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 Halogenoalkanes Lesson 4: Reactivity of Halogenoalkanes, Halogenoarenes and Acyl Chlorides Lesson 5: Applications of Halogenoalkanes Lesson 1 Lesson 2 SN1 and SN2 Lesson 3 Animations of the stereochemical outcomes of SN1 and SN2 Lesson 4 Lesson 5

St Andrew’s Junior College JC2 H2 Chemistry 2026 3 1. INTRODUCTION Halogen derivatives contain a polar C−X bond , where X is a halogen atom (Group 17 – F, Cl, Br or I). Halogenoalkanes (alkyl halides) Halogenoarenes (aryl halides) • Halogen derivative of an alkane • R – X, where R is an alkyl group • General formula: CnH2n+1X • Halogen derivative of an arene • Ar – X, where Ar is an aryl group • Halogen atom is attached directly to the aromatic ring Examples CH3Cl chloromethane CH3CH2Br bromoethane CH3CH2CH2I 1-iodopropane Examples Br I CH3 Cl bromobenzene iodobenzene 3-chloromethylbenzene Is chloromethylbenzene (benzyl chloride) a halogenoalkane or halogenoarene? It is a halogenoalkane. It is not a halogenoarene as the halogen is not directly attached to the aromatic ring. Halogenoalkanes can be classified as primary, secondary or tertiary depending on the number of R groups attached to the C atom bonded to the halogen. Primary (1) RX 1 R group Secondary (2) RX 2 R groups Tertiary (3) RX 3 R groups C X H R H C X R' R H C X R' R R'' chloroethane 2-chloropropane 2-chloro-2-methylpropane

St Andrew’s Junior College JC2 H2 Chemistry 2026 4 2. PHYSICAL PROPERTIES 2.1 Boiling Points Comparison between halogenoalkanes and alkanes Halogenoalkanes have higher boiling points than the corresponding alkanes of the same carbon chain length. E.g. Compound Boiling Point / ○C CH3Cl −24 CH4 −162 • CH4 has simple molecular structure with instantaneous dipole-induced dipole ( id–id) interactions between molecules. • CH3Cl has simple molecular structure with permanent dipole-permanent dipole (pd–pd) interactions between molecules. • More energy is required to overcome the stronger pd-pd interactions, CH 3Cl has higher boiling point than CH4. Furthermore, the id–id between CH3Cl molecules are stronger than that of CH 4 due to the greater number of electrons / larger electron cloud size. Comparison between halogen derivatives with similar number of C atoms For halogenoalkanes with the same alkyl group but different halogen atoms, the boiling points generally increase down Group 17. E.g. Compound Boiling Point / ○C CH3CH2F −37.1 CH3CH2Cl 12.5 CH3CH2Br 38 CH3CH2I 72 • Number of electrons increases from CH3CH2F < CH3CH2Cl < CH3CH2Br < CH3CH2I • Strength of instantaneous dipole-induced dipole (id–id) interactions between molecules increases from CH3CH2F < CH 3CH2Cl < CH 3CH2Br < CH 3CH2I (due to more polarizable electron cloud) • More energy required to overcome the stronger id -id interactions, boiling point increases from CH3CH2F < CH3CH2Cl < CH3CH2Br < CH3CH2I

St Andrew’s Junior College JC2 H2 Chemistry 2026 5 2.2 Solubility Since halogenoalkanes and halogenoarenes are polar molecules, they usually dissolve in polar organic solvents, such as ethanol and propanone , as the energy released on formation of permanent dipole – permanent dipole (pd-pd) interactions between the solute and solvent is sufficient to overcome the pd-pd interactions between the solute molecules and pd-pd interactions between the solvent molecules. However, halogenoalkanes and halogenoarenes are largely insoluble in water as the energy released on formation of pd-pd interactions between the halogen derivatives and water molecules is insufficient to overcome the (stronger) hydrogen bonding between water molecules and pd-pd interactions between halogenoalkane molecules. 3. PREPARATION OF HALOGENOALKANES AND HALOGENOARENES 3.1 Nucleophilic substitution of alcohols (Refer to hydroxy compound notes) Halogenoalkanes are mainly prepared from alcohols. The hydroxyl ( –OH) group is substituted by the halogen atom. • Using phosphorus chloride, PCl5 General equation: R−OH + PCl5 ⎯→ R−Cl + POCl3 + HCl (white fumes) • Using HX(g) (where X = Cl, Br or I) General equation: R−OH + HX ⎯→ R−Cl + H2O Refer to Appendix 1 for more details on the generation of HX. 3.2 Electrophilic addition of alkenes (Refer to alkene notes) • Using HX (g) • Using X2 in CCl4

St Andrew’s Junior College JC2 H2 Chemistry 2026 6 3.3 Free radical substitution of alkanes (Refer to alkanes notes) • Using limited X2(g) in UV light General equation: R−H + X2 ⎯→ R−X + HX FRS is not a recommended method to obtain a specific halogenoalkane as a mixture