Chapter 6 — Haloalkanes and Haloarenes
Put a halogen on carbon and the molecule becomes a starting point for almost anything: alcohols, ethers, amines, alkenes, longer chains. How halogen compounds are made, how nucleophiles replace the halogen by SN1 or SN2, what that does to chirality, and why some of them harm the planet.
Look at the label of almost any medicine cabinet, refrigerator or garden shed and you will find a halogen compound. Chloroquine fights malaria; halothane and its successors made modern surgery possible; PVC carries our water and insulates our wires; Teflon lines our frying pans. The thyroid hormone in your body needs iodine on its benzene rings to work. Chemists value these compounds even more as intermediates: the polar carbon–halogen bond is a handle by which an alkyl group can be attached to oxygen, nitrogen, another carbon or a metal, so a single alkyl halide can lead to dozens of products.
This chapter explains how that handle works. You will learn to classify and name halogen compounds and to make them from alcohols, hydrocarbons and diazonium salts. Then comes the heart of the chapter: nucleophilic substitution, which goes by two different mechanisms, SN2 and SN1. The rate laws of Chapter 3 tell them apart, and so does their effect on chiral molecules, molecules that differ from their mirror images. You will see why bases sometimes remove HX to give alkenes instead, how magnesium turns a haloalkane into a Grignard reagent, why chlorobenzene shrugs off nucleophiles, and how CFCs and DDT, chemically "safe" compounds, turned out to damage the ozone layer and food chains.
| Question | Key idea | Section |
|---|---|---|
| What kind of halide is it? | carbon bearing X: $sp^{3}$ (1°, 2°, 3°, allylic, benzylic) or $sp^{2}$ (vinylic, aryl) | 6.1 |
| How is it made? | $\mathrm{ROH+SOCl_{2}\rightarrow RCl+SO_{2}+HCl}$ | 6.1 |
| How is the halogen replaced? | SN2: $\text{rate}=k[\mathrm{RX}][\mathrm{Nu^{-}}]$; SN1: $\text{rate}=k[\mathrm{RX}]$ | 6.2 |
| What happens at a stereocentre? | SN2 inversion; SN1 racemisation | 6.2 |
| Substitution or alkene? | alcoholic KOH, heat: Saytzeff alkene | 6.3 |
| Why are haloarenes unreactive? | partial C=X double bond by resonance | 6.3 |
- classify halogen compounds (1°, 2°, 3°, allylic, benzylic, vinylic, aryl; gem and vic), name them by IUPAC rules, and explain the polarity and strength of the C–X bond;
- prepare haloalkanes from alcohols, alkanes and alkenes and by halogen exchange (Finkelstein, Swarts), and haloarenes by electrophilic substitution and the Sandmeyer reaction; explain trends in physical properties;
- predict the products of nucleophilic substitution (including ambident nucleophiles), describe the SN2 and SN1 mechanisms with their rate laws, energy profiles and reactivity orders, and choose between them;
- recognise chiral molecules and stereocentres, explain enantiomers, racemic mixtures and optical rotation, and predict inversion or racemisation;
- predict elimination products with Saytzeff's rule, write Grignard, Wurtz, Wurtz–Fittig and Fittig reactions, explain the low reactivity and o/p-directing behaviour of haloarenes, and discuss the uses and environmental effects of polyhalogen compounds.
Sections in this chapter
Classification, Nomenclature and Preparation
Alkyl, allylic, benzylic, vinylic and aryl halides; IUPAC names; the C–X bond; preparation from alcohols, hydrocarbons, halogen exchange and diazonium salts; physical properties.
Nucleophilic Substitution: SN1, SN2 and Stereochemistry
Products with common nucleophiles, ambident nucleophiles, the SN2 and SN1 mechanisms, chirality, enantiomers, inversion and racemisation.
Elimination, Reactions of Haloarenes and Polyhalogen Compounds
β-Elimination and Saytzeff's rule, Grignard and Wurtz reactions, haloarenes towards nucleophiles and electrophiles, freons and DDT.