SAJC 2026 Halogen Derivatives Lecture Notes (Students)

St Andrew’s Junior College JC2 H2 Chemistry 2026 1 St Andrew’s Junior College H2 Chemistry 2026 Lecture Notes 13 Halogen Derivatives Assessment Objectives: Candidates should be able to: (a) recall the chemistry of halogenoalkanes as exemplified by (i) the following nucleophilic substitution reactions of bromoethane: • hydrolysis using NaOH(aq) and heat • formation of nitriles using KCN in ethanol and heat • formation of primary amines by reaction with ammonia in ethanol heated under pressure (ii) the elimination of hydrogen bromide from 2-bromopropane using NaOH in ethanol and heat (b) describe and explain the mechanisms of nucleophilic substitutions in halogenoalkanes: (i) SN1, in terms of stability of the carbocation intermediates (ii) SN2, in terms of steric hindrance in the halogenoalkanes (c) explain the stereochemical outcome in nucleophilic substitution involving optically active substrates (see also (b)): (i) inversion of configuration in SN2 mechanism (ii) racemisation in SN1 mechanism (d) interpret the different reactivities of halogenoalkanes, with particular reference to hydrolysis, and to the relative strengths of the carbon-halogen 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 halogenalkanes (see also (d)) (ii) halogenalkanes and halogenoarenes (see also (e)) e.g. hydrolysis, followed by testing the halide ions (g) explain the uses of fluoroalkanes and fluorohalogenoalkanes in terms of their relative chemical inertness (h) recognise the effect of chlorofluoroalkanes (CFCs) on the ozone layer, and that their proposed replacements, hydrofluoroalkanes (HFCs) and hydrochlorofluoroalkanes (HCFCs), have significant environmental impact too [the mechanistic details of how CFCs and HCFCs deplete the ozone layer are not required]

St Andrew’s Junior College JC2 H2 Chemistry 2026 2 Lecture Outline: 1. Introduction 2. Physical properties 3. Preparation 4. Reactions of halogenoalkanes • Nucleophilic substitution • Elimination 5. Halogenoarenes 6. Distinguishing tests 7. Applications (self read) 8. Appendix References: 1. Chemistry for advanced level by Peter Cann & Peter Hughes 2. ‘A’ Level Chemistry by E N Ramsden 3. Chemistry in Context by G C Hill & J S Holman Useful websites: 1. http://www.chemguide.co.uk/mechanisms/nucsubmenu.html 2. MOE SLS (requires SLS login) Lesson 1: Introduction to Common Reactions of Halogenoalkanes Lesson 2: Nucleophilic Substitution of Halogenoalkanes Lesson 3: Stereochemical Outcomes of Nucleophilic Substitution Reaction of Halogenoalkan