Solutions_Manual_for_Organic_Chemistry_6th_Ed 197

Solutions_Manual_for_Organic_Chemistry_6th_Ed 197 - 8 -72...

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Unformatted text preview: 8 -72 First, we explain how the mixture of stereoisomers results, then why. We have seen many times that the bridged halonium ion permits attack of the nucIeophile only from the opposi te side. expected: Q H '" --- Ci-CI Ph W H Ph CI� �:S:1: � ~ Cl t rans o nly H A mixture of c is a nd trans c ould result only if attack of chloride were possible from both top and hottom, s omethi ng possible only if a carbocation e xisted at this carbon. actual: 0� H H Ph --� +� CI H H Ph trans cis This picture of the p orbitals of benzene show resonance overlap with the p orbital of the carbocation. The chloride nucIeophile can form a bond to the positive carbon from either the top or the bottom. Why d oes a carbocation exist here? Not only is it 3°, it is also next to a benzene ring (benzylic) and therefore resonance-stabilized. This resonance stabilization would be forfeited in a halonium ion intermediate. r'�/ �CI 0: "::: // .. - + � � aCI IIi CI ' Ph H + CC"'H Ph :Ci : 'Ci CI C 0:CI 190 � ...
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