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.

1. 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:

1. When nitrile is heated under reflux with a dilute acid such as dilute hydrochloric acid, carboxylic acid will be formed.
2. Using ethanenitrile as an example, ethanoic acid is formed. CH3CN + 2H2O + H⁺ → CH3COOH + NH4⁺

Alkaline hydrolysis of nitrile:

1. 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
2. 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.