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Unformatted text preview: 6-63 NBS generates bromine which produces bromine radical. Bromine radical abstracts an allylic hydrogen, resulting in a resonance-stabilized a\lylic radical. The a\lylic radical can bond to bromine at either of the two carbons with radical character. H H2C=C-C-CH3 H CH3
I I I + Br- -- HEr + continues propagation
• 1H2C=C-C-CH3 H CH3
I I -- ..f--I� .. .. a\lylic Br + 6-64 The bromine radical from NBS will abstract whichever hydrogen produces the most stable intermediate; in this structure, that is a benzylic hydrogen, giving the resonance-stabilized benzylic radical. < }�HCH) � HBr + 1< }CHCH).
+ H2C=C-C-CH3 H CH3
� Br I � Br2
+ H2C-T ��H CH3
= CH, } H2C-C=C-CH3 H CH3
I I Br I OCHCH) • -- _ ..f--I� .. .. 6-65 Two related factors could explain this observation. First, as carbocation stability increases, the leaving group will be less tightly held by the carbocation for stabilization; the more stable carbocations are more "free" in solution, meaning more exposed. Second, more stable carbocations will have longer lifetimes, allowing the leaving group to drift off in the solvent, leading to more possibility for the incoming nucleophile to attack from the side that the leaving group just left. The less stable carbocations hold tightly to their leaving groups, preventing nucleophiles from attacking this side. Backside attack with inversion is the preferred stereochemical route in this case. 6 -66 < }CHCH3 Br Br· (Even though three carbons of the ring have some radical character, these are minm resonance contributors. The product is most stable when the ring has all three double bonds intact, necessitating that the bromine bond to the benzylic carbon.) < 1 CHCH3 >= a CH3 ----..
� Br Br + 2° carbocation
126 (t� CH3 _--=---H
� mechanisms continued on next page 3° carbocation ...
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- Spring '10