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6.4.1Additional lessons

Nucleophilic Substitution — IB Chemistry SL Revision Notes & Topical Questions

5 min read 6 parts Lesson 97 of 98

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Part 1 of 6Nucleophiles

6.4.1 Nucleophilic Substitution

Halogenoalkanes have a polar carbon–halogen bond because the halogen is more electronegative than carbon.

Definition
  • A nucleophile is an electron-rich species that donates a lone pair to form a new covalent bond.
Charged
  • Examples of charged nucleophiles are OH⁻, CN⁻ and Cl⁻, which carry lone pairs.
Neutral
  • Examples are H₂O, NH₃ and ROH, which have a lone pair on O or N.
Target
  • Nucleophiles attack the electron-deficient (δ+) carbon atom in a polar C–X bond.

Part 2 of 6Reactions of Halogenoalkanes

Reactions of Halogenoalkanes
With aqueous alkali
  • CH₃CH₂Br + OH⁻ → CH₃CH₂OH + Br⁻ when heated under reflux with aqueous NaOH, giving an alcohol.
With ammonia
  • CH₃CH₂Br + 2NH₃ → CH₃CH₂NH₂ + NH₄Br in excess ammonia, giving a primary amine.
With cyanide
  • CH₃CH₂Br + CN⁻ → CH₃CH₂CN + Br⁻, giving a nitrile and lengthening the chain by one carbon.
Leaving group
  • The halide ion that departs, here Br⁻, is called the leaving group.
NucleophileProduct from 1-bromopropane
OH⁻Propan-1-ol
NH₃Propylamine (propan-1-amine)
CN⁻Butanenitrile

Part 3 of 6Mechanism in Words

1
The nucleophile is attracted to the δ+ carbon atom of the polar C–X bond.
2
Its lone pair forms a new bond to carbon, shown with a curly arrow from the lone pair to carbon.
3
At the same time the C–X bond breaks and both electrons go to the halogen, forming the halide ion.
4
The halide is lost as the leaving group, and the nucleophile has replaced the halogen.

Part 4 of 6Rate of Reaction

Bond strength
  • Reactivity increases from chloro- to bromo- to iodo- compounds because the C–X bond gets weaker.
Iodoalkanes
  • They react fastest, because the C–I bond is the weakest to break.
Chloroalkanes
  • They react slowest, with the stronger C–Cl bond.
Polarity
  • Polarity is less important because C–Cl is the most polar bond, yet it reacts the slowest.

Part 5 of 6Worked Example

Question Name the organic product when 1-bromopropane is heated under reflux with (a) aqueous NaOH and (b) KCN in ethanol.

  1. (a) OH⁻ replaces Br: CH₃CH₂CH₂Br + OH⁻ → CH₃CH₂CH₂OH + Br⁻.
  2. (b) CN⁻ replaces Br: CH₃CH₂CH₂Br + CN⁻ → CH₃CH₂CH₂CN + Br⁻.
  3. Count carbonsthe nitrile has four, one more than the starting material.

Answer (a) Propan-1-ol. (b) Butanenitrile.

Part 6 of 6Exam Checklist & Tips

6.4.1 – Key Points for the Exam

  • Nucleophile A species that donates a pair of electrons to form a covalent bond.
  • Substitution A reaction in which one atom or group replaces another.
  • Leaving group The atom or ion that departs with the bonding pair of electrons.
  • Halogenoalkane An alkane in which one or more hydrogen atoms are replaced by halogen atoms.

Common Mistakes

  • Starting the curly arrow from the positive carbon instead of the nucleophile's lone pair.
  • Saying the rate depends mainly on C–X polarity; it depends mainly on bond strength.
  • Using ethanolic KOH for substitution; aqueous alkali is used.
  • Naming the product of the cyanide reaction as a carboxylic acid.

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