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5.6.3Unit 5. Transition Metals and Organic Nitrogen Chemistry

Increasing the Carbon Chain Length — IAL Edexcel Linear (Chemistry) Revision Notes & Topical Questions

5 min read 5 parts Lesson 40 of 43 Unit quiz

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Part 1 of 5Nitrile Route

5.6.3 Increasing the Carbon Chain Length

Cyanide introduces a new carbon–carbon bond.

Reaction
  • A halogenoalkane is heated under reflux with potassium cyanide dissolved in ethanol, giving a nitrile with one extra carbon.
Equation
  • CH₃CH₂Br + KCN → CH₃CH₂CN + KBr, so bromoethane becomes propanenitrile.
Mechanism type
  • The cyanide ion acts as a nucleophile and attacks the δ+ carbon, displacing the halide in nucleophilic substitution.
Why ethanol
  • Using an ethanolic solvent prevents the halogenoalkane being converted to an alcohol by water.
StepReagentProduct from CH₃CH₂Br
1KCN in ethanol, refluxCH₃CH₂CN
2aDilute HCl, refluxCH₃CH₂COOH
2bLiAlH₄ in dry etherCH₃CH₂CH₂NH₂

Part 2 of 5Cyanohydrins

1
Aldehydes and ketones react with hydrogen cyanide, usually made in situ from KCN and dilute acid, to add a carbon.
2
The cyanide ion attacks the partially positive carbon of the C=O group in nucleophilic addition.
3
The intermediate anion then picks up a proton from water or acid, giving a hydroxynitrile.
4
For ethanal the product is 2-hydroxypropanenitrile, CH₃CH(OH)CN.
5
The planar carbonyl is attacked from either side equally, so a racemic mixture of two enantiomers forms.

Part 3 of 5Friedel–Crafts Reactions

Friedel–Crafts Reactions
Catalyst
  • Anhydrous aluminium chloride, AlCl₃, accepts a lone pair from the reagent and generates the electrophile.
Alkylation
  • Benzene with a chloroalkane such as CH₃Cl and AlCl₃ gives an alkylbenzene such as methylbenzene.
Acylation
  • Benzene with an acyl chloride such as CH₃COCl and AlCl₃ gives a phenyl ketone, phenylethanone.
Conditions
  • The reaction needs anhydrous conditions because water would destroy the catalyst.

Part 4 of 5Using the New Carbon

Hydrolysis of nitriles
  • Heating a nitrile with dilute acid converts –CN into –COOH, giving a carboxylic acid with the extra carbon.
Reduction of nitriles
  • Lithium aluminium hydride converts –CN into –CH₂NH₂, making a primary amine.
Hydroxynitrile hydrolysis
  • Acid hydrolysis of the cyanohydrin gives a 2-hydroxy acid such as lactic acid.
Safety
  • Cyanide is highly toxic, so these steps are carried out in a fume cupboard with careful waste disposal.

Part 5 of 5Exam Checklist & Tips

5.6.3 – Key Points for the Exam

  • Nitrile A compound containing the –C≡N group.
  • Nucleophile An electron-pair donor that attacks a partially positive carbon.
  • Hydroxynitrile A compound with both –OH and –CN on the same or adjacent carbons.
  • Friedel–Crafts reaction Electrophilic substitution that attaches an alkyl or acyl group to benzene using AlCl₃.

Common Mistakes

  • Using aqueous KCN, which gives some alcohol by substitution with water instead of the nitrile.
  • Forgetting that the nitrile has one more carbon than the halogenoalkane.
  • Describing the cyanohydrin reaction as substitution rather than nucleophilic addition.
  • Using moist AlCl₃ or wet solvent in a Friedel–Crafts reaction.

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