Solutions_Manual_for_Organic_Chemistry_6th_Ed 84

Solutions_Manual_for_Organic_Chemistry_6th_Ed 84 - 4 -53...

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Unformatted text preview: 4 -53 Assume that chlorine atoms (radic als) are sti l l generated in the initi ati on reaction . Focus on the propagation steps. B ond dissoci ation energies are gi ven below the bonds, in kJ/mol e (kcal/mole). Cl o + H-CH3 435 ( 1 04) + - H-CI + 0 C H3 43 1 ( 1 03) !1H = + 4 k J/mole ( + 1 kcal/mole) 1 09 kJ/mole ( - 26 kcal/mole) CI - C1 242 ( 5 8 ) o CH3 - C I - CH3 + Cl o 3 5 1 (84) + !1H =- What happens when the different radical species react with iodi ne? Cl o + I-I 1 5 1 (36) o CH3 - I - CI 2 1 1 (50) + I-I 1 5 1 (36) - 1 - CH3 + I 0 2 34 (56) 01 !1H = - 60 kJ/mole ( - 14 kcal/mole) - 8 3 kJ/mole ( - 20 kcal/mole) !1H = Compare the second reaction in each pair: methyl radical reacting with c h lorine is more exothermic than methyl radical reacting with i odine ; this does not explain how iodine prevents the chlorination reaction . Compare the first reaction in eac h pair: chlorine atom reacti n g with i odine is very exothermic whereas c h l orine atom reacting with methane is slightly endothermic . Here is the key: chlorine atoms w i l l be scavenged by i odine before they h ave a chance to react with methane. Without chlorine atoms, the reaction comes to a dead stop. 4-54 (a) i nitiation fl ( 2) B r- B r Br 0 + nn f' � " H - hv 2 Br 0 ----- ( 3) propagation (4) � O' H � B' H -SnB u3 H-Br + o SnBu3 H + B r - SnBu3 + 1 . snBU j - --.. ":":\ + H -SnB u 3 n - O· (jH H + o S nBu3 (b) A l l energies are in kJ/mole. The abbreviation "cy" stands for the cyclohexane ring . Step 2 : break H-S n , make H-Br: +3 1 0 + - 368 Step 3 : break cy-Br, make B r-Sn: +28 5 + - 5 5 2 S tep 4 : break H-S n , make c y-H: + 3 1 0 + - 397 The sum of the two propagation steps are : - 267 + - 87 = - 5 8 kJ/mol e - 2 6 7 kJ/mol e WOW ! - 87 kJ/mole - 354 kJ/mole -a h ugely exothermic reaction . = = = 76 ...
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This note was uploaded on 02/27/2010 for the course CHEM 140 taught by Professor Wade during the Spring '10 term at Whitman.

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