Nitriles and hydroxynitriles
Nitriles and hydroxynitriles
- Cyanide can add a carbon to a chain in two ways.
- A nitrile has the –CN group; a hydroxynitrile also has an –OH.
- Nitriles are a useful synthetic hub.
Two ways to add cyanide
- Nitrile: heat a halogenoalkane with KCN in ethanol — a nucleophilic substitution that adds one carbon:
$$\text{C}_2\text{H}_5\text{Br} + \text{KCN} \rightarrow \text{C}_2\text{H}_5\text{CN} + \text{KBr}$$
- Hydroxynitrile: add HCN (KCN catalyst) to an aldehyde/ketone — a nucleophilic addition across the C=O.
Practice
Heating a halogenoalkane with KCN in ethanol makes a nitrile, which usefully:
The CN group adds a carbon (a nucleophilic substitution); HCN with a carbonyl gives a hydroxynitrile instead.
Practice
Adding HCN to an aldehyde or ketone gives a hydroxynitrile by:
CN⁻ adds across the carbonyl; the O⁻ then takes an H⁺ — nucleophilic addition.
What nitriles become
- hydrolysis (warm with dilute acid) turns –CN into –COOH → a carboxylic acid:
$$\text{CH}_3\text{CN} + 2\text{H}_2\text{O} + \text{HCl} \rightarrow \text{CH}_3\text{COOH} + \text{NH}_4\text{Cl}$$
- reduction (H₂ + catalyst) turns a nitrile into an amine.
Practice
Hydrolysing a nitrile with dilute acid turns –CN into:
Hydrolysis converts the nitrile group to a carboxylic acid (with an ammonium salt by-product).
Practice
Reducing a nitrile (with H₂ and a catalyst) gives:
Reduction of a nitrile gives an amine — a route to a longer-chain amine.
You've got it
Key idea
- KCN + halogenoalkane → a nitrile (nucleophilic substitution, adds a carbon)
- HCN + carbonyl → a hydroxynitrile (nucleophilic addition)
- a nitrile hydrolyses to a carboxylic acid, or reduces to an amine