Reactions of benzene ring:

Reactions of benzene ring: Electrophilic aromatic substitution: We have already learnt in XI standard that the groups like -OH, -NH , etc., which when directly attached to the benzene ring, activate the ring towards electrophilic substitution reaction and direct the incoming electrophile to occupy either the ortho or para position. Common electrophilic aromatic substitutions are as follows: i) Nitrosation: Phenol can be readily nitrosoated at low temperature with HNO . HNO 278K NO p-nitroso phenol phenol XII U11-Hydroxy XII U11-Hydroxy - - - - ii) Nitration: Phenol can be nitrated using % nitric acid even at room temperature, a mixture of ortho and para nitro phenols are formed. % HNO 298K - % yield NO % yield NO orthonitrophenol paranitrophenol The ortho and para isomers are separated by steam distillation, as o-nitro phenol is slightly soluble in water and more volatile due to intra molecular hydrogen bonding, whereas p-nitro phenol is more soluble in water and less volatile due to intermolecular hydrogen bonding. Nitration with Conc. HNO +con.H SO gives picric acid. Conc. HNO Conc. H O 298K , , -trinitrophenol (picric acid) phenol + H O NO NO O S iii) Sulphonation: Phenol reacts with con.H SO at 280K to form o-phenol sulphonic acid as the major product. When the reaction is carried out at 373K the major product is p-phenol sulphonic acid. phenol Conc. H SO o-phenol sulphonic acid p-phenol sulphonic acid SO H SO H XII U11-Hydroxy XII U11-Hydroxy - - - - iv) Halogenation: Phenol reacts with bromine water to give a white precipitate of , , -tri bromo phenol. 3Br H O 3HBr Br B Phenol , , -tribromophenol r Br , If the reaction is carried out in CS or CCl at 278K, a mixture of ortho and para bromo phenols are formed. o -bromo phenol phenol p-bromo phenol Br Br 278K Br CCl , v) Kolbe’s (or) Kolbe’s Schmit reaction: In this reaction, phenol is