WHAT IS ORTHA PARA ACTIVATING GROUPS

The substituents or groups which direct the incoming group to ortho and para positions are called ortho and para directiong groups.For ex- -CH3,-C2H5,-OH,-Br,-NH2,-OCH3 etc.
For example nitration of toluene is carried out,then mixture of ortho and para nitrotoluene is formed.
C6H5CH3 + HNO3 + H2SO4 ------------> o-Nitrotoluene + p-Nitrotoluene
All the above groups except the halogens increases the reactivity of benzene ring towards electrophilic substitutions as they increases the e-density at the ring through resonance effect.In case of phenol the -OH group directs the incoming groups to o and p posotions and activates the ring towards electrophilic substitution.So e- density is more concentrated at these position thus electrophilic substitution takes place at ortho and para positions.