electrophilic sub

electrophilic sub - Electrophilic Aromatic Substitution One...

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Electrophilic Aromatic Substitution One of the most important reactions of arenes is electrophilic aromatic substitution , in which an electrophile reacts with the ring, forming a new bond to a ring carbon with the loss of one hydrogen. In general, these reactions require a Lewis acid catalyst, as shown below for the reaction of bromine with benzene, catalyzed by FeBr 3 . The role of the FeBr 3 is to complex the bromine to form a bromonium cation- like species (which is often simply referred to as Br+) which is the actual electrophilic agent. This electrophile first forms a loose complex with the π− cloud, which rearranges to a cationic sigma- complex, in which the electrophile is directly bonded to a ring carbon. Since the ring is a conjugated system, the cationic charge which forms on the adjacent carbon is delocalized over the ring, with partial positive charge developing on the carbons which are ortho- and para- to the position where the electrophile bonded. Loss of H + from the sigma-complex regenerates the aromatic π− system (with its associated stability), and gives bromobenzene and HBr as the final products. Chlorination proceeds by a similar mechanism; for iodination, I 2 /CuCl 2 is typically utilized to generate the electrophilic I+ cation.
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This note was uploaded on 05/08/2009 for the course CHEM 140A taught by Professor Whiteshell during the Spring '04 term at UCSD.

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electrophilic sub - Electrophilic Aromatic Substitution One...

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