AQA A-Level Organic Chemistry Curly Arrow Mechanisms

Introduction

• Focus on drawing mechanisms for marks.
• Not covering reagents or catalysts in detail.

Nucleophilic Substitution

Haloalkanes with Nucleophiles

• Start with haloalkane (bromine or chlorine).
• Nucleophiles:
  • Hydroxide (OH-)
  • Cyanide (CN-)
  • Ammonia (NH3)
• Mechanism:
  • Nucleophile attacks delta positive carbon.
  • Halogen leaves as a halide ion.
  • Curly arrows show electron pair movement.

Hydroxide Ion Example

• OH- attacks carbon.
• Halogen leaves, forming an alcohol and halide ion.

Cyanide Ion Example

• Similar mechanism.
• CN- replaces halogen, forming a nitrile.

Ammonia Example

• NH3 attacks carbon.
• Intermediate ammonium ion formed.
• Final product: primary amine and ammonium halide.
• Subsequent reactions can form secondary, tertiary amines, and quaternary ammonium salts.

Elimination Reactions

Haloalkanes with Hydroxide Ion

• Hydroxide ion attacks hydrogen.
• Double bond forms, creating an alkene.
• Halogen leaves as halide ion.

Electrophilic Addition

Alkenes

• Electrophiles:
  • Diatomic Halogens (e.g., Cl2, Br2)
  • Hydrogen Halides (e.g., HBr, HCl)
  • Sulfuric Acid (H2SO4)
• Mechanism:
  • Electrophile attracted to electron-rich C=C bond.
  • Formation of carbocation intermediate.
  • Final product: halogenoalkane or derivative.

Specific Examples

• Diatomic Halogen: Forms dihaloalkanes.
• Hydrogen Halide: Forms haloalkanes.
• Sulfuric Acid: Forms alkyl hydrogen sulfate.

Hydration of Ethene

• Acid catalyzed with H3PO4.
• Water adds across double bond.
• Final product: alcohol.

Alcohol Elimination

• Objective: Form alkene from alcohol.
• Acid-catalyzed elimination mechanism.
• Final product: alkene and water.

Nucleophilic Addition

Ketones and Aldehydes

• Nucleophiles:
  • Hydride Ion (H-)
  • Cyanide Ion (CN-)
• Mechanism:
  • Nucleophile attacks carbonyl carbon.
  • Electron pair moves to oxygen.
  • Final products: alcohols for hydride, hydroxy nitriles for cyanide.

Nucleophilic Addition-Elimination

• Acyclic Chlorides
  • Four nucleophiles: water, alcohols, ammonia, primary amines.
• Mechanism:
  • Nucleophile attacks acylic carbon.
  • Chlorine leaves as chloride ion.
  • Final products vary with nucleophile: carboxylic acids, esters, primary/secondary amides.

Electrophilic Substitution

Aromatic Compounds

• Reactions:
  • Nitration with nitronium ion (NO2+).
  • Friedel-Crafts Acylation with RCO+.
• Mechanism:
  • Electrophile attacks benzene ring.
  • Intermediate carbocation stabilized by electron donation.
  • Final product reforming electron-dense ring.

Conclusion

• Practice questions over memorization.
• Understand reagents, conditions, and catalysts.
• Best of luck with exams!