Halogen Derivatives

The term “halogen derivatives” refer to the organic compounds of halogens which usually contain covalently-bonded halogens. The electronegativity of elements plays a crucial role in deciding which get to bond with halogens and which don’t. The similarity in electronegativity values of H and C render the latter to be as effective as the former to bind halogens.

Course Contents

  • Halogenoalkanes
  • (i) nucleophilic substitution
  • (ii) elimination
  • Relative strength of the C-Hal bond
  • Unreactivity of halogenoarenes

Learning Outcomes

Candidates should be able to:

  • (a) recall the chemistry of halogenoalkanes as exemplified by:
  • (i) the following nucleophilic substitution reactions of bromoethane: hydrolysis; formation of nitriles; formation of primary amines by reaction with ammonia
  • (ii) the elimination of hydrogen bromide from 2-bromopropane
  • (b) describe and explain the mechanisms of nucleophilic substitutions in halogenoalkanes:
  • (i) SN 1, in terms of stability of the carbocation intermediates
  • (ii) SN 2, in terms of steric hindrance in the halogenoalkanes
  • (c) explain the stereochemical outcome in nucleophilic substitution involving optically active substrates:
  • (i) inversion of configuration in SN 2 mechanism
  • (ii) racemisation in SN 1 mechanism
  • (d) interpret the different reactivities of halogenoalkanes, with particular reference to hydrolysis, and to the relative strengths of the C-Hal bonds
  • (e) explain the unreactivity of chlorobenzene compared to halogenoalkanes towards nucleophilic substitution, in terms of the delocalisation of the lone pair of electrons on the halogen and steric hindrance
  • (f) suggest characteristic reactions to differentiate between:
  • (i) different halogenoalkanes
  • (ii) halogenoalkanes and halogenoarenes e.g. hydrolysis, followed by testing of the halide ions
  • (g) explain the uses of fluoroalkanes and fluorohalogenoalkanes in terms of their relative chemical inertness
  • (h) recognise the concern about the effect of chlorofluoroalkanes (CFCs) on the ozone layer (the mechanistic details of how CFCs deplete the ozone layer are not required)