AMINATION

INTRODUCTION:

The process by which amine group is introduced in an organic compound thereby producing primary, secondary or tertiary amine or its derivatives is known as amination process.

SIGNIFICANCE OF AMINATION

 Amination is a very important unit process as the products Amines are of very great importance as intermediates in the chemical process industries. They are used in production of dyes, rubber chemicals, nylon, pharmaceuticals, gasoline additives, surfactants, textile auxiliaries, photographic chemicals, chelating agents, sweetening agents, agricultural chemicals, polyurethanes, inks, plastics etc.

In recent years, the production of diamines by reductive methods has become increasingly important. Hexamethylenediamine is a prime intermediates, together with adipic acid, in the production of nylon 66. m-phenylenediamine is used as a crosslinking agent in epoxy resin. Toludenediamines are intermediates in the production of toluene disocyanates, from which are made polyurethane foam, rubbers, coatings and adhesives.

TYPES OF AMINATION:

• AMINATION BY Reduction
• Ammonolysis

 Amination by reduction involve the synthesis of amines by reductive methods. Amines can be produced by reducing nitro, nitroso, hydroxylamino, azoxy, azo and hydrazo compounds as well as oximes, amides, nitriles and azides. In each case, a carbon to nitrogen bond already exists.

Amines may also be formed by ammonolysis i.e. reacting compounds containing certain labile groups (e.g. halogens, hydroxyl and sulfonic) with ammonia. It involves substitution of –X, -H,-OH or –SO3H group by –NH2 group as well hydroamonolysis of carbonyl group.

• AMINATION BY Reduction

Amines can be easily produced by a great variety of reduction methods as shown below:

.

• The reduction of nitro, nitroso, hydroxylamino, azoxy, azo and hydrazo compounds.
• The reduction of nitrites, amides, oximes and azides.
• The replacement of labile groups, such as nitro, halogen, hydroxyl and sulfonic acid by reaction with ammonia or ammonia progenitors such as urea.
• Intramolecular replacement of (a) hydrazobenzenes and hydroxylamines (b) amides and (c) secondary and tertiary amines.
• The hydrolysis of N-substituted amides.
• Direct amination by means of hydroxylamine and sulfuric acid.

First four are the most important methods from chemical engineering point of view.

The reduction agents used are enlisted as follows:

1. Metal and acid: Iron and acid (Bechamp method) is the major example in this category, but other metals (tin, zinc) have also been employed. Generally, hydrochloric acid is preferred, but sulphuric acid, acetic acid and formic acid have also been used.
2. Metal and alkali: This method is used mainly for the production of azoxy, azo and hydrazo compounds. The latter are important in the manufacture of the benzidine series.
3. Metal hydrides
4. Catalytic: This method involves the use of hydrogen (or hydrogen - containing gases) and a catalyst such as nickel, copper, platinum, palladium or molybdenum sulfide.
5. Sulfide: This is used mainly for the partial reduction of polynitro aromatic compounds to nitro-amines and for reduction of nitro-anthraquinones to amino- anthraquinones.
6. Sulfite (Piria method): The reaction of sodium sulfite and bisulfite on an aromatic nitro compound leads to a mixture of amine and aminoaryl sulfonic acid.
7. Sodium hydrosulfite (hyposulfite).
8. Electrolytic
9. Sodium and sodium alcoholate.
10. Strong caustic oxidation-reduction.
11. Hydrogenated quinolines and naphthalene.

During the general course of reduction various intermediate reduction products are obtained. By a proper selection of reducing agent and careful regulation of the process, reduction may often be stopped at intermediates states and valuable products other than amines obtained. Metal and acid reduction is most vigorous and usually yields amines as end products. When nitrobenzene is treated with zinc and a mineral acid, the resultant product is aniline. When an alkaline solution is employed, hydrazobenzene is generally obtained, but very vigorous conditions sometimes result in the formation of aniline. When zinc dust and water are used, reaction product is phenyl hydroxylamine as shown below.

Selective reduction is normally carried out in presence of alkaline sulphide. When the compound to be treated contains more than one nitro group, the product of reduction depends upon the agents used. Thus, m-phenylenediamine is obtained by the iron and acid reduction of m-dinitrobenzene, while the alkaline sulfide reduction yields m-nitro aniline.

In case of nitro compounds consisting acid or alkali sensitive groups (e.g. ester or amides), it is necessary to adjust the pH carefully to avoid decomposition and side reactions. In such reductions on the acid side, it is generally advisable to replace the normally used mineral acids with organic acid like acetic acid. An example of the latter is the reduction of nitroanilides, for here the presence of mineral acids would tend to hydrolyze the anilide to an amine.

The method to be used will therefore, depend upon the degree of reduction desired, the sensitivity to the process of both starting material and final product, the need for avoiding contaminants and the overall economics.

The reduction of nitro compounds involves the progressive removal and replacement by hydrogen of the oxygen in the -NO2 group. The intermediate products in the reduction of nitrobenzene, whose interrelationships are shown in the following figure are obtained by control of the reduction potential of the system.

Sulphide reduction: though this is an expensive method of reduction but find uses in case of partial and selective reduction. Like reduction of dinitro compounds to natramines, reduction of nitrophenols,  reduction of nitroantraquinons.

Since alkali sulphide can easily take up oxygen, it causes reduction of nitro compound. The reaction may involve following steps:

4 RNO2 +6 Na2S + H2O → 4 RNH2  + 3 Na2S2O3 + 6 NaOH

RNO2 + Na2S2 + H2O →  RNH2  + Na2S2O3 

4 RNO2 +6 NaHS + H2O → 4 RNH2  + 3 Na2S2O3 

In dye intermediate industry, ammonium sulphide is used extensively to reduce dinitrobenzene derivative to nitroamines.

Catalytic Hydrogenation: This is the process of reducing nitro compounds into amines by hydrogen gas in presence of any catalyst. Catalytic reduction can be carried out in different conditions by using different catalysts, promoter, poison, temperature, pressure solvent etc. A slight modification in any of these, have a marked influence on the course of reduction and rate of hydrogenation.

The general course of reduction follows following order:

Nitrobenzene → nitrosobenzene → hydrazobenzene → aniline

Generally, used in preparation of aniline and xylidenes.