1. By the reaction of an alkyl halide with ammonia (Hofmann method) : Aqueous or alcoholic solution of ammonia is heated in a sealed tube at 100°C.
\(CH_3I \xrightarrow{NH_3}\underset{1°amine}{CH_3NH_2}\xrightarrow{CH_3I}\underset{2°amine}{(CH_3)_2NH}\xrightarrow{CH_3I}\)
\(\underset{3°amine}{(CH_3)_3N}\xrightarrow{CH_3l}\underset{4°salt}{(CH_3)_4N^ +I^-}\)
(i) Reactivity of alkyl halides towards ammonolysis. RI > RBr > RCI
(ii) With excess of ammonia, 1° amine is the main product, while with excess of alkyl halide quaternary ammonium salt is the main product.
(iii) The method cannot be used for preparing arylamines because simple aryl halides do not undergo nucleophilic substitution easily.
Reaction of chlorobenzene and ammonia (Dow's process).
\(2C_6H_5Cl+2NH_3\xrightarrow[475\,K,60atm]{Cu_2O}\)
\(2C_6H_5NH_2+Cu_2Cl_2+H_2O\)
2. By the reaction of alcohol with ammonia : A mixture of the vapours of an alcohol and ammonia is passed over heated alumina or thoria at 350°C.
\(CH_3OH\xrightarrow{NH_3}CH_3NH_2 \xrightarrow{CH_3OH} (CH_3)_2NH\xrightarrow{CH_3OH}\)
\((CH_3)_3N\xrightarrow{CH_3OH}(CH_3)_4N^ +OH^-\)
The mixture can be separated from each other by means of benzenesulphonyl chloride1 C6H5SO2Cl (Hinsberg method), diethyl oxalate, (COOC2H5)2 (Hofmann method) and fractional distillation. Tertiary amines (having no hydrogen on nitrogen) do not react with these reagents.
Reaction of phenol with ammonia constitutes one of the industrial methods for preparing aniline.
\(C_6H_5OH+HNH_2\xrightarrow[300°C]{ZnCl_2} \underset{Aniline}{C_6H_5NH_2}+H_2O\)
In both these methods primary amine may be obtained as the main product by using a large excess of ammonia.
