PREDICTING DISUBSTITUTION PATTERNS ON BENZENE RINGS
Disubstitution on aromatic rings may occur in three different orientations:
The presence of one substituent on an aromatic ring will dictate the orientation of the second subtituent added to the ring during a reaction. The first substituent will generally be either an ortho/para director or a meta director. Using a few rules, one can predict whether an existing substituent directs the next group ortho/para, or meta:
1) an alkyl group
2) if the atom directly attached to the ring has one or more electron pairs
1) if the atom directly attached to the ring has a pi system (multiple bond)
2) if the atom directly attached to the ring has a formal + charge
Let's look at why a substituent is an ortho/para director or a meta director. We know that the --OH group on phenol directs ortho/para due to the lone pair electrons on the oxygen. If we were to examine the resonance contributors for phenol, additional justification for the orientation comes to light:
Look at how the formal negative charge is found in the ortho and para positions in the resonance contributors shown above. An electrophile approaching the ring will thus be attracted to these positions, because it is where the electrophile will encounter the highest amount of electron density.
Let's look at a meta director, such as the aldehyde group in benzaldehyde. Once again, the resonance contributors provide a justification for meta direction:
Notice how a formal positive charge is found in the ortho and para positions. An electrophile approaching the ring will be repelled from these positions. Instead, the only ring locations where the electrophile will encounter electron density is at the meta postions.
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© Copyrght, 2001, L. Ladon. Permission is granted to use and duplicate these materials for non-profit educational use, under the following conditions: No changes or modifications will be made without written permission from the author. Copyright registration marks and author acknowledgement must be retained intact.