11-17 - 11/18/2010 Which C-H bond is the weakest in...

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Unformatted text preview: 11/18/2010 Which C-H bond is the weakest in arachidonic acid? Which C-H bond is the strongest in arachidonic acid? A B C D A B C D Why are C & D the weakest bond in arachidonic acid? 1˚ A 421 Biochemical pathway for the preparation of the prostaglandins and prostacyclins. 2˚ B 410 C 358 D 320 kJ/mol = bond dissociation energies ibuprofen (advil) and aspirin inhibit this enzyme What is the mechanism of this reaction? What is the first step? What is the mechanism for this reaction? O O OOH CO2H PGG2 1 11/18/2010 What is the mechanism for this reaction? What is the mechanism for this reaction? O O OOH CO2H + HO + HO What is the mechanism for this reaction? O O OOH CO2H What is the mechanism for this reaction? O O OOH CO2H What is the mechanism for this reaction? O O OOH CO2H What is the slow or rate determining step for the reaction? 440 kJ/mol H H C H H H C H + Cl Cl H H + Cl H H C H H C H Cl + Cl + H Cl 432 kJ/mol A B ∆H° = 440 – 432 = +8 kJ/mol ∆H° = 243 – 352 = -109 kJ/mol 243 kJ/mol Ea = 16 kJ/mol 352 kJ/mol ΔH° = +8 kJ/mol energy energy ΔH° = -101 kJ/mol ΔH° = -109 kJ/mol reaction 12 2 11/18/2010 Are other mechanisms for the chlorination of methane possible? E = 16 kJ/mol a Which is the correct reaction mechanism? Ea = 16 kJ/mol 440 kJ/mol H H C H H H C H + Cl Cl H + Cl H H C H H H C H Cl + Cl + H Cl 440 kJ/mol H H C H H H C H + Cl Cl H H + Cl H H C H H C H Cl + + 432 kJ/mol H Cl ΔH° = +8 kJ/mol 432 kJ/mol ΔH° = +8 kJ/mol Cl ΔH° = -101 kJ/mol ΔH° = -109 kJ/mol A 243 kJ/mol ΔH° = -101 kJ/mol ΔH° = -109 kJ/mol 243 kJ/mol H H C H H + Cl H H C H 352 kJ/mol 352 kJ/mol H reaction reaction H Cl + H H C H H + Cl H C H Cl + H B + Cl Cl H Mechanism B is incorrect because it is more endothermic (greater overall ∆H˚). H + Cl Cl H Cl + Cl H Cl + Cl 13 A. True B. False 14 What is the ΔH° for the first step of the second reaction mechanism? E = 16 kJ/mol a What is the ΔH° for the second step of the second reaction mechanism? E = 16 kJ/mol a 440 kJ/mol H H C H H H C H + Cl Cl H H + Cl H H C H H C H Cl + + 432 kJ/mol H Cl ΔH° = +8 kJ/mol H 440 kJ/mol H C H H H C H + Cl Cl H H + Cl H H C H H C H Cl + + 432 kJ/mol H Cl ΔH° = +8 kJ/mol Cl ΔH° = -101 kJ/mol ΔH° = -109 kJ/mol Cl ΔH° = -101 kJ/mol ΔH° = -109 kJ/mol 243 kJ/mol 440 kJ/mol H H C H H + Cl H H C H 352 kJ/mol 352 kJ/mol Cl + H H H C H H + 243 kJ/mol 440 kJ/mol H Cl H C H 352 kJ/mol 352 kJ/mol Cl + H H + Cl Cl H Cl + Cl A. B. C. D. +88 -88 +189 -189 15 H + Cl Cl H Cl + Cl A. B. C. D. +88 -88 +189 -189 16 243 kJ/mol 432 kJ/mol Why is the second mechanism unlikely to play a major role in the chlorination of methane? E = 16 kJ/mol a Which Which compound would be predicted to react faster in a free radical chlorination reaction? 440 kJ/mol H H C H H H C H + Cl Cl H H + Cl H H C H H C H Cl + + 432 kJ/mol H Cl ΔH° = +8 kJ/mol Cl ΔH° = -101 kJ/mol ΔH° = -109 kJ/mol 243 kJ/mol 440 kJ/mol H H C H H + Cl H H C H 352 kJ/mol 352 kJ/mol Cl + H A ΔH° = +88 kJ/mol B H + Cl Cl H Cl + Cl ΔH° = -101 kJ/mol ΔH° = -189 kJ/mol 243 kJ/mol 432 kJ/mol reaction 17 18 3 11/18/2010 Do other different C-H bonds show different rates of reactivity in free radical reactions? 400 kJ/mol C C C C H H H H 413 kJ/mol C C C H H H 421 kJ/mol C H C H H 440 kJ/mol H 369 kJ/mol H H 465 kJ/mol H C H H H What would be predicted to be the product yields of the following compound if all C-H bonds had the same reactivity? H H H H H H H H H allyl 3° 2° 1° methyl vinyl CH3 H3C C CH3 H + Cl2 initiator CH3 H3C C CH3 Cl + H3C CH3 C H2C H Cl + H Cl C-H reactivity H H H H H H C C C C H H H C C C H H H C C H H H H H C A H H H 37% 50% 10% 67% 33% 100% 63% 50% 90% 33% 67% 0% 20 B C D E hyperconjugation radical stability 19 F From the product yields which hydrogen, 3° or 1°, is most reactive? A. 3° B. 1° CH3 H3C C CH3 H + Cl2 400 kJ/mol C C C C H H H 421 kJ/mol C H C H H How many times more reactive is 3° compared to a 1° C─H bond? 400 kJ/mol C C C C H H H H 421 kJ/mol C C H H H H H H 3° initiator CH3 H3C C CH3 Cl + 1° CH3 H3C C H2C H Cl + H Cl H3C CH3 C CH3 H + Cl2 3° initiator CH3 H3C C CH3 Cl + 1° CH3 H3C C H2C H Cl + H reactivity 37 63 /9 3° 1° ≈5 Cl 37% 10% 63% 90% actual yield statistical prediction 37% 10% 63% 90% actual yield statistical prediction A ≈ 0.5 21 B≈2 C≈5 D≈9 E ≈ 100 F ≈ 200 22 What is the selectivity of bromination in this reaction? 400 kJ/mol H C C C C H H H H What is the mechanism of these reactions? CH3 H3C C CH3 CH3 H3C C + Cl H+ 421 kJ/mol C C H H H 400 kJ/mol Cl H3C CH3 C CH3 CH3 H3C C CH3 432 kJ/mol Cl +H ∆H° = 400 – 432 = -32 kJ/mol 243 kJ/mol Cl 349 kJ/mol Cl + Cl ∆H° = 243 – 349 = -106 kJ/mol reactivity 3° 1° ≈ 5 1 3° CH3 H3C C CH3 H + Cl2 initiator CH3 H3C C CH3 Cl + 1° CH3 H3C C H2C H Cl + H 3° reactivity 1° Cl CH3 CH3 H3C C CH3 H + Cl2 37(9) 63 ≈5 initiator CH3 H3C C CH3 Cl + H3C CH3 C H2C H Cl + H Cl 37 63 /9 37% 63% 37% 63% Is the first step exothermic? CH3 H3C C CH3 H + Br2 initiator CH3 H3C C CH3 Br + H3C CH3 C H2C H Br 23 A. yes B. no + H Br 99.4(9) ≈ 1600 0.6 24 99.4% 0.6% 4 11/18/2010 What is the ∆H˚ for the overall reaction? CH3 H3C C CH3 CH3 H3C C + Cl H+ 400 kJ/mol Cl H3C CH3 C CH3 CH3 H3C C CH3 432 kJ/mol Cl What is the difference in ∆H° of the first two steps? CH3 H3C C CH3 H+ 400 kJ/mol Cl H3C +H ∆H° = 400 – 432 = -32 kJ/mol CH3 C CH3 432 kJ/mol Cl +H ∆H° = 400 – 432 = -32 kJ/mol 243 kJ/mol Cl 349 kJ/mol Cl + Cl ∆H° = 243 – 349 = -106 kJ/mol CH3 H3C C H2C H+ H 421 kJ/mol Cl H3C CH3 432 kJ/mol C H2C CH3 H3C C CH3 H +H Cl ∆H° = 421 – 432 = -11 kJ/mol CH3 CH3 H3C C CH3 H + Cl2 initiator CH3 H3C C CH3 Cl + H3C CH3 C H2C H Cl + H Cl H3C CH3 C CH3 H + Cl2 initiator CH3 Cl + H3C C H2C H Cl + H Cl energy C H Cl C H−Cl energy C H Cl C H−Cl Δ Δ H° = 21 kJ/mol ΔH° = -138 kJ/mol reaction 25 reaction 26 What is the reactivity of 3˚to 1˚ if ∆∆H* = 21 kJ/mol? CH3 H3C C CH3 CH3 H3C C H2C H+ H H+ 400 kJ/mol Cl H3C CH3 C CH3 432 kJ/mol Cl +H ∆H° = 400 – 432 = -32 kJ/mol 3˚/ 1˚ ∆H° = 421 – 432 = -11A = kJ/mol 421 kJ/mol Cl H3C CH3 432 kJ/mol C H2C CH3 H +H Cl 1 5 50 500 5000 B= C= D= E= CH3 H3C C H2C H H3C C CH3 energy Δ Δ H* = 21 kJ/mol C H Cl C H−Cl transition state theory (Ch 6) Keq = 1021/5.7 = 103.7 ≈ 5000 Δ Δ H° = 21 kJ/mol reaction 27 28 5 ...
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