Лако-красочные материалы — производство

Halogen atoms, usually chlorine or bromine (rarely iodine or fluorine1) are generally introduced by the action of the elementary halogen on the compound to be substituted. It is often necessary to use a catalyst; otherwise chlorine adds instead of substitutes (replacing hydrogen). The catalyst most commonly employed is iron (ferric chloride), sometimes iron with a trace of iodine and, less frequently, antimony, sulfur, or phosphorous compounds.

In place of elementary chlorine (bromine), sodium hypochlorite in the presence of mineral acid is used in certain cases (e. g., chlorination of acet-o-toluidide). The nascent chlorine reacts very energetically and undesirable side reactions do not occur.

If for any reason the direct introduction of chlorine is not possible, or the chlorine does not enter the desired position in the molecule, recourse may be had (but rarely!) to the Sandmeyer reaction (see example on page 161). In some cases, the Sandmeyer reaction can be avoided (it is rather bothersome and expensive) by the use of a trick reaction, such as in the preparation of o-chlorotoluene from p toluene — sulfonic acid2 (page 163).

Other methods may be used for halogenating phenols. Thus, phenol-

1 See, for example, Hoffa and Muller (I. G.), Ger. Pat. 551,882 (1932) [Frdl., 19, 1625 (1934); C. A., 26, 4959 (1932)1; and Osswald, Muller, and Steinhauser (I. G.), Ger. Pat. 575,593 (1933) [Frdl., 20, 475 (1935); C. A., 27, 4813 (1933)1-

2 Badische A. und S. F., Ger. Pat. 294,638 (1916) [Frdl., 12, 908 (1914-1916); CA., 11, 2582 (1917)].

ate and hypochlorite react to produce chiefly the o-chlorophenol (75 to 80 per cent), while the reaction of free phenol and sulfuryl chloride yields p-chlorophenol as the chief product (about 78 per cent)[1] (see page 145).

In special cases, chlorination is brought about by replacement of a sulfonic group by chlorine. This reaction is particularly important with anthraquinone compounds, but it is also known in the benzene series (see page 236).

These reactions do not by any means cover the whole field, but represent only the more important portions of it Later in this book, other less important reactions are mentioned, such as alkylation of amines, introduction of the aldehyde group by the Sandmeyer method, and phenylation. Although some of these reactions are carried out on a large scale, especially in the preparation of alkyl — and benzylanilines and toluidines, they are in terms of quantity of considerably less im­portance than the operations discussed above.

In all industrial operations, the chemist must always strive to achieve the greatest yield at the smallest cost. The methods which are suitable for scientific research frequently are not successful in industry. It must also be remembered that the intermediates should be as pure as possible because small variations in purity often lead to disproportionately large losses in the manufacture of the dye. For this reason, many of the com­mercially available intermediates are chemically pure, and the require­ments in recent years have become even more exacting.