Nitriles and hydroxynitriles

AS Level Chemistry · Nitrogen compounds. A short explanation of the idea, the rules to remember, the mistake to avoid, a worked example and practice questions with answers.

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The idea

A nitrile contains the –C≡N group. It is made by heating a halogenoalkane with KCN in ethanol, a nucleophilic substitution. A hydroxynitrile has –OH and –CN on the same carbon atom. It is made from an aldehyde or ketone and HCN, with KCN as catalyst, by nucleophilic addition.

Both reactions add one carbon atom to the chain, which makes them useful in synthesis. The carbon of the CN group is counted in the name, so CH3CH2CN is propanenitrile.

A nitrile is hydrolysed to a carboxylic acid by heating with dilute acid. With dilute alkali the carboxylate salt forms first and must then be acidified. The –CN group becomes –COOH and the nitrogen leaves as ammonia or an ammonium salt.

Rules to remember

Common mistake

Students forget the extra carbon atom. Bromoethane has two carbon atoms but the nitrile made from it has three, so it is propanenitrile, and its hydrolysis gives propanoic acid, not ethanoic acid.

Worked example

Butanal, CH3CH2CH2CHO, reacts with HCN (KCN catalyst). The product is then heated under reflux with dilute sulfuric acid. Name the final organic product.

  1. HCN adds across the C=O bond: H goes to the oxygen and CN to the carbon, giving CH3CH2CH2CH(OH)CN.
  2. This has five carbon atoms: it is 2-hydroxypentanenitrile.
  3. Acid hydrolysis changes –CN into –COOH and leaves the –OH group unchanged.

Answer: 2-hydroxypentanoic acid, CH3CH2CH2CH(OH)COOH.

Practice questions

Try each one, then open the answer.

1. Which reagents and conditions convert 1-bromopropane into butanenitrile?

  1. A
    HCN with a KCN catalyst at room temperature
  2. B
    KCN in ethanol, heat under reflux
  3. C
    NaOH in water, heat under reflux
  4. D
    NH3 in ethanol, heat under pressure
Show answer

Answer: B. CN− from KCN substitutes for Br, adding one carbon to give CH3CH2CH2CN. Ethanol is the solvent because in water hydroxide substitution would compete.

2. 1-Chlorobutane is heated under reflux with potassium cyanide in ethanol. What is the name of the organic product?

  1. A
    butanenitrile
  2. B
    butylamine
  3. C
    pentan-1-ol
  4. D
    pentanenitrile
Show answer

Answer: D. The CN group adds one carbon to the four-carbon chain, and the nitrile carbon is counted in the name: CH3CH2CH2CH2CN is pentanenitrile.

3. Why is a small amount of KCN added when HCN reacts with a carbonyl compound?

  1. A
    KCN removes the water formed in the reaction
  2. B
    KCN acts as an oxidising agent
  3. C
    HCN is a weak acid, so KCN supplies the CN− ions that act as the nucleophile
  4. D
    KCN reacts with the carbonyl compound to form an ester
Show answer

Answer: C. HCN is only slightly dissociated, so there are very few CN− ions. KCN provides CN− to attack the δ+ carbonyl carbon, and CN− is regenerated in the second step, so it acts as a catalyst.

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