12.4 Hydrolysis of Nitriles

This document details the hydrolysis of nitriles, a significant organic reaction that converts nitriles into carboxylic acids or their salts using either dilute acid or dilute alkali.

Nitriles are organic compounds containing the cyano group ($-C\equiv N$). The hydrolysis of nitriles involves the breaking of the carbon-nitrogen triple bond through reaction with water, typically under acidic or alkaline conditions. This method is particularly useful in organic synthesis because it allows for the extension of a carbon chain by one carbon atom (when starting from an alkyl halide).

1. Acid Hydrolysis of Nitriles

When a nitrile is heated with a dilute acid, such as dilute hydrochloric acid ($HCl$) or dilute sulfuric acid ($H_{2}S{O}_4$), it undergoes hydrolysis to form a carboxylic acid. The acid acts as a catalyst and a reagent to protonate intermediates.

• Reagents: Dilute acid (e.g., $HC{l}_{(aq)}$ or $H_{2}S{O}_{4(aq)}$)
• Conditions: Heated
• Product: Carboxylic acid

General Reaction:

$\mathrm{R}-\mathrm{CN}+2{\mathrm{H}}_2\mathrm{O}+{\mathrm{H}}^{+}\stackrel{\text{heat}}{\to }\mathrm{R}-\mathrm{COOH}+{\mathrm{NH}}_4^{+}$

During the reaction, an amide is formed as an intermediate, which then further hydrolyzes to the carboxylic acid.

Example: Hydrolysis of Ethanenitrile

${\mathrm{CH}}_3{\mathrm{CN}}_{(\mathrm{aq})}+2{\mathrm{H}}_2{\mathrm{O}}_{(\mathrm{l})}+{\mathrm{HCl}}_{(\mathrm{aq})}⟶{\mathrm{CH}}_3{\mathrm{COOH}}_{(\mathrm{aq})}+{\mathrm{NH}}_4{\mathrm{Cl}}_{(\mathrm{aq})}$

• Reactant: Ethanenitrile ($C{H}_{3}CN$)
• Product: Ethanoic acid ($C{H}_{3}COOH$) and ammonium chloride ($N{H}_{4}Cl$)

2. Alkaline Hydrolysis of Nitriles

When a nitrile is heated with a dilute sodium hydroxide ($NaOH$) solution, an initial product, the salt of a carboxylic acid, is produced. This salt is then acidified with dilute hydrochloric acid ($HCl$) to yield the free carboxylic acid. This is a two-step process.

• Step 1: Reaction with alkali

  • Reagents: Dilute sodium hydroxide solution ($NaO{H}_{(aq)}$)
  • Conditions: Heated
  • Product: Salt of carboxylic acid (e.g., sodium carboxylate)

• Step 2: Acidification

  • Reagents: Dilute hydrochloric acid ($HC{l}_{(aq)}$) or another dilute acid
  • Conditions: Room temperature (usually)
  • Product: Free carboxylic acid

General Reaction Scheme:

Step 1: Formation of carboxylate salt

$\mathrm{R}-\mathrm{CN}+{\mathrm{NaOH}}_{(\mathrm{aq})}/{\mathrm{H}}_2\mathrm{O}\stackrel{\text{heat}}{\to }{\mathrm{R}-\mathrm{COONa}}_{(\mathrm{aq})}+{\mathrm{NH}}_3$

Note: Ammonia gas is typically evolved during this step.

Step 2: Acidification of carboxylate salt

${\mathrm{R}-\mathrm{COONa}}_{(\mathrm{aq})}+{\mathrm{H}}_{(\mathrm{aq})}^{+}⟶{\mathrm{R}-\mathrm{COOH}}_{(\mathrm{aq})}+{\mathrm{Na}}_{(\mathrm{aq})}^{+}$

Or, using $HCl$:

${\mathrm{R}-\mathrm{COONa}}_{(\mathrm{aq})}+{\mathrm{HCl}}_{(\mathrm{aq})}⟶{\mathrm{R}-\mathrm{COOH}}_{(\mathrm{aq})}+{\mathrm{NaCl}}_{(\mathrm{aq})}$