Carboxylic acids are compounds that contain the -COOH group.
Examples of carboxylic acids:
Salts of carboxylic acids are called carboxylate salts. The formation of carboxylate salts shows that carboxylic acids are acidic. A carboxylate salt is formed upon the removal of H⁺ from -COOH group, leaving -COO⁻.
Physical properties of carboxylic acids
- Carboxylic acids have higher melting and boiling points than the corresponding alcohols.
- This is because in a pure carboxylic acid, hydrogen bonding can occur between two molecules of acid to produce a dimer.
- This doubles the size of the molecule, making the van der Waals’ forces stronger. iii. Hence more energy is required to overcome these forces of attraction.
- Small carboxylic acids are soluble in water.
- carboxylic acid is capable of forming hydrogen bond with water molecule.
- However, the solubility decreases as the number of carbon atoms increases. This is because the presence of long hydrocarbon tail disrupts the effectiveness of hydrogen bonding.
Preparation of carboxylic acids
- Oxidation of primary alcohol.
- Oxidation of aldehyde.
- Acidic/alkaline hydrolysis of nitrile.
Oxidation of primary alcohol and aldehyde
Acidic hydrolysis of nitrile:
- When nitrile is heated under reflux with a dilute acid such as dilute hydrochloric acid, carboxylic acid will be formed.
- Using ethanenitrile as an example, ethanoic acid is formed. CH3CN + 2H2O + H⁺ → CH3COOH + NH4⁺
Alkaline hydrolysis of nitrile:
- When nitrile is heated under reflux with an alkali such as sodium hydroxide, salt of carboxylic acid(carboxylate salt) is produced. Using ethanenitrile as an example, ethanoate ion is formed. CH3CN + 2H2O + OH⁻ → CH3COO⁻ + NH3
- Ethanoate ion can be converted to ethanoic acid by the addition of acid. CH3COO⁻ + H⁺ → CH3COOH
Reactions of Carboxylic Acids
Acidity of carboxylic acids
Carboxylic acids are acidic because they can donate a proton to form carboxylate ion and hydroxonium ion. The presence of hydroxonium ions makes the solution acidic.
RCOOH + H2O ⇌ RCOO⁻ + H3O⁺
However, carboxylic acids are only weak acids. For instance ethanoic acid has a pH of about 2-3.
Reaction with bases
Since carboxylic acids are acids, they will react with a base such as sodium hydroxide to produce a carboxylate salt and water. This is a simple neutralisation reaction.
- Take ethanoic acid and sodium hydroxide as an example: CH3COOH + NaOH → CH3COONa + H2⁺ + OH⁻ → H2O
Reaction with metals
Carboxylic acids will react with reactive metals such as sodium to produce a carboxylate salt and hydrogen gas.
- Take ethanoic acid and sodium metal as an example:
2CH3COOH + 2Na → 2CH3COONa + H2
Reaction with carbonates and hydrogen carbonates
Carboxylic acids will react with carbonates and hydrogen carbonates
to produce a carboxylate salt, carbon dioxide gas and water.
-
ethanoic acid and sodium carbonate
2CH3COOH + Na2CO3 → 2CH3COONa + CO2 + H2O …and the ionic equation is: 2H⁺ + CO3²⁻ → CO2 + H2O
- This is a useful test to distinguish carboxylic acids from alcohols and phenols. This is because alcohols and phenols are not acidic enough to react with these.
Converting carboxylic acids to acyl chlorides
-
Acyl chlorides have the -OH in the -COOH group of the carboxylic acid replaced by chlorine.
-
ethanoic acid produce ethanoyl chloride.
- Acyl chlorides are very reactive and can be used to produce a range of other organic compounds.
There are three methods to produce acyl chlorides from carboxylic acids:
i. Reaction with phosphorus(III) chloride, PCl3.
Using ethanoic acid, the equation is: 3CH3COOH + PCl3 → 3CH3COCl + H3PO4
ii. Reaction with phosphorus(V) chloride, PCl5 eg ethanoic acid, the equation is:
CH3COOH + PCl5 → CH3COCl + POCl3 + HCl
iii. Reaction with thionyl chloride, SOCl2.
- Take ethanoic acid as an example, the equation is:
CH3COOH + SOCl2 → CH3COCl + SO2 + HCl
- This method is generally preferred because SO2 and HCl are both gases, and can be separated from the mixture easily.