3.3 Haloalkanes

Halogenoalkanes are much more reactive than alkanes. They have many uses, including as
refrigerants, as solvents and in pharmaceuticals. The use of some halogenoalkanes has been
restricted due to the effect of chlorofluorocarbons (CFCs) on the atmosphere.

  • Nucleophilic substitution mechanism of haloalkanes - Haloalkanes contain polar bonds. Haloalkanes undergo substitution reactions with the nucleophiles OH–, CN– and NH3 Students should be able to: • understand and outline the mechanism of nucleophilic substitution in primary haloalkanes • explain why the carbon–halogen bond enthalpy influences the rate of reaction. Assessment and practical opportunities Students could follow instructions when carrying out test-tube hydrolysis of halogenoalkanes to show their relative rates of reaction. Students
  • Elimination mechanism of haloalkanes - The concurrent substitution and elimination reactions of a halogenoalkane (eg 2-bromopropane with potassium hydroxide). Students should be able to: • explain the role of the reagent as both nucleophile and base • outline the mechanisms of these reactions• appreciate the usefulness of this reaction in organic synthesis
  • Mechanism of ozone depletion by haloalkanes - Ozone, formed naturally in the upper atmosphere, is beneficial because it absorbs ultraviolet radiation. Chlorine atoms are formed in the upper atmosphere when ultraviolet radiation causes C–Cl bonds in chlorofluorocarbons (CFCs) to break. Chlorine atoms catalyse the decomposition of ozone and contribute to the hole in the ozone layer. Appreciate that results of research by different groups in the scientific community provided evidence for legislation to