Question

How would you convert:
i) Ethanoic acid to methanamine? (1)
ii) Aniline to 2,4,6-tribromoaniline? (1)
iii) Aniline to benzene diazonium chloride? (1)
iv) Aniline to phenyl isocyanide? (1)
v) Chlorobenzene to chlorobenzene sulphonic acid? (1)
OR
i) Write four chemical equations for the methods of preparation of aniline. (1+1+1+1)
ii) Write the chemical equation of any one method for the preparation of chlorobenzene. (1)

Hint:

Conversions: use Hofmann degradation (acid→amide→amine, one C less), bromine water for tribromoaniline, cold NaNO2/HCl for diazotisation, CHCl3 + KOH for carbylamine, and fuming H2SO4 for sulphonation. Aniline prep: reduce nitrobenzene (Sn/Fe/HCl or H2/Ni).

Answer 1

Option 1 (Conversions):
i) Ethanoic acid → methanamine: First convert the acid to its amide, then apply Hofmann bromamide degradation (which removes one carbon).
\[CH_3COOH + NH_3 \rightarrow CH_3COONH_4 \xrightarrow{\Delta} CH_3CONH_2 + H_2O\]\[CH_3CONH_2 + Br_2 + 4NaOH \rightarrow CH_3NH_2 + 2NaBr + Na_2CO_3 + 2H_2O\]The product methanamine \(CH_3NH_2\) has one carbon less than acetamide.
ii) Aniline → 2,4,6-tribromoaniline: The \(-NH_2\) group is strongly activating and o/p-directing, so aniline reacts with bromine water at all ortho and para positions.
\[C_6H_5NH_2 + 3Br_2 \rightarrow C_6H_2Br_3NH_2 + 3HBr\] (2,4,6-tribromoaniline, white precipitate).
iii) Aniline → benzene diazonium chloride: Diazotisation with nitrous acid (\(NaNO_2 + HCl\)) at 273-278 K.
\[C_6H_5NH_2 + NaNO_2 + 2HCl \xrightarrow{273-278\,K} C_6H_5N_2^{+}Cl^{-} + NaCl + 2H_2O\]
iv) Aniline → phenyl isocyanide: Carbylamine reaction of a primary amine with chloroform and alcoholic KOH.
\[C_6H_5NH_2 + CHCl_3 + 3KOH \xrightarrow{\Delta} C_6H_5NC + 3KCl + 3H_2O\]
v) Chlorobenzene → chlorobenzene sulphonic acid: Sulphonation with concentrated (fuming) sulphuric acid substitutes mainly at the para position.
\[C_6H_5Cl + H_2SO_4(\text{conc.}) \rightarrow ClC_6H_4SO_3H + H_2O\] (p-chlorobenzenesulphonic acid).

Option 2:
i) Four preparations of aniline:
1) Reduction of nitrobenzene with tin and HCl: \[C_6H_5NO_2 + 6[H] \xrightarrow{Sn/HCl} C_6H_5NH_2 + 2H_2O\]
2) Reduction of nitrobenzene with iron and HCl: \[C_6H_5NO_2 + 6[H] \xrightarrow{Fe/HCl} C_6H_5NH_2 + 2H_2O\]
3) Catalytic hydrogenation of nitrobenzene: \[C_6H_5NO_2 + 3H_2 \xrightarrow{Ni} C_6H_5NH_2 + 2H_2O\]
4) Hofmann bromamide degradation of benzamide: \[C_6H_5CONH_2 + Br_2 + 4NaOH \rightarrow C_6H_5NH_2 + 2NaBr + Na_2CO_3 + 2H_2O\]
ii) One preparation of chlorobenzene: Chlorination of benzene in presence of anhydrous \(AlCl_3\) (Lewis acid catalyst):
\[C_6H_6 + Cl_2 \xrightarrow{\text{anhyd. } AlCl_3} C_6H_5Cl + HCl\]