Carboxylic acids are : they only partially ionise in water. Most molecules stay whole, and only a few release H⁺ ions:
CH₃COOH ⇌ CH₃COO⁻ + H⁺
So a solution of ethanoic acid has fewer H⁺ ions, and a , than a solution of a strong acid such as hydrochloric acid at the .
Key terms
Esters
A carboxylic acid reacts with an alcohol, using an (concentrated sulfuric acid), to make an and water:
ethanoic acid + ethanol ⇌ ethyl ethanoate + water
CH₃COOH + C₂H₅OH ⇌ CH₃COOC₂H₅ + H₂O
Esters have the functional group –COO–. They are (evaporate easily) and have fruity smells, so they are used in perfumes and flavourings. Ethyl ethanoate is also used as a solvent, for example in nail varnish remover.