Question

Identify the correct sequence of reagents to carry out the conversion: \[ \mathrm{Aniline} \longrightarrow \mathrm{Fluorobenzene} \]

• A. (i) $\mathrm{NaNO_2 + HCl,\ 273-278\,K}$; (ii) $\mathrm{HBF_4}$, heat
• B. (i) $\mathrm{Cl_2/FeCl_3}$; (ii) $\mathrm{NaF}$
• C. (i) $\mathrm{HNO_3 + H_2SO_4}$; (ii) $\mathrm{HF}$
• D. (i) $\mathrm{NaNO_2 + HCl}$; (ii) $\mathrm{Cu_2F_2}$

Hint:

Remember: \[ \mathrm{Aniline} \rightarrow \mathrm{Diazonium\ Salt} \rightarrow \mathrm{Fluorobenzene} \] The conversion is achieved by the Balz-Schiemann reaction using $\mathrm{HBF_4}$ followed by heating.

Answer 1

The Correct Option is A

Concept: Fluorobenzene is prepared from aniline through the Balz-Schiemann reaction. The reaction involves diazotisation of aniline followed by conversion of the diazonium salt into fluorobenzene.

Step 1: Diazotisation of aniline. Aniline reacts with sodium nitrite and hydrochloric acid at low temperature. \[ \mathrm{C_6H_5NH_2} \xrightarrow[\ 273-278K\ ]{\mathrm{NaNO_2/HCl}} \mathrm{C_6H_5N_2^+Cl^-} \] Benzene diazonium chloride is formed.

Step 2: Formation of diazonium tetrafluoroborate. The diazonium chloride reacts with fluoroboric acid. \[ \mathrm{C_6H_5N_2^+Cl^-} + \mathrm{HBF_4} \rightarrow \mathrm{C_6H_5N_2^+BF_4^-} \]

Step 3: Thermal decomposition. On heating, \[ \mathrm{C_6H_5N_2^+BF_4^-} \rightarrow \mathrm{C_6H_5F} +\mathrm{BF_3} +\mathrm{N_2} \] Fluorobenzene is obtained.

Step 4: Identify the reaction. This is the Balz-Schiemann reaction. Therefore, \[ \boxed{\text{Option (1)}} \]