Chemistry Preparation of Amines

Topics Covered :

● Preparation of Amines
● Reduction of Nitro Compounds
● Ammonolysis of Alkyl Halides
● Reduction of Nitriles
● Reduction of Amides
● Gabriel Phthalimide Synthesis
● Hoffmann Bromamide Degradation Reaction

Preparation of Amines :

There are various methods by which amines are prepared. So the following methods are :

Reduction of nitro compounds :

`=>` Nitro compounds are reduced to amines by passing hydrogen gas in the presence of finely divided nickel, palladium or platinum and also by reduction with metals in acidic medium.

`=>` Nitroalkanes can also be similarly reduced to the corresponding alkanamines.

`=>` Reduction with iron scrap and hydrochloric acid is preferred because `color{red}(FeCl_2)` formed gets hydrolysed to release hydrochloric acid during the reaction. Thus, only a small amount of hydrochloric acid is required to initiate the reaction.

Ammonolysis of alkyl halides

The carbon - halogen bond in alkyl or benzyl halides can be easily cleaved by a nucleophile.

● Hence, an alkyl or benzyl halide on reaction with an ethanolic solution of ammonia undergoes nucleophilic substitution reaction in which the halogen atom is replaced by an amino (`color{red}(–NH_2)`) group.

● This process of cleavage of the `color{red}(C–X)` bond by ammonia molecule is known as ammonolysis.

● The reaction is carried out in a sealed tube at `373 K`.

● The primary amine thus obtained behaves as a nucleophile and can further react with alkyl halide to form secondary and tertiary amines, and finally quaternary ammonium salt. See fig.

`color{red}(underset(1^o)(RNH_2) overset(RX)→ underset(2^o)(R_2NH) overset(RX)→ underset(3^o)(R_3N) overset(RX)→ undersettext(Quaternary ammonium salt)(R_4 overset(+)N overset(-)X))`

`=>` The free amine can be obtained from the ammonium salt by treatment with a strong base :

`color{red}(R - overset(+)NH_3overset(-)X+NaOH → R-NH_2+H_2O +overset(+)Na overset(-)X)`

`color{green}(text(Disadvantage ))` : Ammonolysis yields a mixture of primary, secondary and tertiary amines and also a quaternary ammonium salt.

● However, primary amine is obtained as a major product by taking large excess of ammonia.

● The order of reactivity of halides with amines is `color{red}(RI > RBr > RCl)`.
Q 3063201145

Write chemical equations for the following reactions:
(i) Reaction of ethanolic `NH_3` with `C_2H_5Cl`.
(ii) Ammonolysis of benzyl chloride and reaction of amine so formed with two moles of `CH_3Cl`.


(i). `undersettext(Chloroethane)(C_2H_5 -Cl) overset(NH_3)→ undersettext(Ethanamine)(C_2H_6-NH_2) overset(C_2H_5-Cl)→ undersettext(N-Ethylethanamine)(C_2H_5 - overset(overset(H)(|))N-C_2H_5) overset(C_2H_5-Cl)→ undersettext(N, N- Diethylethanamine)(C_2H_5 - underset(underset(C_2H_5)(|))N-C_2H_5)`

(ii). `undersettext(Benzylchloride)(C_6H_5-CH_2-Cl) overset(NH_3)→ undersettext(Benzylamine)(C_6H_5-CH_2NH_2) overset(2CH_2Cl)→ undersettext(N, N - Dimethylphenylmethanamine)(C_6H_5-CH_2- underset(underset(CH_3)(|))N-CH_3)`

Reduction of nitriles :

`=>` Nitriles on reduction with lithium aluminium hydride `color{red}(LiAlH_4)` or catalytic hydrogenation produce primary amines.

● This reaction is used for ascent of amine series, i.e., for preparation of amines containing one carbon atom more than the starting amine.

`color{red}(R-C equiv N underset(Na(Hg)//C_2H_5OH) overset(H_2//Ni)→ R- CH_2-NH_2)`

Reduction of amides :

The amides on reduction with lithium aluminium hydride yield amines.

`color{red}(R - overset(overset(O)(||))C-NH_2 underset((ii)H_2O) overset((i) LiAlH_4)→ R-CH_2-NH_2)`

Gabriel phthalimide synthesis :

`=>` Gabriel synthesis is used for the preparation of primary amines.

`=>` Phthalimide on treatment with ethanolic potassium hydroxide forms potassium salt of phthalimide which on heating with alkyl halide followed by alkaline hydrolysis produces the corresponding primary amine.

`=>` Aromatic primary amines cannot be prepared by this method because aryl halides do not undergo nucleophilic substitution with the anion formed by phthalimide.

Hoffmann bromamide degradation reaction :

`=>` Hoffmann developed a method for preparation of primary amines by treating an amide with bromine in an aqueous or ethanolic solution of sodium hydroxide.

`=>` In this degradation reaction, migration of an alkyl or aryl group takes place from carbonyl carbon of the amide to the nitrogen atom.

`=>` The amine so formed contains one carbon less than that present in the amide.

`color{red}(R - overset(overset(O)(||))C-NH_2+Br+4NaOH → R-NH_2+Na_2CO_3+2NaBr+2H_2O)`

Q 3063401345

Write chemical equations for the following conversions:
(i) `CH_3–CH_2–Cl` into `CH_3–CH_2–CH_2–NH_2`
(ii) `C_6H_5–CH_2–Cl` into `C_6H_5–CH_2–CH_2–NH_2`


(i) `undersettext(Chloroethane)(CH_3-CH_2-Cl) oversettext(Ethanolic NaCN)→ undersettext(propanenitrile)(CH_3-CH_2 - C equiv N) oversettext(reduction)→ undersettext(Propan -1 - amine)(CH_3-CH_2-CH_2-NH_2)`

(ii). `undersettext{ Chlorophenylmethane (benzyl chloride)} (C_6H_5-CH_2-Cl) oversettext(Ethanolic NaCN)→ undersettext{Phenyethanenitrile (Benzyl cyanide)} (C_6H_5-CH_2 - C equiv N) overset(H_2//Ni)→ undersettext(2-phenylethanamine)(C_6H_5-CH_2-CH_2-NH_2)`
Q 3073101946

Write structures and IUPAC names of
(i) the amide which gives propanamine by Hoffmann bromamide reaction.
(ii) the amine produced by the Hoffmann degradation of benzamide.


(i) Propanamine contains three carbons. Hence, the amide molecule must contain four carbon atoms. Structure and IUPAC name of the starting amide with four carbon atoms are given below: `CH_3-CH_2-CH_2 - underset ( underset(O)(||))C-NH_2` Butanamide

(ii) Benzamide is an aromatic amide containing seven carbon atoms. Hence, the amine formed from benzamide is aromatic primary amine containing six carbon atoms.