917 CHAPTER 23 ARYL HALIDES T he value of alkyl halides as starting materials for the preparation of a variety of organic functional groups has been stressed many times. In our earlier discussions, we noted that aryl halides are normally much less reactive than alkyl halides in reactions that involve carbonhalogen bond cleavage. In the present chapter you will see that aryl halides can exhibit their own patterns of chemical reactivity, and that these reac- tions are novel, useful, and mechanistically interesting. 23.1BONDING IN ARYL HALIDES Aryl halides are compounds in which a halogen substituent is attached directly to an aro- matic ring. Representative aryl halides include Halogen-containing organic compounds in which the halogen substituent is not directly bonded to an aromatic ring, even though an aromatic ring may be present, are not aryl halides. Benzyl chloride (C6H5CH2Cl), for example, is not an aryl halide. The carbonhalogen bonds of aryl halides are both shorter and stronger than the carbonhalogen bonds of alkyl halides, and in this respect as well as in their chemical behavior, they resemble vinyl halides more than alkyl halides. A hybridization effect F Fluorobenzene Cl NO2 1-Chloro- 2-nitrobenzene Br 1-Bromonaphthalene ICH2OH p-Iodobenzyl alcohol BackForwardMain MenuTOCStudy Guide TOCStudent OLCMHHE Website seems to be responsible because, as the data in Table 23.1 indicate, similar patterns are seen for both carbonhydrogen bonds and carbonhalogen bonds. An increase in s char- acter from 25% (sp3hybridization) to 33.3% s character (sp2hybridization) increases the tendency of carbon to attract electrons and strengthens the bond. PROBLEM 23.1Consider all the isomers of C7H7Cl containing a benzene ring and write the structure of the one that has the weakest carbonchlorine bond as measured by its bond dissociation energy. The strength of their carbonhalogen bonds causes aryl halides to react very slowly in reactions in which carbonhalogen bond cleavage is rate-determining, as in nucle- ophilic substitution, for example. Later in this chapter we will see examples of such reac- tions that do take place at reasonable rates but proceed by mechanisms distinctly differ- ent from the classical SN1 and S SN2 pathways. 23.2SOURCES OF ARYL HALIDES The two main methods for the preparation of aryl halideshalogenation of arenes by electrophilic aromatic substitution and preparation by way of aryl diazonium saltswere described earlier and are reviewed in Table 23.2. A number of aryl halides occur natu- rally, some of which are shown in Figure 23.1 on page 920. 23.3PHYSICAL PROPERTIES OF ARYL HALIDES Aryl halides resemble alkyl halides in many of their physical properties. All are practi- cally insoluble in water and most are denser than water. Aryl halides are polar molecules but are less polar than alkyl halides. Since carbon is sp2-hybridized in chlorobenzene, it is more electronegative than the sp3- hybridized carbon of chlorocyclohexane. Consequently, the withdrawal of electron den- sity away from carbon by chlorine is less pronounced in aryl halides than in alkyl halides, and the molecular dipole moment is smaller. Cl Chlorocyclohexane ? 2.2 D Cl Chlorobenzene ? 1.7 D 918CHAPTER TWENTY-THREEAryl Halides TABLE 23.1 CarbonHydrogen and CarbonChlorine Bond Dissociation Energies of Selected Compounds Compound CH3CH2X CH2CHX Hybridization of carbon to which X is attached sp3 sp2 sp2 X ? H 410 (98) 452 (108) 469 (112) X ? Cl 339 (81) 368 (88) 406 (97) Bond energy, kJ/mol (kcal/mol) X Melting points and boiling points for some representa- tive aryl halides are listed in Appendix 1. Compare the electronic charges at chlorine in chlorocy- clohexane and chlorobenzene on Learning By Modeling to ver- ify that the CCl bond is more polar in chlorocyclohexane. BackForwardMain MenuTOCStudy Guide TOCStudent OLCMHHE Website 23.4Reactions of Aryl Halides: A Review and a Preview919 TABLE 23.2Summary of Reactions Discussed in Earlier Chapters That Yield Aryl Halides Reaction (section) and comments Halogenation of arenes (Section 12.5) Aryl chlorides and bromides are con- veniently prepared by electrophilic aro- matic substitution. The reaction is lim- ited to chlorination and bromination. Fluorination is difficult to control; iodi- nation is too slow to be useful. The Sandmeyer reaction (Section 22.18) Diazotization of a primary arylamine followed by treatment of the diazo- nium salt with copper(I) bromide or copper(I) chloride yields the corre- sponding aryl bromide or aryl chloride. Reaction of aryl diazonium salts with iodide ion (Section 22.18) Adding potassium iodide to a solution of an aryl diazonium ion leads to the forma- tion of an aryl iodide. The Schiemann reaction (Section 22.18) Diazotization of an arylamine followed by treatment with fluoroboric acid gives an aryl diazonium fluoroborate salt. Heating this salt converts it to an aryl fluoride. General equation and specific example ?ArH AreneHalogen X2 Aryl halide