9701 · 19.2
Nitriles and hydroxynitriles
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.
Need to know
What you need to know
- 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.
Explanation
Carbon Chain Builders
- 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.
- 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).
- 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.
- 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.