8.1 Functional Group Transformation by Nucleophilic Substitution The carbon-halogen bond in an alkyl halide is polar R文 X=L. Br. CLF nd is cleaved on attack by a nucleophile so that the two electrons in the bond are retained R-Y+: X by the halogen The most frequently encountered nucleophiles in functional group transfo are anions, which are used as their lithium, sodium, or potassium salts. If we use M to represent lithium, sodium, or potassium, some representative nucleophilic reagents are MOR(a metal alkoxide, a source of the nucleophilic anion RO:) MOCr (a metal carboxylate, a source of the nucleophilic anion RC-O:) SH (a metal hydrogen sulfide, a source of the nucleophilic anion HS:) MCn(a metal cyanide, a source of the nucleophilic anion: CEN;) (a metal azide, a source of the nucleophilic anion: N=N=N:) Table8. 1 illustrates an application of each of these to a functional group transfor- mation. The anionic portion of the salt substitutes for the halogen of an alkyl halide. The netal cation portion becomes a lithium, sodium, or potassium halide. M+y:+r R-Y +M+:X Nucleophilic Alky Product of Metal halide halide nucleophilic substitution Notice that all the examples in Table 8. 1 involve alkyl halides, that is, compounds in which the halogen is attached to an sp-hybridized carbon. Alkenyl halides and aryl Alkenyl halides are also re- halides, compounds in which the halogen is attached to sp-hybridized carbons, are ferred to as vinylic halides essentially unreactive under these conditions, and the principles to be developed in this chapter do not apply to them sp-hybridized carbon sp-hybridized carbon Alkyl halide Alkenyl halide I halide To ensure that reaction occurs in homogeneous solution, solvents are chosen that dis solve both the alkyl halide and the ionic salt. The alkyl halide substrates are soluble in organic solvents, but the salts often are not. Inorganic salts are soluble in water, but alkyl halides are not mixed solvents such as ethanol-water mixtures that can dissolve enough of both the substrate and the nucleophile to give fairly concentrated solutions are fre- The use of DMSO as a sok- quently used. Many salts, as well as most alkyl halides, posses cant solubility in vent in dehydrohalogenation eactions was mentioned dimethyl sulfoxide(DMsO), which makes this a good medium for carrying out nucle- earlier, in Section 5.14 Back Forward Main MenuToc Study Guide ToC Student o MHHE Website8.1 Functional Group Transformation by Nucleophilic Substitution 303 The most frequently encountered nucleophiles in functional group transformations are anions, which are used as their lithium, sodium, or potassium salts. If we use M to represent lithium, sodium, or potassium, some representative nucleophilic reagents are Table 8.1 illustrates an application of each of these to a functional group transfor￾mation. The anionic portion of the salt substitutes for the halogen of an alkyl halide. The metal cation portion becomes a lithium, sodium, or potassium halide. Notice that all the examples in Table 8.1 involve alkyl halides, that is, compounds in which the halogen is attached to an sp3 -hybridized carbon. Alkenyl halides and aryl halides, compounds in which the halogen is attached to sp2 -hybridized carbons, are essentially unreactive under these conditions, and the principles to be developed in this chapter do not apply to them. To ensure that reaction occurs in homogeneous solution, solvents are chosen that dis￾solve both the alkyl halide and the ionic salt. The alkyl halide substrates are soluble in organic solvents, but the salts often are not. Inorganic salts are soluble in water, but alkyl halides are not. Mixed solvents such as ethanol–water mixtures that can dissolve enough of both the substrate and the nucleophile to give fairly concentrated solutions are fre￾quently used. Many salts, as well as most alkyl halides, possess significant solubility in dimethyl sulfoxide (DMSO), which makes this a good medium for carrying out nucle￾ophilic substitution reactions. sp2 sp -hybridized carbon 3 -hybridized carbon Alkyl halide C X Alkenyl halide X C C Aryl halide X Nucleophilic reagent M Y R X Alkyl halide R Y Product of nucleophilic substitution X M Metal halide MOR MOCR O X MSH MCN MN3 (a metal alkoxide, a source of the nucleophilic anion ) RO (a metal hydrogen sulfide, a source of the nucleophilic anion ) HS (a metal cyanide, a source of the nucleophilic anion ) CPN (a metal azide, a source of the nucleophilic anion NœNœN ) (a metal carboxylate, a source of the nucleophilic anion RC±O O X ) Y R X R Y X R X X  I, Br, Cl, F   The carbon–halogen bond in an alkyl halide is polar and is cleaved on attack by a nucleophile so that the two electrons in the bond are retained by the halogen Alkenyl halides are also re￾ferred to as vinylic halides. The use of DMSO as a sol￾vent in dehydrohalogenation reactions was mentioned earlier, in Section 5.14. Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website