Topic 10b: Halogenoalkanes - Damaged Chemistry

Introduction

• Focus on halogenoalkanes
• Review past papers and learning outcomes
• Address nomenclature, classification, reactions, mechanisms, and hydrolysis rates
• Core Practicals: 5 and 6 (details not covered in this summary)

Halogenoalkanes Overview

• Homologous series with general formula: $C_nH_{2n+1}X$
  • X = Halogen (F, Cl, Br, I)
• Not hydrocarbons due to presence of halogen
• Nomenclature similar to branched alkanes and alkenes
  • Halogens listed alphabetically
  • Example: 1-bromo-3-chloropropane

Classification of Halogenoalkanes

• Based on alkyl groups or hydrogens attached
  • Primary: One alkyl group
  • Secondary: Two alkyl groups
  • Tertiary: Three alkyl groups

Reactivity of Halogenoalkanes

• Hydrocarbons: Nonpolar, less reactive (similar electronegativities: 2.5, 2.2)
• Halogenoalkanes: Polar bonds due to halogen electronegativity
  • Carbon becomes electron-deficient; attracts nucleophiles

Hydrolysis Reaction

• Splitting molecules using water
• Results in alcohol and hydrogen halide
• Determines reactivity of halogenoalkanes
  • Core Practical 5
• Silver nitrate test to observe reactions

Factors Affecting Reactivity

• Bond enthalpy differences explain reactivity
  • C-I has weakest bond; reacts fastest
  • C-F bond is strong, reacts slowly
• Structure (primary, secondary, tertiary) affects reactivity
  • Tertiary reacts fastest; primary slowest

Key Reactions of Halogenoalkanes

1. Hydrolysis: Already covered
2. Nucleophilic Substitution
   • Heat with aqueous KOH under reflux
   • Forms alcohols
   • Mechanism: Nucleophile attacks electron-deficient carbon, halogen leaves as ion
3. Nitrile Formation
   • Heat with KCN in ethanol
   • Forms nitriles; lengthens carbon chains
4. Formation of Primary Amines
   • Heat with ammonia under pressure
   • Mechanism: Protonated amine formation, avoids salt formation
5. Elimination Reaction
   • Use ethanolic KOH
   • Forms alkenes by eliminating hydrogen and halogen

Past Paper Questions

• Properties of 2-chlorobutane

  • Solubility: Insoluble in water
  • Reacts with charged rods due to polar bonds

• Elimination Products of 2-Bromobutane

  • Form multiple products due to different sites of hydrogen removal

• Preparation and Properties of Iodobutane

  • Balance equations using phosphorus iodide and butanol
  • Consider structural isomers

• Reaction with Silver Nitrate

  • Observes precipitate formation indicative of reaction completion

• Reaction with Ammonia

  • Forms butylamine and ammonium salts

Concluding Remarks

• Emphasized understanding of mechanisms and reactivity
• Encouraged review of the organic section for deeper understanding and preparation