Nitriles and Hydroxynitriles

2026 Syllabus Objectives

By the end of this topic, you should be able to:

  1. Recall the reactions by which nitriles can be produced: (a) reaction of a halogenoalkane with KCN in ethanol and heat
  2. Recall the reactions by which hydroxynitriles can be produced: (a) the reaction of aldehydes and ketones with HCN, KCN as catalyst, and heat
  3. Describe the hydrolysis of nitriles with dilute acid or dilute alkali followed by acidification to produce a carboxylic acid

Main Content

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1. What are Nitriles?

Nitriles are organic compounds that contain a -CN functional group. A functional group is a specific group of atoms that gives a molecule its characteristic chemical properties.

The -CN group consists of a carbon atom triple-bonded to a nitrogen atom (C≡N). This is called a cyanide group or nitrile group.

Example: Propanenitrile has the structure CH₃-CH₂-C≡N

  • The first two carbons are part of a normal carbon chain
  • The third carbon is the nitrile carbon, triple-bonded to nitrogen
  • So there are 3 carbons in total

2. Production of Nitriles from Halogenoalkanes

Nitriles can be made by reacting a halogenoalkane with potassium cyanide (KCN) dissolved in ethanol, with heating.

What is a halogenoalkane?
A halogenoalkane is an organic molecule where one or more hydrogen atoms have been replaced by a halogen atom (like chlorine, bromine, or iodine). For example, bromoethane (CH₃CH₂Br) has a bromine atom attached.

The Reaction

Reagent: Potassium cyanide (KCN) dissolved in ethanol
Conditions: Heat under reflux (heating with a condenser so nothing escapes)
Product: A nitrile

General equation:

Halogenoalkane + KCN (in ethanol) → Nitrile + Potassium halide

Specific example:

CH₃CH₂Br + KCN → CH₃CH₂CN + KBr
Bromoethane         Propanenitrile

How the Reaction Works

This is a nucleophilic substitution reaction. Let's break down what that means:

  • A nucleophile is a species (atom or molecule) that has a lone pair of electrons and can donate them to form a new bond. It's attracted to positive charges.
  • In this reaction, the cyanide ion (CN⁻) is the nucleophile
  • The CN⁻ ion attacks the carbon atom that's bonded to the halogen (because this carbon has a slight positive charge)
  • The halogen (Br, Cl, or I) leaves, and the CN group takes its place

Important point: The negative charge on the cyanide ion is actually on the carbon atom of CN⁻, not on the nitrogen. So the carbon of the cyanide attacks and forms the new bond.

Why Use Ethanol as the Solvent?

If you use water (aqueous KCN) instead of ethanol, you might get an alcohol formed instead of a nitrile. Using ethanol ensures you get the nitrile product you want.

Why This Reaction is Useful

This reaction adds an extra carbon atom to the carbon chain.

In the example above:

  • Bromoethane has 2 carbon atoms
  • Propanenitrile has 3 carbon atoms

This is very useful for chemists who want to build larger molecules from smaller starting materials.

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