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Amines from Alkyl Halides

(a) By ammonolysis of alkyhaldies

Alkyl halides or benzyl halides on reaction with ethanolic solution of ammonia in a sealed tube at 373 K undergo of C – X bond and the halo group (- X) is replaced by amino group ( - NH₂). This cleavage of C – X bond with ammonia is known as ammonolysis.

This reaction is a typical example of nucleophilic substitution reaction. Here, the ammonia molecules in the first step and amine molecules in the subsequent steps act as nucleophiles.

Hence, the primary amine formed in the first step further reacts with alkyl halide to form secondary amine, tertiary amine and finally quaternary ammonium salt.

The composition of the final mixture depends upon the molar ratio of alkyl halide and ammonia at the beginning of the reaction. If alkyl halide is in excess the major product is quaternary ammonium salt whereas if ethanolic ammonia is in excess the major product is primary amine. The reactivity of various halides in this reaction is:

Limitations of ammonolysis

(i) Ammonolysis of alkyl halides does not give single amine but gives a mixture of primary secondary and tertiary amines.

(ii) Ammonolysis is not suitable for preparing aryl amines because of the low reactivity of aryl halides towards nucleophilic substitutions.

(b) By treatment with sodium azide followed by reduction: on treatment with sodium azide, alkyl halides undergo nucleophilic substitution to form alkyl azides, which on catalytic hydrogenation give primary amines with same number of carbon atoms as in parent halide.


(c) Gabriel Phthalimide synthesis: this method is used to prepare pure 1˚ aliphatic amine or 1˚ aryl alkyl amine from alkyl halide or aryl alkyl halide respectively in this method phthalimide is first converted into potassium phthalimide by reaction with KOH. The potassium phthalimide on treatment with KOH. The potassium phthalimide on treatment with alkyl halide gives N-alkyl phthalimide which on subsequent alkaline hydrolysis with 20% NaOH or hydrazenolysis with hydrazine gives pure primary amine.

It may be noted that:

(i) Gabriel synthesis yield only 1˚ amines.

(ii) 2˚ and 3˚ amines cannot be prepared by Gabriel synthesis.

(iii) Aniline cannot be prepared by Gabriel synthesis: It is because of the fact that aryl halides do not undergo nucleophilic substitution reactions under ordinary conditions. Hence, C₆H₅Cl or C₆H₅Br does not react with potassium phthalimide to give N.phenylphthalimide.

Preparation of amines from amides and alcohols

(1) from amides

(a) By reduction of Amides using Na/C₂H₅OH or LiAlH₄: In this reaction amides get reduced to primary amines with the same number of carbon atoms as in parent amide

Secondary or tertiary amides can, however, give secondary or tertiary amines.


(b) By Hoffmann’s Bromamide reaction: primary (1˚) acid amides on reaction with Br₂ in the presence of alkalies at about 343 K give primary amines. It may be noted that amine formed by this method has one carbon atom less than the parent amide.


(2) From alcohols (Industrial method)

Aliphatic amines of low molecular mass are prepared on industrial scale by passing a mixture of an alcohol and ammonia in the vapour phase over heated alumina at 575 K. if ammonia is in excess; the primary amine is the major product. However, if the alcohol is in excess, then primary, secondary and tertiary amines are formed together.