ASRJC 2025 H2 Chem Halogen Derivatives Notes

2025 JC2 H2 Halogen Derivatives 2025/ASRJC/Chemistry 1 Anderson Serangoon Junior College JC2 H2 Chemistry HALOGEN DERIVATIVES 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) SN1, in terms of stability of the carbocation intermediates (ii) SN2, 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 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 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] References 1. Chemistry for Advanced Level by P. Cann & P. Hughes 2. Organic Chemistry (6th edition) by McMurry 3. A−Level Chemistry by E.N. Ramsden 4. Understanding Chemistry for Advanced Level by Ted Lister and Janet Renshaw 5. Chemistry in Context by Graham Hill and John Holman

2025 JC2 H2 Halogen Derivatives 2025/ASRJC/Chemistry 2 1. INTRODUCTION • Halogen derivatives are a homologous serie s of organic compounds containing one or more halogen atoms. • General formula: CnH2n+1X (where X = F, Cl, Br or I). • They are important intermediates in organic synthesis. • 2 types: halogenoalkanes and halogenoarenes Halogenoalkanes (or alkyl halides) Halogenoarenes (or aryl halides) R−X where R is an alkyl group such as −CH3, −CH2CH3 X X is a substituent in the benzene ring X is a halogen atom, an element from Group 17 e.g. F, Cl, Br, I. Checkpoint 1: Circle the correct classification for the following compound. CH2Br Alkyl halide Aryl halide Note: The C atom in the −CH 2Br group, being the immediate carbon bonded to the benzene ring, is known as a benzyl carbon. • Halogenoalkanes are classified as primary (1º), secondary (2º) or tertiary (3º), depending on the number of R groups/carbon atoms attached to the carbon bonded to the halogen. Methyl (1o) Primary (1o) Secondary (2o) Tertiary (3o) No R group 1 R group 2 R groups 3 R groups C H H H X C H H XR C H XR R' C XR R' R'' Note: the groups R, R’ and R’’ may or may not be the same. or Ar-X

2025 JC2 H2 Halogen Derivatives 2025/ASRJC/Chemistry 3 2. NOMENCLATURE • The halogen atom is considered as a substituent. • Prefixes (fluoro−, chloro−, bromo− and iodo−) are used together with numbers, where necessary, to show the position of the halogen atom in the molecule. CH3CH(Br)CH3 chloroethane 2−bromopropane chlorobenzene • If there are 2 or more halogen atoms of the same kind, prefixes (di−, tri−, tetra−, etc.) are used to indicate the number of halogen atoms present. 1,2−dichloroethane 1,1,2−trichloropropane 1,2,4−trichlorobenzene • When a compound contains 2 or more different halogen atoms the substituents are numbered in alphabetical order. 3−chloro−2−iodohexane Checkpoint 2: Write the structural formula and IUPAC names of the following compounds. 2−chloro−2−methylpropane 2,4−dichloromethylbenzene (bromomethyl)benzene/ benzyl bromide

2025 JC2 H2 Halogen Derivatives 2025/ASRJC/Chemistry 4 3. PHYSICAL PROPERTIES OF HALOGENOALKANES AND HALOGENOARENES 3.1 Boiling Point 1. Halogenoalkanes have higher boiling points (lower volatility) than alkanes of the same number of carbon atoms e.g. Compound CH3CH2CH3 CH3CH2CH2Br Boiling point/ K 231 344 There is stronger permanent dipole – permanent dipole attractions between polar CH3CH2CH2Br molecules. In addition, the g reater number of electrons / greater size of the electron cloud results in greater ease of distortion of electron cloud in CH3CH2CH2Br. Hence there is stronger instantaneous dipole−induced dipole attractions (id-id) between CH3CH2CH2Br molecules. More energy is required to overcome the stronger intermolecular forces between CH3CH2CH2Br than the weaker id-id attractions between CH3CH2CH3. 2. Boiling point of halogenoalkanes in a homologous series increases with the length of the carbon chain e.g. Compound CH3Cl CH3CH2Cl CH3CH2CH2Cl Boiling point/ K 249 285 320 As the length of the alkyl chain increases, the number of electrons / electron cloud size increases, which results in a greater ease of distortion of the electron cloud . More energy is required to overcome the stronger instantaneous dipole –induced dipole attractions between the molecules. 3. Boiling point increases down the halogen group in halogenoalkanes and halogenoarenes e.g. Compound CH3CH2F CH3CH2Cl CH3CH2Br CH3CH2I Boiling point/ K 235 285 312 345 e.g. Compound C6H5F C6H5Cl C6H5Br C6H5I Boiling point/ oC 85 132 156 189 The number of electrons / electron cloud size increases , which results in a greater ease of distortion of the electron cloud . More energy is required to overcome the stronger instantaneous dipole–induced dipole attractions between the molecules.

2025 JC2 H2 Halogen Derivatives 2025/ASRJC/Chemistry 5 4. For structural isomers, the order of the boiling point is 1o > 2o > 3o halogenoalkanes e.g. 1o 2o 3o Compound CH3CH2CH2CH2Br CH3CH2CHBrCH3 C(CH3)3Br Boiling point/ K 375 364 346 The surface area for contact between molecules decreases as the shapes of the molecules change from elongated (1o) to more spherical