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4.5.2Core Organic Chemistry

Nucleophilic Substitution of Haloalkanes — OCR A Level Chemistry A H432 Revision Notes & Topical Questions

5 min read 5 parts Lesson 21 of 29

📚 OCR A Level Chemistry A H432 Hub / Topical Revision
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Part 1 of 5The Mechanism

4.5.2 Nucleophilic Substitution of Haloalkanes
1
The nucleophile uses a lone pair of electrons to approach the δ+ carbon atom, shown with a curly arrow from the lone pair to the carbon.
2
As the new bond to the nucleophile forms, the carbon-halogen bond breaks heterolytically.
3
A second curly arrow goes from the C–X bond to the halogen atom, which leaves as a halide ion carrying the bonding pair.
4
For a primary haloalkane the two events happen together in one step, via a transition state, without forming an intermediate.
5
The product contains the nucleophile in place of the halogen, and the halide ion is released.

Part 2 of 5Reagents and Products

Aqueous NaOH
  • Warming with aqueous sodium hydroxide gives an alcohol, such as propan-1-ol from 1-bromopropane.
Ethanolic KCN
  • Heating under reflux with potassium cyanide dissolved in ethanol gives a nitrile with one more carbon atom.
Excess NH₃
  • Heating with excess ammonia in ethanol in a sealed tube gives a primary amine.
Why sealed
  • Ammonia is a gas, so a sealed tube prevents loss of ammonia and allows the mixture to be heated safely.
Excess ammonia
  • A large excess makes it less likely that the amine formed will substitute again to give secondary amines.
NucleophileReagentOrganic product from 1-bromopropane
OH⁻NaOH(aq), heatPropan-1-ol
CN⁻KCN in ethanol, refluxButanenitrile
NH₃Excess NH₃ in ethanol, heatPropylamine

Part 3 of 5Hydroxide Substitution Equation

Hydroxide Substitution Equation
Full equation
  • CH₃CH₂Br + NaOH → CH₃CH₂OH + NaBr, or ionic: CH₃CH₂Br + OH⁻ → CH₃CH₂OH + Br⁻.
Arrow one
  • The first curly arrow begins at a lone pair on oxygen of OH⁻ and ends at the δ+ carbon.
Arrow two
  • The second arrow begins at the C–Br bond and ends at the bromine atom, giving Br⁻.
Dipole labels
  • Mark the δ+ on carbon and δ− on bromine in the haloalkane before drawing the arrows.

Part 4 of 5Using the Reactions in Synthesis

Alcohol route
  • Hydroxide substitution turns a haloalkane into an alcohol, which can then be oxidised.
Chain extension
  • Cyanide substitution is valuable because it adds a carbon atom, building a longer carbon skeleton.
Nitrile follow-up
  • The nitrile group can later be converted to an amine or carboxylic acid, increasing the range of products.
Amine formation
  • Ammonia substitution is a direct way to make a primary amine from a haloalkane.

Part 5 of 5Exam Checklist & Tips

4.5.2 – Key Points for the Exam

  • Nucleophilic substitution Replacement of a halogen by a nucleophile through attack on the δ+ carbon.
  • Nitrile A compound containing the –C≡N functional group.
  • Lone pair A pair of outer electrons not used in bonding, available for forming a new bond.
  • Leaving group The atom or ion, here the halide, that departs with the bonding pair of electrons.

Common Mistakes

  • Starting the curly arrow from the negative charge of OH⁻ instead of from the lone pair on oxygen.
  • Drawing the second arrow from the carbon instead of from the C–X bond.
  • Omitting ethanol as the solvent when reacting with cyanide or ammonia.
  • Forgetting that the halide ion leaves with the pair of electrons and carries a negative charge.

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