CHAPTER 23 ARYL HALIDES SOLUTIONS TO TEXT PROBLEMS 23.1 There are four isomers of CH,CI that contain a benzene ring, namely, o, m, and p-chlorotoluene and benzyl chloride CH3 CH CH,CI o-Chlorotoluene m-Chlorotoluene p-Chlorotoluene Benzyl chloride Of this group only benzyl chloride is not an aryl halide; its halogen is not attached to the aromatic ring but to an sp-hybridized carbon Benzyl chloride has the weakest carbon-halogen bond, its measured carbon-chlorine bond dissociation energy being only 293 kJ/mol (70 kcal/mol) Homolytic cleavage of this bond produces a resonance-stabilized benzyl radical CH+ Benzyl chloride Benzyl radical Chlorine aton 23.2 (b) The negatively charged sulfur in C6HSCH2S: Na is a good nucleophile, which displaces SCH,CSH O2 O, 1-Chloro-2.4- Benzyl 2, 4- dinitrobenzene dinitrophenyl sulfide 656 Back Forward Main Menu TOC Study Guide Toc Student OLC MHHE Website
CHAPTER 23 ARYL HALIDES SOLUTIONS TO TEXT PROBLEMS 23.1 There are four isomers of C7H7Cl that contain a benzene ring, namely, o, m, and p-chlorotoluene and benzyl chloride. Of this group only benzyl chloride is not an aryl halide; its halogen is not attached to the aromatic ring but to an sp3 -hybridized carbon. Benzyl chloride has the weakest carbon–halogen bond, its measured carbon–chlorine bond dissociation energy being only 293 kJ/mol (70 kcal/mol). Homolytic cleavage of this bond produces a resonance-stabilized benzyl radical. 23.2 (b) The negatively charged sulfur in is a good nucleophile, which displaces chloride from 1-chloro-2,4-dinitrobenzene. Cl NO2 NO2 1-Chloro-2,4- dinitrobenzene SCH2C6H5 NO2 Cl NO2 Benzyl 2,4- dinitrophenyl sulfide C6H5CH2S Na C6H5CH2S Na Benzyl chloride Benzyl radical CH2 Chlorine atom CH Cl 2 Cl Cl CH3 p-Chlorotoluene CH2Cl Benzyl chloride Cl CH3 m-Chlorotoluene CH3 Cl o-Chlorotoluene 656 Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website
ARYL HALIDES 657 (c) The nitrogen in ammonia has an unshared electron pair and is nucleophilic; it displaces chloride from 1-chloro-2 4-dinitrobenzene N 1-Chloro-2, 4- 2.4-Dinitroaniline (d) As with ammonia, methy lamine is nucleophilic and displaces chloride NO2 CHANH 1-Chloro-2. 4. N-Methyl- dinitrobenzene 23.3 The most stable resonance structure for the cyclohexadienyl anion formed by reaction of methox ide ion with o-fluoronitrobenzene involves the nitro group and has the negative charge on oxygen 23. 4 The positions that are activated toward nucleophilic attack are those that are ortho and para to the group. Among the carbons that bear a bromine leaving group in 1, 2, 3-tribromo-5-nitrobenzene C-2 satisfies this requirement. Br Br NaOCH, CH isniobahaenoay 23.5 Nucleophilic addition occurs in the rate-determining step at one of the six equivalent carbons of hexafluorobenzene to give the cyclohexadienyl anion intermediate. F+: oc Hexafluorobenzene Methoxide Cyclohexadienyl anion Back Forward Main Menu TOC Study Guide Toc Student OLC MHHE Website
ARYL HALIDES 657 (c) The nitrogen in ammonia has an unshared electron pair and is nucleophilic; it displaces chloride from 1-chloro-2,4-dinitrobenzene. (d) As with ammonia, methylamine is nucleophilic and displaces chloride. 23.3 The most stable resonance structure for the cyclohexadienyl anion formed by reaction of methoxide ion with o-fluoronitrobenzene involves the nitro group and has the negative charge on oxygen. 23.4 The positions that are activated toward nucleophilic attack are those that are ortho and para to the nitro group. Among the carbons that bear a bromine leaving group in 1,2,3-tribromo-5-nitrobenzene, only C-2 satisfies this requirement. 23.5 Nucleophilic addition occurs in the rate-determining step at one of the six equivalent carbons of hexafluorobenzene to give the cyclohexadienyl anion intermediate. Hexafluorobenzene F F F F F F Methoxide ion OCH3 Cyclohexadienyl anion intermediate F F F F F F OCH3 1,2,3-Tribromo- 5-nitrobenzene Br Br Br NO2 1,3-Dibromo-2-ethoxy- 5-nitrobenzene OCH2CH3 Br Br NO2 NaOCH2CH3 CH3O F N O O Cl NO2 NO2 1-Chloro-2,4- dinitrobenzene NHCH3 NO2 NO2 N-Methyl-2,4- dinitroaniline CH3NH2 Cl NO2 NO2 1-Chloro-2,4- dinitrobenzene NH2 NO2 NO2 2,4-Dinitroaniline NH3 Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website
658 ARYL HALIDES Elimination of fluoride ion from the cyclohexadienyl anion intermediate restores the aromaticity of he ring and completes the reaction. F CF OCH3 Cyclohexadienyl anion 2.3, 4.5, 6-Pentafluoroanisole Fluoride 23.6 4-Chloropyridine is more reactive toward nucleophiles than 3-chloropyridine because the anior intermediate formed by reaction of 4-chloropyridine has its charge on nitrogen. Because nitrogen is more electronegative than carbon, the intermediate is more stable. Y intermediate 23.7 The aryl halide is incapable of elimination and so cannot form the benzyne intermediate necessary for substitution by the elimination-addition pathway 2-Bromo.1.3. 23.8 The aryne intermediate from p-iodotoluene can undergo addition of hydroxide ion at the position meta to the methyl group or para to it. The two isomeric phenols are m-and p-methylphenol NaOH. H,O NaOH. H-O OH m-Methylphenol Back Forward Main Menu TOC Study Guide Toc Student OLC MHHE Website
658 ARYL HALIDES Elimination of fluoride ion from the cyclohexadienyl anion intermediate restores the aromaticity of the ring and completes the reaction. 23.6 4-Chloropyridine is more reactive toward nucleophiles than 3-chloropyridine because the anionic intermediate formed by reaction of 4-chloropyridine has its charge on nitrogen. Because nitrogen is more electronegative than carbon, the intermediate is more stable. 23.7 The aryl halide is incapable of elimination and so cannot form the benzyne intermediate necessary for substitution by the elimination–addition pathway. 23.8 The aryne intermediate from p-iodotoluene can undergo addition of hydroxide ion at the position meta to the methyl group or para to it. The two isomeric phenols are m- and p-methylphenol. CH3 OH p-Methylphenol CH3 OH m-Methylphenol CH3 I p-Iodotoluene CH3 NaOH, H2O (elimination phase) NaOH, H2O (addition phase) CH3 Br CH3 2-Bromo-1,3- dimethylbenzene (No protons ortho to bromine; elimination is impossible.) 3-Chloropyridine N Cl Y Anionic intermediate (less stable) N Cl Y 4-Chloropyridine N Cl Y Anionic intermediate (more stable) N Y Cl 2,3,4,5,6-Pentafluoroanisole F F F F OCH3 F Fluoride ion F Cyclohexadienyl anion intermediate F F F F F F OCH3 Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website
ARYL HALIDES 659 23.9 The"triple bond"of benzyne adds to the diene system of furan Mg THF 23.10(a) dIb CICHCH (e) (f) O,N 2-Bromo-l-chloro-4. (Note: This compound is not an aryl halide. CHCI ,CI 2-Chloronaphthalene p-Bromobenzyl Cl CI (i) 9-Fluorophenanthrene 23.11 (a) Chlorine is a weakly deactivating, ortho, para-directing substituent CCH AlCI HCcI CH Chlorobenzene o-Chloroacetophenone p-Chloroacetophenone Back Forward Main Menu TOC Study Guide Toc Student OLC MHHE Website
23.9 The “triple bond” of benzyne adds to the diene system of furan. 23.10 (a) (b) (c) (d) (e) ( f ) (g) (h) (i) ( j) 23.11 (a) Chlorine is a weakly deactivating, ortho, para-directing substituent. Chlorobenzene Cl AlCl3 o-Chloroacetophenone Cl CCH3 O Acetyl chloride CH3CCl O p-Chloroacetophenone Cl C O CH3 9-Fluorophenanthrene F 3 2 1 10 9 8 7 6 5 4 1,8-Dichloronaphthalene Cl Cl 1 2 3 5 4 6 7 8 2-Chloronaphthalene Cl 1 2 3 5 4 6 7 8 CH2Cl Br p-Bromobenzyl chloride ClCHCH3 1-Chloro-1-phenylethane (Note: This compound is not an aryl halide.) Br O2N Cl 2-Bromo-1-chloro-4- nitrobenzene 4,4-Diiodobiphenyl I I 1 1 3 3 2 2 4 4 5 5 6 6 CH CH2 F p-Fluorostyrene OCH3 Br Br 2,6-Dibromoanisole CH3 Cl m-Chlorotoluene F Br Mg, THF heat O O ARYL HALIDES 659 Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website
660 ARYL HALIDES (b) Bromobenzene reacts with magnesium to give a gri H-Br diethyl ether C6HsMgBr Bromobenzene (c) Protonation of the Grignard reagent in part(b)converts it to benzene C6HSMgBr ChO Benzene bromide (d) Aryl halides react with lithium in much the same way that alkyl halides do, to form organo- lithium reagents. diethyl ether CHI C6HLi lil Iodobenzene Lithium Phenyllithium Lithium (e) With a base as strong as sodium amide, nucleophilic aromatic substitution by the elimination-addition mechanism takes place NaNH NaNH, Bromobenzene Benzyne C) The benzyne intermediate from p-bromotoluene gives a mixture of m-and p-methylaniline H3 NH, p-Bromotoluene 4-Methylbenzyn Methylaniline (g) Nucleophilic aromatic substitution of bromide by ammonia occurs by the addition-elimination NO (h) The bromine attached to the benzylic carbon is far more reactive than the one on the ring and is the one replaced by the nucleophile NaCN CH, Br Bromobenzyl bromide Bromobenzyl cyanide Back Forward Main Menu TOC Study Guide Toc Student OLC MHHE Website
(b) Bromobenzene reacts with magnesium to give a Grignard reagent. (c) Protonation of the Grignard reagent in part (b) converts it to benzene. (d) Aryl halides react with lithium in much the same way that alkyl halides do, to form organolithium reagents. (e) With a base as strong as sodium amide, nucleophilic aromatic substitution by the elimination–addition mechanism takes place. (f ) The benzyne intermediate from p-bromotoluene gives a mixture of m- and p-methylaniline. (g) Nucleophilic aromatic substitution of bromide by ammonia occurs by the addition–elimination mechanism. (h) The bromine attached to the benzylic carbon is far more reactive than the one on the ring and is the one replaced by the nucleophile. NaCN p-Bromobenzyl bromide Br CH2Br p-Bromobenzyl cyanide Br CH2CN NH3 1-Bromo-4- nitrobenzene Br NO2 p-Nitroaniline NO2 NH2 NaNH2 NH3 NaNH2 NH3 p-Bromotoluene Br CH3 4-Methylbenzyne CH3 m-Methylaniline NH2 CH3 p-Methylaniline NH2 CH3 NaNH2 NH3 NaNH2 NH3 Bromobenzene Benzyne Br Aniline NH2 diethyl ether Iodobenzene C6H5I Phenyllithium C6H5Li Lithium 2Li Lithium iodide LiI H2O HCl Benzene C6H6 Phenylmagnesium bromide C6H5MgBr diethyl ether Bromobenzene C6H5Br Phenylmagnesium bromide Mg C6H5MgBr 660 ARYL HALIDES Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website