In simple terms
A friendly intro before the formal notes — no formulas yet.
Carbon Chain Builders
Nitriles and hydroxynitriles are your go-to tools for adding a single carbon atom to an organic molecule. These reactions typically involve a cyanide ion acting as a nucleophile to create a new carbon-carbon bond.
Think of building a train. Your starting molecule is the main engine and carriages. To make the train longer, you need to add a new carriage. The cyanide ion (CN⁻) is that new carriage, and the reaction conditions (like reflux or an acid catalyst) are the coupling mechanism that securely attaches it, extending your molecular 'train' by one unit.
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A halogenoalkane reacts with potassium cyanide (KCN) in ethanol under reflux. The cyanide ion (CN⁻) acts as a nucleophile, replacing the halogen to form a nitrile.
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Nitriles can be hydrolysed by heating with dilute acid (H⁺/H₂O). This breaks the C≡N triple bond and converts the group into a carboxylic acid (–COOH).
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An aldehyde or ketone reacts with hydrogen cyanide (HCN). The CN⁻ nucleophile adds across the C=O double bond, forming a hydroxynitrile which has both an –OH and a –CN group.
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The formation of nitriles and hydroxynitriles is a vital synthetic step. It extends the carbon chain, allowing for the subsequent creation of other functional groups like carboxylic acids or amines.
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Key formulas
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Full topic notes
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Formation of Nitriles from Halogenoalkanes
Nitriles can be synthesised from primary halogenoalkanes by heating them with a solution of potassium cyanide (KCN) in ethanol. This is a classic example of a nucleophilic substitution reaction. The cyanide ion, :C≡N⁻, is a potent nucleophile which attacks the electron-deficient (δ+) carbon atom of the C-Halogen bond, displacing the halide ion.
Reaction Type: Nucleophilic Substitution.
Reagent: Potassium cyanide, KCN.
Solvent: Ethanol. This is crucial as it minimises the competing elimination reaction that would be favoured in an aqueous solution where CN⁻ can also act as a base.
Conditions: Heat under reflux. This allows the reaction to be heated for a prolonged period without losing volatile reactants or solvent.
Outcome: The carbon chain is extended by one carbon atom.
Reactions of Nitriles
Once formed, the nitrile group is a versatile intermediate that can be converted into other functional groups. The two key reactions you need to know are hydrolysis and reduction.
1. Hydrolysis to Carboxylic Acids
Nitriles can be hydrolysed to form carboxylic acids by heating them under reflux with a dilute aqueous acid, such as hydrochloric acid or sulfuric acid. During this reaction, the C≡N triple bond is broken, and the carbon atom becomes part of a carboxyl group (–COOH), while the nitrogen atom forms an ammonium ion ().
Alternatively, alkaline hydrolysis can be performed by heating the nitrile with an aqueous alkali like sodium hydroxide. This initially produces a carboxylate salt (e.g., R-COO⁻Na⁺) and ammonia gas. To obtain the final carboxylic acid, the resulting solution must be acidified with a strong acid.
2. Reduction to Primary Amines
Nitriles can be reduced to primary amines using a strong reducing agent. The standard reagent for this at A-level is lithium tetrahydridoaluminate (LiAlH₄) in a dry ether solvent, followed by addition of water (aqueous workup). The reaction adds four hydrogen atoms across the C≡N triple bond.
For exam purposes, when asked for the reagents for the reduction of a nitrile, stating 'LiAlH₄ in dry ether' is sufficient. The '[H]' in the equation represents the hydrogen atoms supplied by the reducing agent. Remember this reaction also extends the carbon chain and produces a primary amine.
Formation of Hydroxynitriles from Carbonyls
Aldehydes and ketones undergo nucleophilic addition with hydrogen cyanide (HCN) to form hydroxynitriles (also known as cyanohydrins). These molecules contain both a hydroxyl (–OH) group and a nitrile (–CN) group on the same carbon. Due to the high toxicity of HCN gas, it is generated in situ by reacting sodium or potassium cyanide with a dilute acid, such as H₂SO₄.
Reaction Type: Nucleophilic Addition.
Reagents: NaCN(aq) or KCN(aq) with dilute H₂SO₄(aq).
Mechanism: The :CN⁻ nucleophile attacks the δ+ carbonyl carbon, and the intermediate O⁻ is protonated by H⁺ from the acid or HCN.
Product: A hydroxynitrile.
Stereochemistry: The reaction with aldehydes (except methanal) and unsymmetrical ketones produces a racemic mixture of enantiomers, as the planar carbonyl group can be attacked from either side with equal probability.
Worked examples
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Devise a two-step synthesis for butanoic acid starting from 1-bromopropane. For each step, state the reagents, conditions, and write a balanced equation.
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Step 1: Formation of butanenitrile from 1-bromopropane.
Ethanal reacts with acidified potassium cyanide. Name the organic product and outline the mechanism for its formation.
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Product Name: 2-hydroxypropanenitrile.
How it all connects
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Glossary
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Revision flashcards
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What is the functional group of a nitrile?
The –C≡N group, where a carbon atom is triple-bonded to a nitrogen atom.
Key takeaways
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Reaction Type: Nucleophilic Substitution.
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Reagent: Potassium cyanide, KCN.
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Solvent: Ethanol. This is crucial as it minimises the competing elimination reaction that would be favoured in an aqueous solution where CN⁻ can also act as a base.
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Conditions: Heat under reflux. This allows the reaction to be heated for a prolonged period without losing volatile reactants or solvent.
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Outcome: The carbon chain is extended by one carbon atom.
Practice — then mark it
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Practice Questions: Nitriles and Hydroxynitriles
Practice Questions: Nitriles and Hydroxynitriles
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