Halogeno- compounds

Nucleophile substitution reaction

- The C-X bond is polar. The carbon atom in the C-X bond acts as an electron deficient centre (or a

   positive site) which is susceptible to the attack by a nucleophile.

- A nucleophile may be an anion or neutral molecule which has lone pair of electrons. In nucleophilic

   substitution, X is represented by the nucleophile which donates a lone-pair of electrons to a partial

   positive carbon atom.

Comparison of rate of hydrolysis of haloalkanes and halobenzene

- The effect of AgNO3 (aq) on bromine-containing compounds:

- Halobenzene is unreactive towards nucleophilic substitution reaction because

  (a) high electron density of benzene ring repels the approaching nucleophile

  (b) the lone pair electrons of halogen atom interacts with the pi-electrons of the benzene ring; the

       carbon-halogen bond carries some double bond character.

Uses of halogeno-compounds (97 essay question - 6 marks)

- as solvents and thinners:

  chlorofluorocarbons are used as solvent for cleaning of electronic compounds and metals.

  1,1,1-trichloroethane: thinner for correcting fluids.

- as aerosol propellants:

  CFCs are normally packed with materials such as paint in pressurized aerosol can. Upon

  decompression,  the liquid vapourize and expel the materials inside the can in the form of aersol

  spray.

- as fire extinguishing agents:

  Bromochlorodifluoromethane and bromotrifluoromethane form a dense layer of vapour over fire.

  Some advantages includes non-flammable, low in toxicity, very unreactive, denser than air and

  vapourizes easily.

  Some disadvantages includes decomposition on heating to form highly toxic and corrosive vapours.

- as refrigerants:

  Freon is non-flammable, low in toxicity, inert and absorbs heat of vapourization on evapourization

  resulting in cooling of the surroundings.

- as raw materials for producting plastics

- as blowing agents in manufacture of foam plastics.

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