Once again, the resonance contributors provide a justification for meta direction: Let's look at a meta director, such as the aldehyde group in benzaldehyde. 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. Look at how the formal negative charge is found in the ortho and para positions in the resonance contributors shown above. If we were to examine the resonance contributors for phenol, additional justification for the orientation comes to light: We know that the -OH group on phenol directs ortho/para due to the lone pair electrons on the oxygen. Let's look at why a substituent is 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:Ģ) if the atom directly attached to the ring has one or more electron pairsġ) if the atom directly attached to the ring has a pi system (multiple bond)Ģ) if the atom directly attached to the ring has a formal + charge The first substituent will generally be either an ortho/para director or a meta director. The presence of one substituent on an aromatic ring will dictate the orientation of the second subtituent added to the ring during a reaction. Ompsub.htm PREDICTING DISUBSTITUTION PATTERNS ON BENZENE RINGSĭisubstitution on aromatic rings may occur in three different orientations:
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