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6.7.1Organic Chemistry and Analysis

Nitriles

5 min read 5 parts Lesson 23 of 33 Virtual lab

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Part 1 of 5Making Nitriles

6.7.1 Nitriles

Nitriles contain the –C≡N group and are named with the suffix -nitrile, with the nitrile carbon counted in the chain.

Nucleophilic substitution
  • A haloalkane is heated under reflux with ethanolic KCN, and the cyanide ion replaces the halogen.
Equation
  • CH₃CH₂Br + CN⁻ → CH₃CH₂CN + Br⁻; the product propanenitrile has one more carbon than bromoethane.
Alternative route
  • Aldehydes and ketones add HCN to give hydroxynitriles, which also lengthen the carbon chain by one.
Solvent
  • Ethanol is used rather than water so that hydroxide ions do not compete and form an alcohol.
Chain length
  • Counting carbons matters herepropanenitrile has three carbons in its chain, including the one in the –C≡N group.

Part 2 of 5Reactions of Nitriles

Reactions of Nitriles
Acid hydrolysis
  • Heating under reflux with dilute HCl gives the carboxylic acid: CH₃CH₂CN + 2H₂O + HCl → CH₃CH₂COOH + NH₄Cl.
Alkaline hydrolysis
  • Heating with NaOH(aq) gives the carboxylate salt and ammonia, and acidifying then gives the acid.
Reduction
  • LiAlH₄ in dry ether, or H₂ with a nickel catalyst, gives a primary amine: RCN + 4[H] → RCH₂NH₂.
Result
  • The same two-carbon nitrile can be converted to either a carboxylic acid or an amine, with one extra carbon.
Mechanism of formation
  • The cyanide ion attacks the δ+ carbon of the C–Br bond using its lone pair, and bromide leaves in a single step.
Starting materialReagentsProduct
BromoethaneKCN in ethanol, refluxPropanenitrile
PropanenitrileDilute HCl, refluxPropanoic acid
PropanenitrileLiAlH₄ in dry etherPropylamine

Part 3 of 5Lengthening a Carbon Chain

1
Start with bromoethane, a two-carbon haloalkane.
2
Reflux with ethanolic potassium cyanide to form propanenitrile, which has three carbons.
3
Hydrolyse with dilute acid to give propanoic acid for a carboxylic acid route.
4
Alternatively reduce with LiAlH₄ in dry ether to give propylamine as an amine route.
5
All cyanide reagents are highly toxic, so these steps are carried out in a fume cupboard with great care.

Part 4 of 5Worked Example

Question

  • 10.9 g of bromoethane (M = 109.0 g mol⁻¹) is refluxed with excess ethanolic KCN and gives propanenitrile (M = 55.1 g mol⁻¹) in 70% yield.
  • Calculate the mass of propanenitrile obtained.

  1. Moles of bromoethane = 10.9 ÷ 109.0 = 0.100 mol.
  2. The equation is 1 : 1, so the theoretical moles of propanenitrile = 0.100 mol.
  3. Theoretical mass = 0.100 × 55.1 = 5.51 g.
  4. Actual mass = 5.51 × 0.70 = 3.86 g.

Answer 3.86 g of propanenitrile

Part 5 of 5Exam Checklist & Tips

6.7.1 – Key Points for the Exam

  • Nitrile An organic compound containing the –C≡N group.
  • Nucleophilic substitution Replacement of a halogen by a nucleophile.
  • Hydrolysis Breakdown by reaction with water, catalysed by acid or alkali.
  • Reduction Addition of hydrogen, shown as [H] in organic equations.

Common Mistakes

  • Using aqueous KCN, which can give alcohol as well as nitrile.
  • Forgetting that the nitrile carbon is counted when naming the chain.
  • Reducing a nitrile with NaBH₄, which is not strong enough to do so.
  • Treating cyanide as safe because the reaction occurs in solution.
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