Substituents
An atom or group of atoms takes the place of another atom or group or occupies a specified molecule position. Substitution in Monosubstituted Benzene: All hydrogen atoms of the benzene ring are equivalent. Therefore, only one mono-substitution product(C6H5-G) is possible.
A Second substituent can occupy any of the remaining five positions.
1. The positions 2 and 6 are equivalent and would give the ortho product.
2. The positions 3 and 5 are equivalent and would give the Meta product.
3. The position 4 is unique and would give the Para product.
Two Types of Influence of Substituents:
A substituent’s (S) already present on the benzene ring exercises two types of influence on further substitution.
1. Directive or Orientation Effect:
The first substituent (S) may direct the next incoming substituent (E) to ortho, meta or para position, depending on the nature of the substituent. This is called the Directive or the Orientation (Orient= to arrange) Effect.
2. Activity Effects:
The present substituents may activate or deactivate the benzene ring toward further substitution. These effects are called the Activate Effects.