haloalkanes

• Halogenoalkanes


  Alkanes with one or more halogen atoms (F, Cl, Br, I) attached
• Prefixes for halogenoalkanes


  • Fluoro for F
  • Chloro for Cl
  • Bromo for Br
  • Iodo for I
• Halogenoalkanes


  • 1-bromopropane
  • 2-chloro-2-methylbutane
• Halogenoalkanes


  • Can be classified as primary, secondary or tertiary depending on the number of carbon atoms attached to the C-X functional group
• Nucleophilic substitution reactions of halogenoalkanes

  1. Nucleophile attacks positive carbon

  2. Halogen leaves as X-

  3. New group bonds to carbon
• Nucleophile


  Electron pair donator e.g. :OH-, :NH3, CN-
• Halogenoalkanes undergo either substitution or elimination reactions
• Substitution

  Swapping a halogen atom for another atom or groups of atoms
• Strength of C-X bond

  Determines rate of substitution reaction
• Bond enthalpies of C-X bonds

  • C-I: 238 kJ/mol
  • C-Br: 276 kJ/mol
  • C-Cl: 338 kJ/mol
  • C-F: 484 kJ/mol
• Iodoalkanes are fastest to substitute, fluoroalkanes are slowest
• Hydrolysis


  Splitting of a molecule (halogenoalkane) by reaction with water
• Aqueous silver nitrate added to halogenoalkane

  Forms silver halide precipitate, rate depends on reactivity of halogenoalkane
• Silver halide precipitates


  • AgI (s) - yellow
  • AgBr(s) - cream
  • AgCl(s) - white
• Nucleophilic substitution with aqueous hydroxide ions

  Halogenoalkane + KOH -> Alcohol + KBr
• Aqueous conditions needed, if solvent changed to ethanol an elimination reaction occurs
• Alternative mechanism for tertiary halogenoalkanes

  1. Br first breaks away to form carbocation

  2. Hydroxide nucleophile then attacks positive carbon
• Nucleophilic substitution with cyanide ions

  Halogenoalkane + KCN -> Nitrile + KBr
• Nucleophilic substitution with ammonia

  Halogenoalkane + 2NH3 -> Amine + NH4Br
• Amines


  Compounds with the -NH2 group
• Nitriles


  Compounds with the -CN group
• Excess ammonia used to minimise further substitution reactions
• Elimination reaction with alcoholic hydroxide ions

  Halogenoalkane + KOH -> Alkene + KBr + H2O
• Primary halogenoalkanes tend towards substitution, tertiary tend towards elimination
• Often a mixture of substitution and elimination products
• Ozone in upper atmosphere

  • Beneficial as it filters out harmful UV radiation
• Ozone in lower atmosphere

  • Pollutant that contributes to smog formation
• CFCs caused hole in ozone layer
• Chlorine radical formation and ozone depletion

  1. UV causes C-Cl bond in CFCs to break, releasing Cl radicals

  2. Cl radicals catalyse decomposition of ozone
• Legislation to ban CFCs supported by chemists, HFCs developed as safer alternatives
• HFCs


  Hydrofluorocarbons, do not contain C-Cl bond
• 1. F bond is stronger than C-Cl and not affected by UV
• Uses of halogenoalkanes


  • Solvents
  • Refrigerants
  • Pesticides
  • Aerosol propellants
• Many uses of halogenoalkanes stopped due to toxicity and environmental impact