Halogen Derivatives

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Halogen Derivatives

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● 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]