Halogenoalkanes

• A halogen takes the place of a H atom in an alkane
• The general formula is CnH2n+1X, where n= number of carbon atoms and x = a halogen
• Bpt increases down the group

  Halogen is larger

  Has more electrons

  Stronger vdw forces
• Reactivity increases down the group
• Nucleophile

  • Has a lone pair of electrons on an electronegative atom
  • They are negatively charged
  • They use their lone pair of electrons to form a new bond with a positively charged atom
• CH3Cl + OH -> CH3OH + Cl-
• CH3Cl + CN -> CH3CN + Cl-
• CH3Cl + 2NH3 -> CH3NH2 + NH4Cl
• Halogenolalkane -> alcohol

  Reagent: KOH/ NaOH

  Conditions: in aqueous solution, heat under reflux

  Mechanism: Nucleophilic substitution
• Halogenoalkane -> nitrile 


  Reagent: KCN dissolved in ethanol/ water mixture

  Conditions: Heating under reflux

  Mechanism: Nucleophilic substitution
• Halogenoalkane -> amine

  Reagent: NH3 dissolved in ethanol

  Conditions: Heating under pressure in a sealed tube

  Mechanism: Nucleophilic substitution
• Elimination: removal of small molecule (often water) from the organic molecule
• Halogenoalkane -> alkene


  Reagent: K/NaOH

  type of reagent: base

  Conditions: in ethanol (heat)

  Mechanism: Elimination
• Halogenoalkanes have been used as refrigerants, pesticides and aerosol propellants
• Chloroalkanes and chlorofluoroalkanes can be used as solvents
• CH3CCl3 was used as the solvent in dry cleaning.
• Many uses have now been stopped due to the toxicity of halogenoalkanes and also their detrimental effect on the atmosphere.
• The naturally occurring ozone (O3 ) layer in the upper atmosphere is beneficial as it filters out much of the sun’s harmful UV radiation
• Chlorine radicals are formed in the upper atmosphere when energy from ultra-violet radiation causes C–Cl bonds in chlorofluorocarbons (CFCs) to break.

  CF2Cl2 -> CF2Cl ' + Cl'
• The chlorine free radical atoms catalyse the decomposition of ozone, due to these reactions, because they are regenerated. (They provide an alternative route with a lower activation energy)
• The regenerated Cl radical means that one Cl radical could destroy many thousands of ozone molecules.
• the weaker the bond...

  the easier it is to break, the faster the reaction
• why are HFCs used?

  safer as they do not contain C-Cl bond

  • The C-F bond is stronger than C-Cl bond and is not affected by UV