Acid hydrolysis of nitriles converts the –C≡N functional group into a carboxylic acid, providing a key route for carbon-chain extension and functional group transformation in synthetic chemistry.

Acid Hydrolysis of Nitriles

Acid hydrolysis of nitriles is an essential transformation in organic synthesis, allowing the conversion of the nitrile functional group into a carboxylic acid. According to the OCR specification, nitriles are hydrolysed under acidic conditions to form carboxylic acids, making this reaction a crucial step in many multi-stage synthetic pathways. The reaction is particularly valuable because nitriles themselves are formed through chain-extending reactions, meaning hydrolysis enables the incorporation of an additional carbon into the resulting acid.

Introducing the Nitrile Group

A nitrile contains a carbon–nitrogen triple bond, written as –C≡N, which is strongly polar and reactive towards hydrolysis. The carbon atom is electrophilic, allowing water or hydroxonium ions to attack during hydrolysis.

Hydrolysis under acidic conditions provides a controlled and predictable route to form a carboxylic acid, ensuring full conversion of intermediate species.

This scheme summarises the conversion of a nitrile (R–C≡N) into a carboxylic acid (R–COOH) using aqueous acid and heat under reflux, highlighting the key conditions required by the OCR specification. Source

Mechanism Overview of Acid Hydrolysis

The hydrolysis proceeds in several steps, ultimately replacing the nitrile’s nitrogen with an oxygen-containing functional group. Although students do not need to memorise every mechanistic arrow, understanding the key transformations improves conceptual clarity.

Stepwise Process of Acid Hydrolysis

Under acidic conditions, nitriles convert into carboxylic acids through the following progression:

• Protonation of the nitrile nitrogen increases electrophilicity of the carbon.

• Nucleophilic attack by water, forming an imidic acid intermediate.

• Tautomerisation to form an amide.

• Further acid-catalysed hydrolysis of the amide to produce a carboxylic acid and an ammonium ion.

The formation of an amide intermediate is important conceptually, as it demonstrates that nitrile hydrolysis proceeds through two linked hydrolyses: nitrile → amide → carboxylic acid.

Conditions Required for Acid Hydrolysis

Acid hydrolysis requires prolonged heating and strongly acidic media to ensure complete conversion, as the nitrile group is relatively resistant to attack compared with other functional groups.

Essential Conditions

• Concentrated dilute strong acid, typically hydrochloric acid or sulfuric acid

• Reflux to maintain consistent reaction temperature

This figure illustrates a standard reflux setup, emphasising correct condenser water flow (in at the bottom, out at the top) to maintain efficient cooling during prolonged heating; the incorrect clamping example provides additional practical context beyond the syllabus. Source

• Excess water to drive hydrolysis to completion

• Sufficient time for conversion of both the nitrile and the intermediate amide

Products of Acid Hydrolysis

The final organic product is the carboxylic acid, while the nitrogen atom is released as ammonium ions (NH4⁺) when hydrochloric acid is used.

This equation illustrates the stoichiometric requirement for water and acidic protons, but real reactions often use excess acid to ensure complete hydrolysis.

Why Acid Hydrolysis Is Important in Organic Synthesis

Nitriles are widely used to lengthen carbon chains, and their hydrolysis is a direct route to generating new carboxylic acids with an additional carbon atom. This transformation is often used in synthetic design, especially when routes require strategic functional group interconversions.

Synthetic Utility

• Enables formation of carboxylic acids from haloalkanes via nitrile intermediates

• Provides access to acids without using strong oxidising agents

• Creates versatile intermediates that undergo further reactions such as esterification or reduction

• Supports multi-step synthesis planning by linking chain extension to subsequent functional group transformations

Chain extension followed by hydrolysis is an essential strategy in assembling more complex molecules, particularly in pathways outlined in organic synthesis sections of the specification.

Acid Hydrolysis vs Base Hydrolysis (Brief Context)

Although this subsubtopic focuses on acid hydrolysis, understanding its distinction from base hydrolysis strengthens comprehension. Acid hydrolysis yields a carboxylic acid directly, whereas base hydrolysis forms a carboxylate salt, requiring further acidification to produce the free acid. This difference highlights the practical and mechanistic importance of acidic conditions.

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Key Features to Remember

• Acid hydrolysis converts a nitrile (–C≡N) into a carboxylic acid (–COOH).

• Reaction requires acid, heat under reflux, and excess water.

• Process proceeds through an amide intermediate before full hydrolysis.

• Nitrogen is released as ammonium ions in strongly acidic solutions.

• This reaction is central to carbon-chain extension and synthetic design across the OCR A-Level Chemistry specification.