Question

How will you obtain the following from phenol? (i) Picric acid (ii) Benzoquinone.

Hint:

Picric acid = trinitration of phenol with conc. \(HNO_3/H_2SO_4\); benzoquinone = oxidation of phenol with acidified \(Na_2Cr_2O_7\).

Answer 1

Step 1: (i) Picric acid from phenol. Picric acid is 2,4,6-trinitrophenol. The -OH group strongly activates the ring and directs incoming groups to the ortho and para positions, so nitration puts three nitro groups at positions 2, 4 and 6.
Reaction: Phenol is treated with concentrated \(HNO_3\) in the presence of concentrated \(H_2SO_4\).
\[C_6H_5OH + 3HNO_3 \xrightarrow{conc.\,H_2SO_4} C_6H_2(NO_2)_3OH + 3H_2O\]
The product \(C_6H_2(NO_2)_3OH\) is picric acid (2,4,6-trinitrophenol).
Step 2: (ii) Benzoquinone from phenol. Phenol is oxidised. On treatment with a strong oxidising agent such as sodium dichromate \((Na_2Cr_2O_7)\) and sulphuric acid, phenol is converted into benzoquinone (1,4-benzoquinone / p-benzoquinone).
Reaction:
\[C_6H_5OH \xrightarrow{Na_2Cr_2O_7/\,H_2SO_4} C_6H_4O_2\ (\text{1,4-benzoquinone})\]
Here the -OH carbon and the para carbon are oxidised to two \(C=O\) groups.
Step 3: Summary. Nitration gives picric acid; oxidation gives benzoquinone.
\[\boxed{\text{(i) conc. }HNO_3/H_2SO_4;\ \text{(ii) }Na_2Cr_2O_7/H_2SO_4}\]