3.2halogenoalkanes

• 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

  • 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 the positive carbon atom

  2. The rate depends on the strength of the C-X bond - the weaker the bond, the faster the reaction
• Fluoroalkanes are very unreactive due to the strength of the C-F bond
• As the C-X bond gets weaker


  The rate of substitution reaction increases (I > Br > Cl > F)
• Hydrolysis of halogenoalkanes

  1. Splitting of halogenoalkane by reaction with water

  2. Aqueous silver nitrate test can be used to compare reactivity
• Nucleophilic substitution with aqueous hydroxide ions

  Halogenoalkane + KOH -> Alcohol + KBr

  Mechanism: Nucleophilic substitution
• Nucleophilic substitution with cyanide ions

  Halogenoalkane + KCN -> Nitrile + KBr

  Mechanism: Nucleophilic substitution
• Nucleophilic substitution with ammonia

  Halogenoalkane + 2NH3 -> Amine + NH4Br

  Mechanism: Nucleophilic substitution
• Nitriles

  Functional group with a C≡N bond, must be at the end of the carbon chain
• Elimination reaction of halogenoalkanes

  Halogenoalkane + KOH (in ethanol) -> Alkene + KX + H2O

  Mechanism: Elimination
• Primary halogenoalkanes tend towards substitution, tertiary tend towards elimination
• Ozone

  • Naturally occurring ozone layer in upper atmosphere filters out harmful UV radiation
• Ozone depletion by CFCs

  CFCs break down in upper atmosphere to release Cl radicals

  Cl radicals catalyse decomposition of ozone

  Legislation banned CFCs, HFCs are now used instead
• Uses of halogenoalkanes

  • Solvents

  Refrigerants

  Pesticides

  Aerosol propellants
• nucleophilic substitution (:NH3) what causes lower yields of amines

  • haloalkanes can react with the amine formed via substitution reaction which will lower the yeild of amines
  • one way to reduce this is adding excess ammonia
• nucleophilic substitution (OH) condition - reagent - product

  • heat under reflux
  • aqueous sodium/pottasium hydroxide
  • alcohol
• nucleophilic substitution (:CN) - condition , reagent , product

  heat under reflux , KCN dissolved in alcohol or water , nitriles
• nucleophilic substitution ( :NH3)- condition , reagent , product

  heat under pressure , excess ammonia , amines
• elimination reactions - condition , reagent , product

  conditions: heat,

  reagent : sodium or potassium hydroxide dissolved in ethanol (base) products : alkene
• during an elimination reaction , an unsymmetrical secondary or tertiary haloalkane will produce structural isomers
• how did the ozone decompose?

  through the use of chlorine radicals which act as a catalyst
• what are alternatives to chloroalkanes and chloroflouroakanes?

  hydroflourocarbon
• during a HBr electrophyllic addition, which carbon will the bromine join?

  the one with the least amount of carbons
• draw the mechanism for nucleophilic substitution of aqueous :OH-

  mechanism

  
• draw the nucleophilic substitution mechanism for potassiumcyanide ( KCN )

  nucleophilic substitution

  
• how are chlorine radicals made?


  when the uv radiation in the atmosphere causes the bonds between the c-cl bonds to break
• why are HFCs better than CFCs?


  the c-f bond is stronger than the c-cl bond so wont be broken by uv rays
• write out the reactions of the destruction of the ozone?


  reaction