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6.4.1Additional lessons

Nucleophilic Substitution — IB Chemistry HL Revision Notes & Topical Questions

5 min read 4 parts Lesson 150 of 172

📚 IB Chemistry HL Hub / Topical Revision
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Curriculum Equation Key قوانين الرقم الهيدروجيني والاتزان الأيوني - pH & Acid-Base Equilibrium FormulasFast Exam Solution
pH = -log[H⁺] | pOH = -log[OH⁻] | pH + pOH = 14 | [H⁺][OH⁻] = 10⁻¹⁴
Word Equation & Conditions: الرقم الهيدروجيني = -لوغاريتم تركيز أيون الهيدروجين | pH + pOH = 14
Location & Specifications: الاتزان الأيوني، كيمياء 3 ثانوي، التحصيلي، و AP Chemistry.
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Part 1 of 4Nucleophiles and Halogenoalkanes

A nucleophile donates a pair of electrons to form a new covalent bond.

Nucleophile
  • Nucleophile A species with a lone pair of electrons that is attracted to a positively charged or partially positive atom.
Examples
  • Examples Common nucleophiles are OH⁻, CN⁻, NH₃ and H₂O, each of which has a lone pair.
Polar bond
  • Polar bond In a halogenoalkane the carbon–halogen bond is polar, so the carbon is slightly positive, δ+.
Substitution
  • Substitution The nucleophile replaces the halogen, which leaves as a halide ion.

Part 2 of 4Reaction with Hydroxide

1
The halogenoalkane is heated under reflux with aqueous sodium hydroxide.
2
The hydroxide ion uses a lone pair to attack the δ+ carbon, shown by a curly arrow from the lone pair to the carbon.
3
At the same time the C–Br bond breaks heterolytically, shown by an arrow from the bond to the bromine atom.
4
The products are an alcohol and a halide ion: CH₃CH₂Br + OH⁻ → CH₃CH₂OH + Br⁻.
5
This reaction can be followed by adding silver nitrate, which gives a cream precipitate of silver bromide.

Part 3 of 4Other Nucleophiles

Other Nucleophiles
Ammonia
  • Ammonia Excess ammonia in ethanol gives a primary amine: CH₃CH₂Br + 2NH₃ → CH₃CH₂NH₂ + NH₄Br.
Cyanide
  • Cyanide KCN in ethanol gives a nitrile, CH₃CH₂Br + CN⁻ → CH₃CH₂CN + Br⁻, which lengthens the carbon chain by one.
Water
  • Water Water is a weak nucleophile and reacts slowly to form an alcohol and hydrohalic acid.
Rate trend
  • Rate trend The C–I bond is the weakest, so iodoalkanes react fastest and chloroalkanes slowest.
NucleophileReagentProduct type
OH⁻NaOH(aq), refluxAlcohol
CN⁻KCN in ethanol, refluxNitrile
NH₃NH₃ in ethanol, sealed tubeAmine

Part 4 of 4Exam Checklist & Tips

6.4.1 – Key Points for the Exam

  • Nucleophile An electron-pair donor that attacks regions of low electron density.
  • Substitution A reaction in which one atom or group is replaced by another.
  • Leaving group The atom or group, such as Br⁻, that departs with the bonding pair.
  • Reflux Heating with a condenser to prevent loss of volatile substances.

Common Mistakes

  • Drawing the curly arrow from the carbon to the nucleophile, which is the wrong direction.
  • Omitting the lone pair on the attacking nucleophile in the mechanism.
  • Using ethanolic hydroxide, which favours elimination rather than substitution.
  • Saying nucleophiles attack electron-rich centres.

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