What happens when phenol is treated with conc. nitric acid?

2,4,6-Trinitrophenol; is produced

The correct answer is option 4. 2,4,6-Trinitrophenol; is produced.

When phenol (\(\text{C}_6\text{H}_5\text{OH}\)) is treated with nitric acid (\(\text{HNO}_3\)), it undergoes nitration, resulting in the introduction of nitro groups onto the benzene ring. The specific products depend on the conditions and concentration of nitric acid used.

Nitration of Phenol

Reaction Conditions:

Dilute Nitric Acid: When phenol is treated with dilute nitric acid, the primary product is 2-nitrophenol. This is because phenol is an activated aromatic compound, and the hydroxyl group (-OH) is an electron-donating group, which makes the ring more reactive towards electrophilic substitution. The nitro group preferentially adds to the ortho position relative to the hydroxyl group.

Concentrated Nitric Acid:

When phenol is treated with concentrated nitric acid, further nitration occurs due to the increased reactivity of the phenol ring. The primary products are 2,4-dinitrophenol and, under some conditions, 2,4,6-trinitrophenol. The additional nitro groups add to the remaining available positions on the benzene ring.

For the nitration of phenol under typical conditions (using concentrated nitric acid), the product  formed is: 2,4,6-Trinitrophenol.