VJC 2025 Lecture Notes for RX

1 VICTORIA JUNIOR COLLEGE CHEMISTRY DEPARTMENT Halogen Derivatives (Halogenoalkanes and Halogenoarenes) Lesson outline (I) Halogenoalkanes 1 Introduction 2 Physical Properties 2.1 Boiling point 2.2 Solubility 3 Laboratory Preparation 3.1 From alcohols 3.2 From alkenes 4 Reactions 4.1 Nucleophilic substitution 4.2 Elimination 5 Effect of halogen atoms 5.1 Distinguishing tests for RX 5.2 Effect of different halogen atoms on reactivity of halogenoalkanes 6 Nucleophilic Substitution Mechanism 7 Fluoroalkanes and Fluorohalogenoalkanes 7.1 Uses 7.2 Effect of CFCs on the Ozone Layer (II) Halogenoarenes 1 Introduction 2 Preparation 3 Chemical Properties 3.1 Reactions involving halogen atom 3.2 Reactions involving benzene ring Lesson 1 2 Pages 1 – 8 9 – 18 Complete by 09/02/25 09/02/25 Tutorial Qns 1 – 4 5 – 10

2 Learning Objectives Students should be able to: (a) recall the chemistry of halogenoalkanes as exemplified by: (i) the following nucleophilic substitution reac tions 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) S N1, in terms of stability of the carbocation intermediates (ii) S N2, in terms of steric hindrance of the halogenoalkanes (c) explain the stereochemical outcome in nucleophilic substitution involving optically active substrates: (i) inversion of configuration in S N2 mechanism (ii) racemisation in S N1 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 delocal isation 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]

3 I HALOGENOALKANES 1 Introduction  General formula: CnH2n+1X  Halogenoalkanes are also known as alkyl halides.  They can be classified as primary, secondary or tertiary halogenoalkanes depending on the number of alkyl or aryl groups attached to the C atom of the C–X bond. Type of halogenoalkane Primary Secondary Tertiary Structure Carbon bonded to halogen X is bonded to 1 alkyl or aryl group 2 alkyl or aryl groups 3