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.