ARYL HALIDES 661 (i) The aromatic ring of N, N-dimethylaniline is very reactive and is attacked by p-chlorobenzene diazonium ion (CH,2N 》+=>c (CH3)2N N, N-Dimethylaniline p-Chlorobenzenediazonium 4-(4-Chlorophenylazo)-N, N-dimethylaniline ( Hexafluorobenzene undergoes substitution of one of its fluorines on reaction with nucleo- philes such as sodium hydrogen sulfide NaSH F F Hexafluorobenzene hydrogen sulfide 3.12 (a) Since the tert-butoxy group replaces fluoride at the position occupied by the leaving group. substitution likely occurs by the addition-elimination mechanism CH OC(CH3) + Ch3)3co OC(CH3)3 o-Fluorotoluene rt-Butoxide tert-Butyl o-methylphenyl (b) In nucleophilic aromatic substitution reactions that proceed by the addition-elimination mechanism, aryl fluorides react faster than aryl bromides. Because the aryl bromide is more reactive in this case, it must be reacting by a different mechanism, which is most likely elimination-addition OC(CH KOC(CH,) DMSO 23.13 (a) Two benzyne intermediates are equally likely to be formed. Reaction with amide ion can occur in two different directions with each benzyne, giving three possible products. They are formed in a 1: 2. 1 ratio H, Ratio: 2 Asterisk(*)refers toc Back Forward Main Menu TOC Study Guide Toc Student OLC MHHE Website
(i) The aromatic ring of N,N-dimethylaniline is very reactive and is attacked by p-chlorobenzenediazonium ion. ( j) Hexafluorobenzene undergoes substitution of one of its fluorines on reaction with nucleophiles such as sodium hydrogen sulfide. 23.12 (a) Since the tert-butoxy group replaces fluoride at the position occupied by the leaving group, substitution likely occurs by the addition–elimination mechanism. (b) In nucleophilic aromatic substitution reactions that proceed by the addition–elimination mechanism, aryl fluorides react faster than aryl bromides. Because the aryl bromide is more reactive in this case, it must be reacting by a different mechanism, which is most likely elimination–addition. 23.13 (a) Two benzyne intermediates are equally likely to be formed. Reaction with amide ion can occur in two different directions with each benzyne, giving three possible products. They are formed in a 1:2:1 ratio. Asterisk (*) refers to 14C. NaNH2 NH3 Cl * * * : 1 : NH2 * 2 NH2 * NH2 Ratio: 1 * Bromobenzene Br tert-Butyl phenyl ether OC(CH3)3 Benzyne KOC(CH3)3 DMSO KOC(CH3)3 DMSO tert-Butoxide ion (CH3)3CO o-Fluorotoluene CH3 F F tert-Butyl o-methylphenyl ether CH3 OC(CH3)3 CH3 F OC(CH3)3 Hexafluorobenzene Sodium hydrogen sulfide NaSH F F F F F F 2,3,4,5,6-Pentafluorobenzenethiol SH F F F F F N,N-Dimethylaniline (CH3)2N 4-(4-Chlorophenylazo)-N,N-dimethylaniline (CH3)2N N N Cl p-Chlorobenzenediazonium ion N N Cl ARYL HALIDES 661 Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website
662 ARYL HALIDES (b) Only one benzyne interme possible, leading to two products in a 1: I ratio Nh t Ratio D refers to H(deuterium) 23. 14(a) o-Chloronitrobenzene is more reactive than chlorobenzene, because the cyclohexadienyl anion intermediate is stabilized by the nitro group CHC1‖ CH3O Comparing the rate constants for the two aryl halides in this reaction reveals that o-chloro- nitrobenzene is more than 20 billion times more reactive at 50%C (b) The cyclohexadienyl anion intermediate is more stable, and is formed faster, when the electron-withdrawing nitro group is ortho to chlorine o-Chloronitrobenzene reacts faster than m-chloronitrobenzene. The measured difference is a factor of approximately 40,000 at 50.C (c) 4-Chloro-3-nitroacetophenone is more reactive, because the ring bears two powerful electron withdrawing groups in positions where they can stabilize the cyclohexadienyl anion CHO CH O CI CH O CI (d) Nitro groups activate aryl halides toward nucleophilic aromatic ution best when they to the leavi is more reactive than 2-Fluoro-1, 3 I-Fluoro-3,5. Back Forward Main Menu TOC Study Guide Toc Student OLC MHHE Website
(b) Only one benzyne intermediate is possible, leading to two products in a 1:1 ratio. D refers to 2 H (deuterium). 23.14 (a) o-Chloronitrobenzene is more reactive than chlorobenzene, because the cyclohexadienyl anion intermediate is stabilized by the nitro group. Comparing the rate constants for the two aryl halides in this reaction reveals that o-chloronitrobenzene is more than 20 billion times more reactive at 50°C. (b) The cyclohexadienyl anion intermediate is more stable, and is formed faster, when the electron-withdrawing nitro group is ortho to chlorine. o-Chloronitrobenzene reacts faster than m-chloronitrobenzene. The measured difference is a factor of approximately 40,000 at 50°C. (c) 4-Chloro-3-nitroacetophenone is more reactive, because the ring bears two powerful electronwithdrawing groups in positions where they can stabilize the cyclohexadienyl anion intermediate. (d) Nitro groups activate aryl halides toward nucleophilic aromatic substitution best when they are ortho or para to the leaving group. is more reactive than 2-Fluoro-1,3- dinitrobenzene F O2N NO2 1-Fluoro-3,5- dinitrobenzene O2N NO2 F CH3O Cl C N CH3 O CH3O Cl N O C CH3 CH3O Cl C N CH3 O O O O O O O CH3O Cl N O O O CH3O Cl N O D Cl D D NH2 D NH2 D NaNH2 NH3 D same as Ratio: 1 : 1 662 ARYL HALIDES Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website