Solutions_Manual_for_Organic_Chemistry_6th_Ed 133

Solutions_Manual_for_Organic_Chemistry_6th_Ed 133 - 6-63...

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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 I H2C=C-C-CH3 I I H CH3 + Br- -- HEr + 1H2C=C-C-CH3 I I H CH3 continues propagation Br I Br + H2C=C-C-CH3 I I H CH3 a\lylic } .. .. .. f- -- -I� - H2C - T = �� - CH, H CH3 Br2 Br I + H2C-C=C-CH3 I I H CH3 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). OCHCH) 1 Br Br2 < }CHCH3 + Br· .... .. f- -- -I� _ < >= CHCH3 (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
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