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 alkyl or aryl groups Note: R groups may or may not be the same.  Nomenclature Halogenoalkanes are named in a similar manner to alkanes where the suffix ends in –ane and the halogens are treated as substituents and named as prefixes such as chloro–, bromo–, iodo–. Examples: (i) CH3CH2Br bromoethane (ii) 2–iodopropane (iii) 2–chloro–2–methylpropane Quick Check 1: Give the IUPAC name of the following compounds. (a) CH 3CH2CH(CH3)CH2Cl (b) CH3CH=CHCH2Br ( c )

4 2 Physical Properties 2.1 Boiling Point  Halogenoalkanes have a higher boiling point than alkanes with the same number of carbon atoms as there are permanent dipole–permanent dipole attractions between polar R X molecules and also stronger instantaneous dipole–induced dipole interactions due to the greater number of electrons present.  For a given alkyl group, boiling point increases with increasing A r of the halogen. i.e. boiling point of RCl < RBr < RI 2.2 Solubility  In spite of the C– X bond being polar, halogenoalkanes are insoluble in water due to their inability to form hydrogen bonds with water.  They are soluble in organic solvents whereby the energy released from solute–solvent interactions is sufficient to overcome the solute–solute and the solvent–solvent interactions. 3 Preparation of halogenoalkanes 3.1 Nucleophilic substitution of alcohols (using ethanol as an example)  CH 3CH2OH + PCl5 → CH 3CH2Cl + HCl + POCl3 Reagents and conditions: PCl5 Note: PCl5 provides the nucleophile, Cl―  3CH 3CH2OH + PX3 → 3CH3CH2X + H3PO3 ( X= Cl, Br) Reagents and conditions: PCl3 or PBr3, heat PX3 provides the nucleophile, X― Note:  Phosphorus triiodide, P I3 is very unstable and so it is prepared in situ (at site of reaction). The alcohol is heated with red P and I2 to produce P I3 in situ which then reacts with the alcohol to form the iodoalkane. 2P + 3I2 → 2PI3  CH 3CH2OH + SOCl2 → CH 3CH2Cl + HCl + SO2 Remember the by-product H3PO3 Remember the by-products HCl (steamy white fumes) and SO 2 Remember the by-products HCl (steamy white fumes) and POCl 3

5 Reagents and conditions: SOCl2, heat Note: SOCl2 provides the nucleophile, Cl―  CH 3CH2OH + HX → CH3CH2X + H2O (where X=Cl, Br, I) Reagents and conditions: HX(g), heat Note: For HCl(g), ZnCl2 catalyst is needed. 3.2 Electrophilic addition of hydrogen halides (H X) to alkenes Reagents and conditions: HX(g) Note: Markovnikov’s rule is followed. You are ready to attempt Qn 1 of tutorial. 4 Reactions of halogenoalkanes  C–X bond is polar due to the presence of the electronegative halogen atom. Hence, halogenoalkanes are chemically very reactive.  Halogenoalkanes undergo nucleophilic substitution as the electron–deficient C atom in the C–X bond carries a partial positive charge which enabled it to attract a nucleophile, leading to substitution of the halogen.  Halogenoalkanes also undergo elimination (using alcoholic OH–). 4.1 Nucleophilic substitution (a) Formation of alcohol CH 3CH2Br + OH– → CH3CH2OH + Br–  Nucleophile: OH –  Reagents and conditions: NaOH(aq) or KOH(aq), heat  This reaction is also known as alkaline hydrolysis. Learning objective: (a)(i) Remember the by-product H2O Remember the by-product Br– MUST specify the state symbol (aq)

6  For geminal–dihalides (where two halogen atoms are attached to the same carbon atom), it would form a germinal diol which would undergo spontaneous dehydration to form a carbonyl compound. CH3CH2CH Cl Cl NaOH(aq), heat CH3CH2CH unstable loss of water CH3CH2CH aldehyde OH OH O CH3CCH3 Cl Cl NaOH(aq), heat CH3CCH3 OH OH unstable loss of water CH3CCH3 O ketone  For geminal–trihalides (where three halogen atoms are attached to the same carbon atom), it would form a germinal trio l which would undergo spontaneous dehydration to form a carboxylic acid. CH3CH2C Cl Cl NaOH(aq), heat CH3CH2C unstable loss of water CH3CH2C carboxylic acid OH OH O Cl OH OH (b) Formation of nitrile CH3CH2Br + CN– → CH3CH2CN + Br–  Nucleophile: CN –  Reagent and condition: KCN in ethanol or ethanolic KCN, heat  This reaction can be applied to lengthen the carbon chain known as a step–up reaction.  Nitriles are useful intermediates in synthesis. They can be hydrolysed to carboxylic acids. Remember the by-product Br– MUST include the solvent eg ethanol

7 RCN + 2H2O + H+ → RCO2H + NH4+ Reagents and conditions: H2SO4(aq), heat  Nitriles can also be reduced to primary amines. RCN + 4[H] → RCH2NH2 Reagents and conditions: LiAlH4 in dry ether : H2, Ni catalyst, heat