# 11-18 - When a bond is broken two radicals are produced...

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Unformatted text preview: 11/18/2009 When a bond is broken two radicals are produced. Which radical is more stable? A H H C H H C H H C H B H H H C H H C H H C H H C H H H H H H C H H C H C H C C H 1° H + H H C H H A H C H H Which C-H bond of propane, A or B, is stronger? 2° H C H H+ H B H 1 2 Which radical is more stable? H H C H H C H C H H Which reaction has the larger BDE ( H)? Which H H C H H C H H C H H C H H H C H C H H H H H H C H H C H H C H H C C 1° H + H H H H C H H C H H C H H C H H H H H H C H H C H H C H H C H C H C H C H H A. A. 1° H + H H BDE ( H) 423 423 kJ/mol H C H H A H C C H H 2° H C H H+ H B. 2° H C H H+ H 413 413 kJ/mol B H H H H C H C H H C H 3 4 H Using Using bond dissociation energies (BDE) estimate the H° of the following reaction. H + The mechanism for the chlorination of methane has been proposed to involve two steps? 440 kJ/mol H C HH H + 432 kJ/mol C HH 243 kJ/mol Cl + + 440 kJ/mol H C +H HH Cl + Cl H C HH bonds of reactants (440 + 243) 683 bonds of products (352 + 432) 784 C. -101 + - = heat of reaction 243 kJ/mol C HH Cl + 352 kJ/mol = -101 kJ/mol E. +32 5 F. 0 What is the H° of the first step in the accepted mechanism of the chlorination of methane? A. -440 B. +109 C. -8 D. +8 E. -109 F. +440 A. -784 B. +682 D. +439 1 11/18/2009 The chlorination of methane involves two steps? 440 kJ/mol + What is the overall methane? + H° for the chlorination of H C HH H + H C HH + 432 kJ/mol H° = +8 H H° = +8 H C HH + H C 243 kJ/mol Cl + C HH + HH 352 kJ/mol C HH Cl + H° = -109 H° = -101 What is the H° of the second step in the accepted mechanism of the chlorination of methane? A. -440 B. +109 C. -8 D. +8 E. -109 F. +440 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 Are other mechanisms for the chlorination of methane possible? E = 16 kJ/mol a 432 kJ/mol H° = 440  432 = +8 kJ/mol H 440 kJ/mol H C H H H + Cl H H C H H + Cl Cl H C H Cl + + 432 kJ/mol H Cl H° = +8 kJ/mol H° = 243  352 = -109 kJ/mol H C H Cl H° = -101 kJ/mol H° = -109 kJ/mol 243 kJ/mol Ea = 16 kJ/mol 352 kJ/mol H° = +8 kJ/mol H H C H H + 243 kJ/mol H Cl H C H 352 kJ/mol reaction reaction Cl + H energy energy H° = -101 kJ/mol H° = -109 kJ/mol H + Cl Cl H Cl + Cl reaction 9 10 What is the H° for the first step of the second reaction mechanism? E = 16 kJ/mol a What is the H° 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 H° = +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 H° = +8 kJ/mol Cl H° = -101 kJ/mol H° = -109 kJ/mol Cl H° = -101 kJ/mol H° = -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 H + Cl Cl H Cl + Cl A. B. C. D. +88 -88 +189 -189 243 kJ/mol 432 kJ/mol 2 11/18/2009 Why is the second mechanism unlikely to play a major role in the chlorination of methane? E = 16 kJ/mol a 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 H° = +8 kJ/mol Cl H° = -101 kJ/mol H° = -109 kJ/mol H 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 H H H H A H H H B H° = +88 kJ/mol H H + Cl Cl H Cl + Cl H° = -101 kJ/mol H° = -189 kJ/mol H H H H 243 kJ/mol 432 kJ/mol reaction 13 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% 16 B C D E hyperconjugation radical stability 15 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 17 0.5 B 2 C 5 D 9 E 100 F 200 18 3 11/18/2009 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 this reaction? 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 63 /9 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 19 A. yes B. no + H Br 99.4 1600 0.6/9 What is the H° for the overall reaction? 99.4% 0.6% 20 Why isn't chlorination more selective? 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 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° of the first two steps? H° = 400  432 = -32 kJ/mol +H 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 Cl + 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 H3C C H2C H Cl + H Cl energy C H Cl C H Cl energy C H Cl C H Cl H° = 21 kJ/mol H° = -138 kJ/mol reaction 21 reaction 22 What is the reactivity of 3to 1 if CH3 H3C C CH3 CH3 H3C C H2C H+ H H+ 400 kJ/mol Cl H3C CH3 C CH3 432 kJ/mol Cl H* = 21 kJ/mol? Why isn't chlorination more selective? 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 H° = 400  432 = -32 kJ/mol 421 kJ/mol Cl H3C CH3 432 kJ/mol C H2C CH3 H +H Cl 3/ 1 A= B= C= D= E= 1 5 50 500 5000 5000 energy 421 kJ/mol Cl H3C CH3 432 kJ/mol C H2C H +H Cl 3/ 1 A= B= C= D= E= 1 5 50 500 5000 5000 CH3 H3C C H2C H H3C C CH3 C H C H Cl Cl C H Cl 3° 1° 5 1 energy H* = 21 kJ/mol C H Cl C H Cl transition state theory (Ch 6) Keq = 1021/5.7 = 103.7 5000 C H Cl C H Cl H° = 21 kJ/mol H° = 21 kJ/mol reaction 23 reaction 24 4 11/18/2009 5 ...
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