Page 171 - Pharm.Org.Chem-I 02-06-05-101

How Do Substituents Affect Orientation?

Substituents fall into two general classes:
1) Ortho–para directors predominantly direct the incoming group to a position ortho
or para to itself.

2) Meta directors predominantly direct the incoming group to a position meta to itself.

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Remember

Inductive Electron Withdrawal Electron Donation by
Hyperconjugation

Withdrawal of electrons through a σ Alkyl substituents (such as CH3)
bond. stabilize carbocations by hyperconjugation
that is, by donating electrons to an empty p-
e.g. +NH3 group
orbital

Electron Withdrawal by Resonance Electron Donation by Resonance

Substituents such as C=O , C=N , SO3 H, Substituents such as NH2 , OH, OR, and Cl
and NO2 withdraw electrons by donate electrons by resonance.
resonance.

These substituents also withdraw electrons inductively because the atom attached
to the benzene ring has a full or partial positive charge and is therefore more
electronegative than a hydrogen.

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