Question

How will you prepare phenol from the following reagents (write only chemical equation)?
i) Benzene
ii) Aniline

Hint:

Benzene: sulphonate, fuse with NaOH, then acidify. Aniline: diazotise at low temperature, then warm the diazonium salt with water.

Answer 1

i) Phenol from benzene (sulphonation route).
Step 1: Benzene is sulphonated with concentrated sulphuric acid to give benzenesulphonic acid.
\[ C_6H_6 + H_2SO_4 \rightarrow C_6H_5SO_3H + H_2O \]
Step 2: Benzenesulphonic acid is heated (fused) with molten sodium hydroxide to give sodium phenoxide.
\[ C_6H_5SO_3H \xrightarrow{NaOH,\ fuse} C_6H_5ONa \]
Step 3: Acidification of sodium phenoxide gives phenol.
\[ C_6H_5ONa + H^{+} \rightarrow C_6H_5OH + Na^{+} \]
ii) Phenol from aniline (diazotisation route).
Step 1: Aniline is treated with nitrous acid \( (NaNO_2 + HCl) \) at 273 to 278 K to form benzenediazonium chloride.
\[ C_6H_5NH_2 + NaNO_2 + 2HCl \xrightarrow{273-278\,K} C_6H_5N_2^{+}Cl^{-} + NaCl + 2H_2O \]
Step 2: The diazonium salt is warmed with water, where it is hydrolysed to phenol with the release of nitrogen gas.
\[ C_6H_5N_2^{+}Cl^{-} + H_2O \xrightarrow{warm} C_6H_5OH + N_2\uparrow + HCl \]