Ginsburg MD / Ginsburg / Robinson Concerning Amines

Their Properties, Preparation and Reactions
1. Auflage 2014
ISBN: 978-1-4831-8590-3
Verlag: Elsevier Science & Techn.
Format: EPUB
Kopierschutz: 6 - ePub Watermark

E-Book, Englisch, 208 Seiten

Concerning Amines: Their Properties, Preparation and Reactions attempts to describe the accumulated knowledge on the properties of amines. This text first discusses nitrogen atom contained in amines, along with the basicity, geometry, nomenclature, occurrence, reaction, and preparation of these derivatives of ammonia. This book then explains the stereochemistry of the amino group and of quaternary ammonium compounds. Amine salts; absolute configuration of asymmetric amines; and influence of amino group are also tackled. This text also looks into the amino group in heterocyclic aromatic systems. This book concludes by explaining the syntheses of heterocyclic amines, enamines, and further reactions of carbonyl compounds with amines, including the Eschweiler, Vilsmeyer, and von Braun reactions. This publication will be invaluable to those interested in studying the properties of amines, such as chemists and students of chemistry.

Autoren/Hrsg.

Ginsburg MD, David

Ginsburg, David

Robinson, Robert

Leseproben

PRACTICAL CONSIDERATIONS IN PREPARATION OF AMINES

Publisher Summary

This chapter discusses the practical considerations in preparation of amines. Another consideration in the preparation of amines is the economics of the process involved. In preparing the same compound on a bench scale for some further laboratory-scale synthetic sequence, convenience and time spent by the investigator involved are usually the overriding factors in the choice of the synthetic method. The reduction of nitro compounds is definitely a reasonable and a practicable method for preparing primary amines. Aniline is prepared industrially by the reduction of nitrobenzene, which in turn is obtained by nitration of benzene. This is an industrial process today and has been for many years. Benzene is the most reasonable starting material from the viewpoint of economics. It is nitrated with mixed acid, a mixture of nitric and sulfuric acids, in high yield. The nitrobenzene is readily isolated and the final reduction stage is accomplished with cheap iron filings in the presence of hydrochloric acid.

Not all the classes of compounds described in the previous section constitute practical starting materials for the preparation of amines. For example, the best way to prepare the aromatic diazo compounds is from the corresponding primary amine.

Clearly, therefore, this would not be a logical nor practical starting material for the preparation of the primary amine concerned even if one found a good method for reduction of the diazo compound to the corresponding primary amine.

We shall therefore be selective and discuss only the more practicable methods for the preparation of amines and not the behaviour of all of the abovementioned classes of compounds upon attempted reduction.

Another consideration in the preparation of amines (or, for that matter, of any other class of compounds) is the economics of the process involved. This aspect will be exemplified as our discussion proceeds but one must always realize that the industrial preparation of organic compounds will place overwhelming emphasis on the economics of the process because the management presumably wishes to keep their jobs by paying dividends to the shareholders. In preparing the same compound on a bench scale for some further laboratory-scale synthetic sequence, convenience and time spent by the investigator involved are usually the overriding factors in the choice of the synthetic method.

Let us take a case in point. We shall see that reduction of nitro compounds is definitely a reasonable and a practicable method for preparing primary amines.

Aniline is prepared industrially by reduction of nitrobenzene which is, in turn, obtained by nitration of benzene.

This is an industrial process today and has been for many years. First, benzene is the most reasonable starting material from the viewpoint of economics. It is nitrated with “mixed acid”, a mixture of nitric and sulphuric acids, in high yield. The nitrobenzene is readily isolated and the final reduction stage is accomplished with cheap iron filings in the presence of hydrochloric acid, another heavy inorganic chemical available in commerce in large quantities and therefore relatively cheap.

Now, undoubtedly the reduction step may be accomplished by means of other reagents. There is no doubt that nitrobenzene may be catalytically reduced in the presence of hydrogen to give aniline. This would, however, require an investment in the catalyst; even if this were recoverable, its recovery would cost us a certain sum. It might also require the use of pressure equipment, since hydrogen is a gas, and this equipment would require a capital expenditure which a manager of an aniline production plant would probably find it unwise to make. It is doubtful, in fact, if any chemist would consider synthesizing aniline in the laboratory by a method other than the industrial one. He certainly would not reduce the nitrobenzene catalytically even when equipment for such reduction were available nor would he use lithium aluminium hydride for the reduction step because of the reagent cost even on a laboratory scale.

If, however, one wanted to prepare allylamine on a laboratory scale one would definitely reduce acrylonitrile with lithium aluminium hydride and save the time of the investigator rather than use a cheaper reagent and a different starting material.

On the other hand, it is quite unlikely that any industrialist would consider this as the most economical method for manufacturing allylamine.

REDUCTIO OF NITRO COMPOUNDS

Aliphatic nitro compounds are industrially available as starting materials for the preparation of amines. They are best prepared by vapour-phase nitration of alkanes.

Aromatic nitro compounds are readily available by direct nitration of aromatic hydrocarbons and certain of their derivatives, using a mixture of nitric and sulphuric acids. Indeed this reaction is the first step of one of the most versatile synthetic sequences in the chemistry of aromatic compounds. The sequence is

The reduction of nitro compounds to amines may be accomplished by metal–acid combinations, by catalytic reduction and in specific cases other reducing agents may be used. Because of its particular importance and because of the chemical insight obtained in this reaction, we shall first discuss in detail the preparation of aniline by reduction of nitrobenzene. Analogous intermediates are perhaps obtained in other reductions of nitro compounds but no other case has been investigated so thoroughly as the reduction of nitrobenzene.

In the industrial preparation of aniline, nitrobenzene is reduced with iron filings and 30% hydrochloric acid. After completion of the reaction the mixture must be neutralized with sodium carbonate and the aniline is removed by steam distillation. Aniline is somewhat soluble in water, sufficiently so that this factor must be considered in the industrial process. The soluble aniline is salted out by the addition of common salt. (Indeed, sodium chloride is formed during the neutralization step.) Two layers are then obtained, one of 20% sodium chloride solution and the other of aniline. Finally, pure aniline may be obtained by distillation. The yield in this process is nearly quantitative. Particular care must be taken in handling aniline as it is poisonous; the first symptom is cyanosis, the lips, for example, appearing bluish.

Aniline is also prepared industrially by amination of chlorobenzene with ammonia in the presence of a copper oxide catalyst and at high temperature and pressure. This requires pressure equipment for carrying out the reaction. Although pressure equipment would be considered uneconomical for catalytic hydrogénation of nitrobenzene to aniline, it is economically feasible to make a capital investment in such equipment for the amination of chlorobenzene. This cost is weighed against the lower cost of chlorine used for the production of chlorobenzene as against the higher cost of nitric acid used in the other process to make nitrobenzene.

Studies have been made of the reduction of nitrobenzene under a variety of reaction conditions. In this way discrete intermediate steps could be isolated. A suitable means for such study is to resort to electrolytic reduction because conditions are easily controlled by varying such factors as current density, potential and pH of the various steps. The reduction is carried out by cathodic hydrogen. It was shown that nitrobenzene is first reduced to nitrosobenzene (uptake of two hydrogen atoms), this then gives phenylhydroxylamine (uptake of two additional hydrogen atoms). The first reduction product, nitrosobenzene, is reduced too quickly to phenylhydroxylamine to permit its actual isolation but its presence can be shown, for example, by adjusting the pH to the alkaline side when nitrosobenzene and phenylhydroxylamine condense with each other and give azoxybenzene. We have here addition of phenylhydroxylamine to the nitroso group under the catalytic influence of base.

The presence of nitrosobenzene which requires a lower cathodic potential for its reduction than does nitrobenzene may also be shown by carrying out the reduction in the presence of substances which interact with nitrosobenzene to form coloured dyes.

Azoxybenzene may be further reduced, two hydrogen atoms being taken up and its oxygen atom being removed, the product being azobenzene. This reduction may be accomplished either electrolytically or by iron in water. The next stage of the reduction is hydrazobenzene (uptake of another two hydrogen atoms) and finally hydrogenolysis of the hydrazobenzene (two additional hydrogen atoms) gives 2 moles of aniline.

Azoxybenzene may be obtained in good yield by boiling nitrobenzene with a methanolic solution of sodium methoxide or with alcoholic potassium hydroxide solution.

Phenylhydroxylamine may be obtained in good yield by reduction of nitrobenzene in neutral solution. Zinc dust or aluminium amalgam in water may be used as the reducing agents. Ammonium chloride is used as a buffer in order to avoid the solution becoming too alkaline.

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