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Unformatted text preview: 4 -25 (a) Mechanism initiation B r- B r nn propagation a
H H IU �
+ + --- hv 2 Br 0 B ,·
- ( b) Energy calcul ation uses the value for the al lylic C-H bond from Table 4-2.
. F irst p ropagatIOn step S econd p ropagation s tep T he first propagation step is rate-limiting. i i �H UH r-' � \n B ,-B,
- 6=: C(
• H H + HBr H H A U Br + B ,· b reak allylic H-CH[ri ng] make H-B r overall allylic H abstraction b re ak B r-Br make 2° C-B r overall C-B r formati on slmclu;c of Ihc transHi on slateo (c) The Hammond postul ate tel ls us that, in an exothermic reaction, the transition state is closer to the reactants in energy and in structure . Since the first propagation step is exothermic (although not by much), the transition state is c loser to cyclohexene + b romine radic al . This is indicated in the transi tion state structure by showing the H closer to the C th an to the Br. (d) A bromine radical wi l l abstract the hydrogen with the lowest bond dissoc i ation energy at the fastest rate . The al lylic hydrogen of cyc lohexene is more easi ly abstracted than a hydrogen of cyclohexane because the radical produced is stabilized by resonance. (Energy values below are per mole . ) aliphatic: 397 kJ (95 kcal ) - H H [ MfD WO M-/ 0 WO WO M-/0 = = -4 kJ/mole ( -1 kcaUmole) + 3 64 kJ/mole (+ 8 7 kcallmole) - 368 kJ/mole ( - 88 kc allmole) = = = + 1 92 kJ/mole (+ 46 kcallmole) 28 5 kJ/mole ( - 68 kcallmole) - 93 kJ/mole ( -22 kcaUmole)
- 0- H - - - H - - - - - - - - - B, 80 8 OJ t SLOWER 66
----- - 3 64 kJ (87 kcal) : allylic FASTER 4 -26 O CH3 B HA radical ¢x ¢x ~ if
----- .. .. .. .. I h- • ----- :::--..
• I OCH3 OCH3 OCH3 67 ...
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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.
- Spring '10