Nitriles and hydroxynitriles

· Nitrile functional group = –C≡N.
· General formula: R–C≡N.
· The carbon of –C≡N counts as part of the main carbon chain.
· Naming: use the suffix –nitrile, e.g. CH₃CH₂CN = propanenitrile.
· Nitriles are important in synthesis because they can be converted into carboxylic acids.

This diagram shows the nitrile functional group, where carbon is triple-bonded to nitrogen. The R group represents an alkyl group. In exam structures, make sure the carbon of –C≡N is included in the carbon chain. Source

Making nitriles from halogenoalkanes

· Reagent: KCN.
· Conditions: ethanol and heat.
· Reaction type: nucleophilic substitution.
· General equation: R–X + KCN → R–CN + KX.
· CN⁻ replaces the halogen atom X.
· Important exam point: the product has one more carbon atom than the original halogenoalkane.
· Example: CH₃CH₂Br + KCN → CH₃CH₂CN + KBr.
· Product name: propanenitrile, not ethanenitrile, because the nitrile carbon is counted.

Making hydroxynitriles from aldehydes and ketones

· Reagents: HCN with KCN as catalyst.
· Conditions: heat.
· Starting compounds: aldehydes or ketones.
· Product: hydroxynitrile.
· Reaction type: addition across the C=O bond.
· General change: C=O → C(OH)(CN).
· The OH and CN groups attach to the original carbonyl carbon.
· Example from an aldehyde: CH₃CHO + HCN → CH₃CH(OH)CN.
· Example from a ketone: CH₃COCH₃ + HCN → (CH₃)₂C(OH)CN.
· Exam tip: draw the product by replacing the C=O with C–OH and adding –CN to the same carbon.

This diagram shows the overall formation of a hydroxynitrile from a carbonyl compound. The key structural change is that –OH and –CN both become attached to the carbonyl carbon. This is the product pattern students must recognise in CIE exam questions. Source

Hydrolysis of nitriles

· Nitriles hydrolyse to form carboxylic acids.
· Reagents/conditions option 1: dilute acid and heat/reflux.
· Reagents/conditions option 2: dilute alkali, then acidification.
· General conversion: R–CN → R–COOH.
· With dilute acid, the final organic product is directly a carboxylic acid.
· With dilute alkali, a carboxylate salt forms first; acidification is needed to produce the carboxylic acid.
· Example: CH₃CH₂CN → CH₃CH₂COOH.
· Name change: propanenitrile → propanoic acid.
· Exam tip: hydrolysis of nitriles keeps the same carbon chain length because the nitrile carbon becomes the carboxyl carbon.

Key reaction summary

· Halogenoalkane → nitrile: use KCN, ethanol, heat.
· Aldehyde/ketone → hydroxynitrile: use HCN, KCN catalyst, heat.
· Nitrile → carboxylic acid: use dilute acid or dilute alkali followed by acidification.
· R–X → R–CN adds one carbon to the chain.
· R–CN → R–COOH converts the nitrile carbon into the carboxyl carbon.
· In equations, show –CN bonded through carbon, not nitrogen.

Common exam traps

· Do not name CH₃CH₂CN as ethanenitrile; it is propanenitrile.
· Do not forget ethanol and heat for making nitriles from halogenoalkanes.
· Do not write only alkali hydrolysis for nitriles without acidification if the final product required is a carboxylic acid.
· Do not place OH and CN on different carbons when forming a hydroxynitrile; both attach to the original carbonyl carbon.
· Do not confuse cyanide ion, CN⁻, with the nitrile functional group, –C≡N.