18 августа, 2015 
Pokraskin The term intermediates refers to those compounds which are prepared from the original coal tar constituents by various chemical procedures and which, in turn, can be converted into commercial dyes by relatively simple further transformations. A typical example is aniline, which is prepared from benzene in various ways, and which can be converted into numerous dyes.
The reactions used in the preparation of intermediates are, for the most part, simple operations. Frequently, they proceed quantitatively according to the rules of stoichiometry. In other cases, side reactions are encountered which complicate the reaction and greatly reduce the yield. It is one of the important tasks of the dye chemist to study these undesirable side reactions sufficiently to understand their nature and then, if possible, to select the reaction conditions which will favor only the main reaction leading to the desired intermediate. This end is not always attained, because often the set of conditions which will eliminate the side reactions is not known, but the chemist must always bear in mind the possibility of achieving these conditions by further study. The preparation of l,8-aminonaphthol-3,6-disulfonic acid (H acid) illustrates this point. This compound has been known for nearly fifty years and is still being studied extensively in many laboratories, yet to this day has not been prepared in satisfactory yield.
In many cases, so-called quantitative yields are obtained but the product is not a pure compound. Thus, the reaction yields the calculated quantity of product, but this is a mixture of isomeric or analogous compounds which must be separated by some type of physical method. The isomeric nitrotoluenes (ortho, meta, and para),
for example, are always obtained in mixture, and special methods have had to be evolved to separate the pure individuals economically. Sometimes a circuitous route can be followed to arrive at an uncontaminated intermediate. For example, a substituent (usually a sulfo group) may be introduced and split out later (e. g., the preparation of o-chloro — toluene, page 163. In other cases, the reactions are selected so as to prevent the formation of undesirable isomers — as in the preparation of “Tobias acid” (2-naphthylamine-l-sulfonic acid). This intermediate cannot be obtained directly from /З-naphthylamine, but can be prepared under the proper conditions from /J-naphthol (see page 198).
As already mentioned, the basic operations of dye chemistry utilize simple chemical reactions. An intermediate can frequently be prepared in several entirely different ways and, in these cases, careful calculations must be made to determine which procedure is most advantageous. The least expensive process is often not necessarily the best when other factors are taken into account. For example, the question of apparatus may enter, and calculations may show that it is uneconomical to purchase an expensive apparatus for the process if a small quantity of the material is to be produced. Furthermore, consideration must be given to the usability of the side products formed. These often cannot be used at all (e. g., primuline), but may be valuable or even indispensable in another process (e. g,, chromium sulfate in the production of anthraquinone).
In evaluating a manufacturing procedure, the apparatus in which the operations are carried out must always be considered. Unlike preparations done in the laboratory, those in the plant cannot be carried out in glass equipment — except in unusual cases. Furthermore, it must be remembered that the chemicals often attack the apparatus, so its amortizement is an important consideration.
Most of the intermediates entering into the preparation of commercial organic dyes are members of the aromatic series. The substituents most frequently present are methyl, halogen (usually chlorine), nitro, amino, hydroxyl, alkoxyl, sulfo, carboxy, and (in some cases) aldehyde and ketone groups —the latter also in the form of quinone groups. These substituents, and other less common ones, may be introduced into the molecule either singly or in combination, and their introduction may be made in various sequences and in different manners, so that the number of possibilities is practically unlimited. Obviously, however, practice is governed by general principles, and the chemist who knows the fundamentals and has a command of the methods can easily determine the simplest method for preparing a desired compound.
Stoichiometric quantities of reactants are almost always used, and only in the most unusual cases is it necessary to use more or less of a reactant than the quantity demanded by the chemical equation. On the other hand, diluents must often be used in order to have the correct mixture during the reaction. It should also be emphasized that intensive mixing (stirring) is usually necessary for the satisfactory progress of the reaction. Also, the reaction temperature often plays an important role, for example, in preventing the formation of undesirable isomers.
In view of the factors cited, it is frequently quite unnecessary to give an exact “recipe” for the preparation of a product. Instead, the chemist is given an indication of the general principles involved and he, on the basis of his own experience, is immediately in a position to set up his own satisfactory procedure.
Accordingly, in this book the compounds in the tables are arranged in a genetic system, the derivatives being listed under their parent substances. This form of presentation has the advantage of giving the beginner a clear picture of the similarities in the reactions and, at the same time, giving information to the expert as to the preparation of the compound he desires. In order to increase the usefulness of these tables, there are included references, either to a procedure given in this book or to a literature source such as patents, scientific publications, etc., which describe the type of preparation for most of the compounds. Obviously, since it has not been possible to consider all possibilities in a relatively small laboratory book, the tables are necessarily incomplete and give only q general outline. The book is designed principally for the beginner, and it is hoped that it will serve for teaching purposes in the industrial chemical laboratory. The expert, however, may also find some use for the tables, particularly those relating to fields which are not his special provinces.
The subject material for the text of the book has also been arranged genetically. It covers, first, the most important operations: chlorination, nitration and reduction, sulfonation and alkali fusion, starting with the simplest basic structure, benzene. From the intermediates thus obtained, more complicated derivatives are built up by reaction series serving as illustrative examples. Following this, the derivatives of benzene homologs are treated in a similar manner, then the derivatives of naphthalene, and finally those of anthraquinone. Such an arrangement simplifies the study since not only is each intermediate traced from its parent structure but also its possible further transformations into more complicated intermediates are shown. In this way the beginner is taught to consider each operation both by itself and in connection with the total synthesis.
Опубликовано в рубрике 