Substitution in Monosubstituted Benzene Derivatives:- a substituent already present on the benzene ring exercises two types of influence on further substitution.
Directive or Orientation Effect:- The substituent already present on the benzene ring directs the incomeing substituent to occupy ortho position. This direction depends on the nture of the first substituent and is called directive or the orientation effect.
Activity Effects:- The substituent already present can increase or decrease the rate of further substitution i.e., it either activates or deactivates the benzene ring towards further substitution. These effects are called activity effects.
Directive Effects of Substituents:- On the basis of extensive experiments it has been shown that when the second substituent, i.e., incoming group enters the benzene nucleus, the main product is either a mixture of ortho= and para isomers or the meta isomer, depending on the nature of the first substituent, i.e., the group already present to the benzene nucleus (host). Thus, there are two types of substituents (i) those wihich direct the incoming group to ortho and para positions simultaneously (neglecting meta all together.
(ii) Those which direct the incoming group to meta position only (neglecting ortho and para positions all together).