MCQ Detailed View

Phenol is an aromatic compound with a hydroxyl group (-OH) attached to a benzene ring. The hydroxyl group makes phenol highly reactive towards electrophilic substitution reactions, such as nitration. When phenol is treated with dilute nitric acid under mild conditions, mononitration occurs and products like ortho-nitrophenol and para-nitrophenol are formed. However, when phenol is heated with concentrated nitric acid, a more vigorous reaction takes place, and multiple nitro groups enter the benzene ring.

In this case, phenol undergoes trinitration. The hydroxyl group strongly activates the benzene ring, especially at the ortho and para positions. Due to the strong nitrating conditions of concentrated nitric acid, three nitro groups (-NO₂) attach at the 2, 4, and 6 positions of the benzene ring relative to the hydroxyl group. The final compound formed is 2,4,6-trinitrophenol, which is commonly known as picric acid.

Picric acid is a yellow crystalline solid, highly acidic due to the strong electron-withdrawing effect of the nitro groups, which make the hydrogen of the hydroxyl group more easily ionizable. It has been historically used as an explosive, a dye, and in chemical laboratories.

The other options are not correct for this reaction. Ortho-nitrophenol and para-nitrophenol are formed only when dilute nitric acid is used under controlled conditions. 1,3,5-trinitrobenzene is not formed because the hydroxyl group directs substitution to specific positions, leading instead to picric acid.

Therefore, heating phenol with concentrated nitric acid always gives picric acid (2,4,6-trinitrophenol) as the major product. This reaction is a key example of electrophilic substitution in aromatic chemistry and demonstrates the activating influence of the hydroxyl group on the benzene ring.