Question

Write down the method of preparation of phenol. Write chemical equations of the reaction of phenol with the following: (i) Conc. HNO$_3$ (ii) Bromine water (iii) Zinc

Hint:

Phenol is highly reactive towards electrophilic substitution due to the activating effect of the –OH group. Therefore, it readily gives nitration (picric acid) and bromination (2,4,6-tribromophenol) without needing catalysts. With reducing agents like Zn, phenol can be deoxygenated to benzene.

Answer 1

Preparation of phenol:
One important laboratory method is from diazonium salts (benzene diazonium chloride).
\[ \mathrm{C_6H_5NH_2 \xrightarrow{NaNO_2/HCl,\ 273K} C_6H_5N_2^+Cl^- \xrightarrow{H_2O,\ \Delta} C_6H_5OH + N_2\uparrow + HCl} \] Thus, phenol is obtained by hydrolysis of benzene diazonium chloride.
Reactions of phenol:
(i) With conc. HNO$_3$ (Nitration):
Phenol reacts with concentrated nitric acid to give picric acid (2,4,6-trinitrophenol).
\[ \mathrm{C_6H_5OH + 3HNO_3 \xrightarrow[\,]{conc.\ H_2SO_4} 2,4,6\!-\!(NO_2)_3C_6H_2OH + 3H_2O} \] (ii) With bromine water (Bromination):
Phenol reacts with bromine water to give a white precipitate of 2,4,6-tribromophenol.
\[ \mathrm{C_6H_5OH + 3Br_2(aq) \;\longrightarrow\; 2,4,6\!-\!C_6H_2Br_3OH\,(ppt) + 3HBr} \] (iii) With zinc (Reduction):
Phenol reacts with zinc on heating to give benzene.
\[ \mathrm{C_6H_5OH + Zn \;\xrightarrow{\Delta}\; C_6H_6 + ZnO} \] \[ \boxed{\text{Hence, phenol undergoes nitration, bromination, and reduction with Zn.}} \]