918 CHAPTER TWENTY-THREE Aryl Halides TABLE 23.1 Carbon-Hydrogen and Carbon-Chlorine Bond Dissociation Energies of Selected Compounds Bond energy Hybridization of kJ/mol (kcal/mol) carbon to which Compound X is attached XEH X=C CH3CH2X 410(98) (81) CH2=CHX 452(108)368(88) X 469(112)406(97) ns to be responsible because, as the data in Table 23. 1 indicate, similar patterns are for both carbon-hydrogen bonds and carbon-halogen bonds. An increase in s char- acter from 25%(Sp hybridization)to 33.3%o s character(sp- hybridization)increases the tendency of carbon to attract electrons and strengthens the bond PROBLEM 23.1 Consider all the isomers of C,H,Cl containing a benzene ring and write the structure of the one that has the weakest carbon -chlorine bond as measured by its bond dissociation energy The strength of their carbon-halogen bonds causes aryl halides to react very slowly in reactions in which carbon-halogen 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 SNI and SN2 pathways 23.2 SOURCES OF ARYL HALIDES The two main methods for the preparation of aryl halides--halogenation of arenes by electrophilic aromatic substitution and preparation by way of aryl diazonium salts\ described earlier and are reviewed in Table 23. 2. A number of aryl halides occu rally, some of which are shown in Figure 23. 1 on page 920. 23.3 PHYSICAL PROPERTIES OF ARYL HALIDES Aryl halides resemble alkyl halides in many of their physical properties. All are practi- points for some representa cally insoluble in water and most are denser than water. tive aryl halides are listed in Aryl halides are polar molecules but are less polar than alkyl halides Chlorocyclohexane Chlorobenzene Compare the electro harges at chlorine in chlorocy. Since carbon is sp-hybridized in chlorobenzene, it is more electronegative than the sp zene hybridized carbon of chlorocyclohexane. Consequently, the withdrawal of electron den- more sity away from carbon by chlorine is less pronounced in aryl halides than in alkyl halides, and the molecular dipole moment is smaller. Back Forward Main MenuToc Study Guide ToC Student o MHHE Websiteseems to be responsible because, as the data in Table 23.1 indicate, similar patterns are seen for both carbon–hydrogen bonds and carbon–halogen bonds. An increase in s char￾acter from 25% (sp3 hybridization) to 33.3% s character (sp2 hybridization) increases the tendency of carbon to attract electrons and strengthens the bond. PROBLEM 23.1 Consider all the isomers of C7H7Cl containing a benzene ring and write the structure of the one that has the weakest carbon–chlorine bond as measured by its bond dissociation energy. The strength of their carbon–halogen bonds causes aryl halides to react very slowly in reactions in which carbon–halogen 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 SN2 pathways. 23.2 SOURCES OF ARYL HALIDES The two main methods for the preparation of aryl halides—halogenation of arenes by electrophilic aromatic substitution and preparation by way of aryl diazonium salts—were 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. 23.3 PHYSICAL 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 918 CHAPTER TWENTY-THREE Aryl Halides TABLE 23.1 Carbon–Hydrogen and Carbon–Chlorine Bond Dissociation Energies of Selected Compounds Compound CH3CH2X CH2œCHX 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 C±Cl bond is more polar in chlorocyclohexane. Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website