Chapter_15 - P1: OXT/SRB JWDD052-15 P2: QZI...

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P1: OXT/SRB P2: QZI Printer: Hamilton JWDD052-15 JWDD052-Solomons-v3 June 1, 2007 20:19 15 REACTIONS OF AROMATIC COMPOUNDS SOLUTIONS TO PROBLEMS 15.1 + + E H A HA HA HA E E E H A E H A + E 15.2 The rate is dependent on the concentration of NO 2 + ion formed from protonated nitric acid. H HA (where HA = HNO 3 or HOSO 3 H) H + ++ + ON O 2 NO 2 + H 3 O + A Because H 2 SO 4 (HOSO 3 H) is a stronger acid, a mixture of it and HNO 3 will contain a higher concentration of protonated nitric acid than will nitric acid alone. That is, the reaction, Protonated nitric acid HOSO 3 H HSO 4 + + H H + O 2 HON O 2 produces more protonated nitric acid than the reaction, H + H NO 3 + + O 2 O 2 O 2 284
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P1: OXT/SRB P2: QZI Printer: Hamilton JWDD052-15 JWDD052-Solomons-v3 June 1, 2007 20:19 REACTIONS OF AROMATIC COMPOUNDS 285 15.3 Step 1 CH 3 CH CH 2 CH 3 CHCH 3 F HF + + HF + + Step 2 CH 3 CH 3 δ+ HC CH 3 CH 3 H CH + + Step 3 F CH 3 CH 3 CH CH 3 CH 3 H CH 15.4 CH 3 CH 3 + + + + O C O C AlCl 3 CH 3 C O O AlCl 3 AlCl 3 O CH 3 CH 3 O C O C O CH 3 CO + CH 3 15.5 The carbocation formed by the action of AlCl 3 on neopentyl chloride is primary. This carbocation rearranges to the more stable tertiary carbocation before it can react with the benzene ring: ++ + AlCl 3 AlCl 4 Cl + AlCl 3 Cl (Several steps) 15.6 + BF 3 H OH O + BF 3 Rearrangement to the secondary carbocation occurs as the leaving group departs. BF 3 O + H H hydride shift + + HOBF 2 + F
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P1: OXT/SRB P2: QZI Printer: Hamilton JWDD052-15 JWDD052-Solomons-v3 June 1, 2007 20:19 286 REACTIONS OF AROMATIC COMPOUNDS Both carbocations are then attacked by the ring. H H CH 2 CH 2 CH 3 H CHCH 3 CH 3 CH 3 CHCH 3 + CH 3 CH 2 CH 2 + A + CH 2 CH 2 CH 3 + HA + A CHCH 3 CH 3 HA 15.7 AlCl 3 Cl O Zn(Hg) HCl reflux (a) O AlCl 3 Zn(Hg) HCl reflux (b) O Cl O O AlCl 3 Cl C 6 H 5 O (c) AlCl 3 HO O O + O O O Zn(Hg) AlCl 3 (d) AlCl 3 SOCl 2 Zn(Hg) HCl O Cl O HO O
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P1: OXT/SRB P2: QZI Printer: Hamilton JWDD052-15 JWDD052-Solomons-v3 June 1, 2007 20:19 REACTIONS OF AROMATIC COMPOUNDS 287 15.8 If the methyl group had no directive effect on the incoming electrophile, we would expect to obtain the products in purely statistical amounts. Since there are two ortho hydrogen atoms, two meta hydrogen atoms, and one para hydrogen, we would expect to get 40% ortho (2/5), 40% meta (2/5), and 20% para (1/5). Thus, we would expect that only 60% of the mixture of mononitrotoluenes would have the nitro group in the ortho or para position. And we would expect to obtain 40% of m -nitrotoluene. In actuality, we get 96% of combined o - and p -nitrotoluenes and only 4% m -nitrotoluene. This shows the ortho-para directive effect of the methyl group. 15.9 SO 3 H 2 SO 4 CH 3 SO 3 H + SO 3 H CH 3 CH 3 Br 2 FeBr 3 Br NO 2 NO 2 AlCl 3 OH R + O O OH R Cl R O OH HNO 3 H 2 SO 4 CO 2 H NO 2 CO 2 H (a) (b) (c) (d) 15.10 As the following structures show, attack at the ortho and para positions of phenol leads to arenium ions that are more stable (than the one resulting from meta attack) because they are hybrids of four resonance structures, one of which is relatively stable. Only three resonance structures are possible for the meta arenium ion, and none is relatively stable.
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Chapter_15 - P1: OXT/SRB JWDD052-15 P2: QZI...

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