The Chemistry of Dyestuff. Intermediate Compounds. III. Nitro Compounds.

A Manual for Students of Chemistry and Dyeing
By
M. Fort, M.Sc. (Leeds) Late Lecturer in Dyeing in the Bradford Technical College and L. L. Lloyd, Ph.D. (Bern) Lecturer in Organic and Technical Chemistry in the Bradford Technical College
Cambridge: at the University Press 1919
(First edition 1917, reprinted 1919)The ordinary chemical reactions that are employed for the preparation of the intermediate compounds may be classified under nitration, sulphonation, chlorination, amidation, alkylation, arylation, acylation, hydroxylation, reduction, oxidation, condensation, and diazotisation.

Nitration. The introduction of nitro groups into aromatic compounds is one of the most important reactions, and is capable of application to a large variety of compounds. There are many methods of nitration, the selection of the method depending upon the nature of the compound to be nitrated. For example, phenol is more easily nitrated than benzene. As nitrating agents one employs nitric acid, potassium nitrate and sulphuric acid, nitric acid and sulphuric acid. The temperature at which nitration is carried out is, as a rule, very important, since the higher the temperature, the more readily will tar be formed, and, generally, the higher will be the degree of nitration. In many cases the sulphuric acid is added as a dehydrating agent in order to prevent the dilution of the nitric acid, so that the whole of the nitric acid is utilised for nitration purposes. In other cases it is desirable that sulphonation should precede nitration, to facilitate the latter, the sulpbonic acid group being replaced by the nitro group. Of great importance is the nature and position of groups present in the aromatic compound.

In some cases, owing to the oxidising action of nitric acid, it is necessary to shield certain groups, as in the preparation of nitranilines from acetanilide.

Nitration of benzene. Nitrobenzene C₆H₅NO₂ (BP. 205°C.) is manufactured from commercial pure benzene (thiophene present) in iron vessels by means of nitric and sulphuric acid. Sulphuric acid is added to combine with the water originally present in the nitric acid and also that formed during nitration. The quantity of sulphuric acid should be sufficient to produce the hydrate H₂ SO₄. 2H₂O. In this manner the whole of the nitric acid added is utilised for nitration. The acid mixture is slowly run into the benzene, and well agitated by means of iron stirring gear. The vessel is provided with a leaden or earthenware cooling worm, through which cold water may be run if the temperature of the contents of the vessel rises too high, During the beginning of the nitration the temperature is seldom allowed to exceed 40°C., but towards the finish the temperature may be raised to about 75°C. After the necessary time for nitration, the contents are allowed to stand until separated and the lower acid layer run off. This is concentrated, and again employed for the same purpose.

The nitrobenzene that collects above the acid liquor is separated and well washed to remove acid, and is then distilled, either directly or by steam, "pure" nitrobenzene or Mirbane oil. It may be obtained pure enough to use as a cheap scent by repeated steam distillation. It has an odour resembling bitter almond oil. The "pure" oil is used in the manufacture of aniline for the production of aniline black; also for the preparation of azobenzol, benzidine, Induline, etc. Nitrobenzene containing nitrotoluene is termed "nitrobenzene for red," as it is used in the manufacture of Magenta, and also Phosphine. Nitrobenzene is largely used as a solvent when reactions are to be conducted at fairly high temperatures. It may also be employed as an oxidising agent.

Nitrotoluenes are obtained in a similar manner to the manufacture of nitrobenzene. The crude nitrotoluene should not be distilled with direct fire on account of the liability to explosion. In this case there are two bodies, orthoand para-nitrotoluene, produced. The two isomers are separated by fractional distillation in vacuum, the para compound being further purified by crystallisation, o-nitrotoluol B.P. 223°C., p-nitrotoluol M.P. 54°C. They are employed for the production of the corresponding toluidines, etc. In some cases the two nitro compounds are not separated, but are converted into .mixed toluidines, the toluidines being more readily separated from one another than the nitro compounds.

Toluene is more easily nitrated than benzene.

Meta-Dinitrobenzene [-] (M.P. 90°C.) is obtained by a similar but stronger nitration in iron vessels; the nitrating acid is added in two portions, and the temperature is finally raised to about 100°C. The lower acid layer is run off, and the dinitrobenzene washed well with cold water and then with hot water, to prevent it from solidifying during washing. The latter washing will contain some dinitrobenzene and is used after cooling for the cold washing of a fresh batch. The dinitrobenzene is run into iron trays about 2 to 4 inches deep and allowed to solidify.

Small quantities of orthoand para-dinitrobenzenes are also produced.

Dinitrotoluene is obtained similarly, the chief product being the 2.4-dinitrotoluene.

Nitrochloro-benzenes are obtained similarly to the nitrotoluenes, and are separated by similar methods. Ortho-nitrochloro-benzene B.P. 246°C., M.P. 32°C. Para-nitrochloro-benzene B.P. 239°C., M.P. 83°C.

Dinitrochloro-benzene is obtained by further nitration of mono -chloronitrobenzene. The main product is 2.4-Dinitrochloro-benzene M.P. 50°C., B.P. 315C. This compound is chiefly used tor the production of dinitrophenol.

Nitronaphthalene [-] (M.P. 61°C.). Naphthalene, in a fine state of division, is well mixed with nitrating acid, the temperature during the action being maintained at 45° to 50°C. After cooling, the cake of ot-nitronaphthalene is washed similarly to the purification of meta-dinitrobenzene.

On further nitration of nitronaphthalene a mixture of 1.5 and 1.8-dinitronaphthalenes is obtained. This
consists roughly of one part of the 1.5 and two parts of the 1.8-dinitronaphthalenes. For many purposes the crude dinitronaphthalene is directly employed. These isomers may be separated from each other by means of cold pyridine, the 1.5-derivative being less soluble (1:125) than the 1.8-isomer (1:10).

1.5-Dinitronaphthalene M.P. 216°C.
1.8- " " M.P. 170°C.

After the introduction of two nitro groups a third may be introduced in the ß-position arid thus by the further nitration of naphthalene, triand tetra-nitronaphthalenes may be obtained.

Nitration of phenol. By the careful nitration of phenol with dilute nitric acid a mixture of orthoand para-nitrophenol is obtained. The temperature during nitration is not allowed to exceed 25°C., in order to prevent formation of tar. The dilute nitric acid is obtained by the action of dilute sulphuric acid on sodium or potassium nitrate. The ortho derivative is then separated from the para by steam distillation.

Ortho-nitrophenol (M.P. 44°C.) is a light yellow body used in the manufacture of ortho-nitroanisol for the preparation of anisidine and dianisidine.

Para-nitrophenol (M.P. 114C.) is non-volatile ir steam, and is colourless.

The dinitrophenols may be obtained by nitration of phenol, but as a rule the 2.4-dinitrophenol is obtained by boiling l-chlor-2.4-dinitrobenzene with a strong solution of sodium carbonate under a reflux condenser. A simple and effective type of condenser is an upright iron pipe with cold water running down the outside. The dinitrophenol is then precipitated by acidifying with sulphuric acid. It is a yellow compound, M.P. 114°C., and is employed for the preparation of sulphide blacks.

Picric acid [-](M.P. 122,5°C.) or symmetrical trihitrophenol is manufactured by sulphonating phenol at 100°C., generally in earthenware vessels. To the sulphonated phenol nitric acid is added with stirring, the picric acid is allowed to crystallise out, the acid liquor run off, the crystals washed with cold water and then recrystallised from hot water. Owing to the explosive nature of picric acid and its salts, especially in presence of traces of acids, or metallic oxides, the purification must be carefully attended to, explosions having been brought about by traces of such substances as whitewash, lime, etc.

Picramic acid  [-](M.P. 165°C.) is obtained by reducing a warm aqueous solution of picric acid with zinc dust and ammonia; when the solution remains distinctly alkaline the excess of ammonia is removed by boiling, the solution filtered and concentrated, and the picramic acid precipitated by addition of acetic acid. It is a red crystalline compound.