Curly Arrow Mechanisms for A-Level Chemistry

May 10, 2025

Summary of Curly Arrow Mechanisms for AQA A-Level Chemistry

Introduction

  • Overview of curly arrow mechanisms in organic chemistry as per AQA A-level Chemistry specification.
  • Focus on drawing mechanisms for exam marks, not on reagents or catalysts.

Nucleophilic Substitution

  • Starting Point: Haloalkane (e.g., bromine or chlorine).
  • Process: Nucleophile attacks the delta positive carbon attached to the halogen, substituting halogen for the nucleophile.
  • Key Nucleophiles:
    • Hydroxide ion (OH⁻)
    • Cyanide ion (CN⁻)
    • Ammonia (NH₃)
  • Mechanism Insights:
    • Curly arrows represent electron pair movement.
    • Formation of dative covalent bonds with lone pair donation.
    • Leaving group: the halogen, forming a bromide or chloride ion.

Hydroxide Ion

  • Formation of an alcohol and a bromide ion.

Cyanide Ion

  • Similar mechanism to hydroxide, forming a nitrile.

Ammonia

  • Intermediate stage forming an ammonium ion and eventually primary amine.
  • Possibility of subsequent nucleophilic substitutions forming secondary and tertiary amines, and quaternary ammonium salts.

Elimination Reactions

  • Starting Point: Haloalkanes with hydroxide ions.
  • Process: Formation of an alkene by attacking a hydrogen, forming a double bond, and removing the halogen as a leaving group.
  • Product: Alkene and water.

Electrophilic Addition (Alkenes)

  • Electrophiles:
    • Diatomic halogens (e.g., Cl₂, Br₂)
    • Hydrogen halides (e.g., HBr)
    • Sulfuric acid (H₂SO₄)

Diatomic Halogens

  • Formation of dihaloalkanes.

Hydrogen Halides

  • Formation of haloalkanes.

Sulfuric Acid

  • Formation of alkyl hydrogen sulfate.

Acid-Catalyzed Addition (Hydration of Ethene)

  • Catalyst: Phosphoric acid (H₃PO₄).
  • Process: Formation of alcohol.

Alcohols to Alkenes (Elimination)

  • Mechanism: Acid-catalyzed elimination forming alkenes.
  • Catalyst: Sulfuric or phosphoric acid.

Nucleophilic Addition (Ketones and Aldehydes)

  • Nucleophiles:
    • Hydride ion (H⁻)
    • Cyanide ion (CN⁻)
  • Product: Alcohols or hydroxy nitriles.
  • Reagents: Strong acids for cyanide reactions.

Nucleophilic Addition-Elimination (Acyl Chlorides)

  • Nucleophiles:
    • Water, Alcohol, Ammonia, Primary Amine
  • Products: Carboxylic acids, esters, primary amides, secondary amides.

Electrophilic Substitution (Benzene)

  • Reactions:
    • Nitration (NO₂⁺ ion)
    • Friedel-Crafts Acylation (RCO⁺ ion)
  • Products: Nitrobenzene, phenyl ketones.

Conclusion

  • Practice mechanisms through questions to understand better than mere memorization.
  • Focus on reagents, conditions, and equations, especially regarding catalysts.
  • Best of luck with revision and exams.