Cyanide is one of the few reagents that makes a new carbon-carbon bond. React it with a halogenoalkane or a carbonyl compound and you get a nitrile, which can then be hydrolysed to a carboxylic acid or reduced to an amine.
A nitrile contains the –C≡N group, and its carbon counts in the name: CH3CH2CN is propanenitrile. Heating a halogenoalkane under reflux with KCN in ethanol substitutes CN− for the halogen, adding one carbon to the chain. Aldehydes and ketones react with HCN and a trace of KCN by nucleophilic addition to give 2-hydroxynitriles, such as CH3CH(OH)CN from ethanal. Nitriles are hydrolysed by heating under reflux with dilute acid to carboxylic acids, and reduced by H2/Ni or LiAlH4 to primary amines. These steps let you build a longer carbon chain and then turn it into the functional group you need.
React a halogenoalkane with KCN in ethanol in the simulator. Count the carbon atoms in the starting material and the product.
| Starting compound | Reagent and conditions | Product | Reaction type |
|---|---|---|---|
| bromoethane, CH3CH2Br | KCN in ethanol, heat under reflux | propanenitrile, CH3CH2CN | nucleophilic substitution |
| 1-bromobutane | KCN in ethanol, heat under reflux | pentanenitrile, C4H9CN | nucleophilic substitution |
| ethanal, CH3CHO | HCN with a trace of KCN, 10 to 20 °C | 2-hydroxypropanenitrile, CH3CH(OH)CN | nucleophilic addition |
| propanone, CH3COCH3 | HCN with a trace of KCN | 2-hydroxy-2-methylpropanenitrile, (CH3)2C(OH)CN | nucleophilic addition |
Four quick checks. Each correct answer earns XP and records this skill on your profile.
What is the name of CH3CH2CN?
Which reagent converts bromoethane into propanenitrile?
What type of reaction occurs between ethanal and HCN?
What is formed when propanenitrile is heated under reflux with dilute hydrochloric acid?
Add HCN to a carbonyl compound in the simulator and follow the curly arrows. Watch which face of the planar C=O group the cyanide ion attacks.
Pick a nitrile in the simulator and hydrolyse or reduce it, then use the reaction map in Fig. 1 to plan two-step routes. Each starts by adding a carbon with cyanide.
| Reaction | Reagent and conditions | Equation |
|---|---|---|
| hydrolysis | dilute HCl (or H2SO4), heat under reflux | CH3CH2CN + 2H2O + HCl → CH3CH2COOH + NH4Cl |
| reduction | H2 with Ni catalyst, or LiAlH4 in dry ether | CH3CH2CN + 4[H] → CH3CH2CH2NH2 |
| hydroxynitrile hydrolysis | dilute acid, heat under reflux | CH3CH(OH)CN → CH3CH(OH)COOH (2-hydroxypropanoic acid) |
A nitrile is named by counting the C of the C≡N group: CH3CH2CN is propanenitrile, not ethanenitrile. KCN must be in ethanol; in water you get an alcohol. HCN adds to C=O by nucleophilic addition, not electrophilic addition as for alkenes. Reducing a nitrile gives an amine with the same number of carbons as the nitrile, CH2NH2 at the end; hydrolysis turns the CN carbon into COOH and releases the nitrogen as NH4+.
Unlocks once the four checks above are done. Worth more XP, written to AS Paper 1 and Paper 2 standard.
Which sequence converts bromoethane into propylamine, CH3CH2CH2NH2?
Ethanal reacts with HCN and a trace of KCN. Why is the product optically inactive?
2.20 g of ethanal (Mr 44.0) gives 2.84 g of CH3CH(OH)CN (Mr 71.0). What is the percentage yield?
Which compound is formed by heating 2-hydroxypropanenitrile under reflux with dilute sulfuric acid?
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