Cresol.—The term cresol used without further qualification indicates a mixture of the three isomers in substantially the same proportions in which they are found in coal tar. The United States Pharmacopoeia describes cresol, a mixture of isomeric cresols obtained from coal tar, as a colorless or yellowish to brownish-yellow or a pinkish, highly refractive liquid, becoming darker with age and on exposure to light. It is widely used in synthetic resins, antiseptics and disinfectants, and in medicine.
The xylenols.—Shortly after the original patents on Bakelite resins expired extensive research was begun for raw materials that would give different properties to the resultant resins. This work led to a study of the high-boiling tar acids, and methods of recovery for some of them were commercially developed. Among those obtained from coal tar are the six isomeric xylenols, methylethyl phenol, and one of the trimethyl phenols (see table [56]). Coal tar contains about 0.2 percent xylenols and 0.5 percent other high-boiling tar acids.
The principal uses for these products have been in the preparation of high-phenol coefficient disinfectants, and recently in the replacement of phenol and cresols in synthetic resins. It was found, for example, that 3: 5 xylenol reacts with formaldehyde faster than either metacresol or phenol. Numerous patents have been granted on the use of these high-boiling acids in the production of synthetic resins.
The xylenols, when pure, are colorless, crystalline substances boiling between 211° and 225° C. They are usually marketed in mixtures containing from 50 to 80 percent xylenols and 20 to 50 percent cresols. There is commercial production of at least one of the separated xylenols (3: 5). An appreciable part of our imports of crude cresylic acid and of our production of cresylic acid contains high percentages of the xylenols.
Other high-boiling tar acids.—The other high-boiling tar acids are ortho ethylphenol, meta ethylphenol, para ethylphenol, methyl ethylphenol, and the three isomeric trimethyl phenols. Several of these have been isolated from coal tar. All of them, when pure, are crystalline compounds with boiling points ranging between 206° and 235° C. There has been little, if any, commercial production of this group up to this time. They are known, however, to have very high phenol coefficients, a property which would make them suitable for use in disinfectants. Little is known as yet concerning their application in synthetic resins.
Cresylic acid.—Cresylic acid is a generic term now applied to mixtures of tar acids in widely varying proportions. As defined in the literature and as formerly used in commerce the term identified a mixture of ortho, meta, and para cresols in the proportions in which they occur in coal tar. This proportion is approximately 40 percent metacresol, 35 percent orthocresol, and 25 percent paracresol. But in recent years the designation cresylic acid has been applied to all sorts of mixtures of tar acids boiling above 190° C. Practically every maker of synthetic resins, antiseptics, and disinfectants has his own specifications for cresylic acid; it may be any mixture in almost any proportions of the three cresols, the six isomeric xylenols, and the higher boiling tar acids. Imports of crude cresylic acid are understood to be largely xylenol mixtures containing low percentages of the cresols. This loose application of cresylic acid in recent years is due to the increased commercial application of the high-boiling tar acids, especially the xylenols.
Under the Tariff Act of 1930 refined cresylic acid, that having a purity of 75 percent or more, is dutiable under paragraph 27 at 3½ cents per pound and 20 percent ad valorem based on American selling price or United States value; while crude cresylic acid, that having a purity of less than 75 percent, is free under paragraph 1651. The provision in paragraph 27 reads, “cresylic acid which on being subjected to distillation yields in the portion distilling below two hundred and fifteen degrees centigrade, a quantity of tar acids equal to or more than 75 per centum of the original distillate.” Under this provision cresylic acid may include an endless number of combinations of tar acids and may or may not contain any of the isomeric cresols. Of the 17 or more tar acids known to exist in coal tar (see table [56]), only 8 have boiling points above 215° C. It would seem to be more accurate and more in line with present day usage to have the tariff drop the designation cresylic acid in favor of more definite terms based on composition, such as cresols and cresol mixes, xylenol and xylenol mixes, etc.
About 60 percent of our consumption of cresylic acid is in synthetic resins and the remainder in the manufacture of insecticides, antiseptics, disinfectants, and other coal-tar products, such as intermediates for dyes, plasticizers for nitrocellulose, etc.