The term coal-tar acids is applied to certain organic compounds either obtained from or known to be present in coal tar. Probably the best known is phenol or carbolic acid, produced in large quantities in the United States and abroad. Others of commercial importance are ortho, meta, and para cresol and the xylenols. All of these are definite chemical compounds available as such or in mixture with other tar acids. Cresylic acid is a term widely used in commerce for almost any mixture of tar acids. Formerly it was used to designate a mixture of ortho, meta, and para cresols in the proportions in which they are found in coal tar. The higher boiling tar acids (listed in table [56] below the xylenols) have little or no commercial importance at this time.

Table [56] lists the tar acids by commercial name, chemical name, boiling point of the pure compound, and average percentage present in coal tar. Boiling point is shown because the several tariff classifications covering tar acids under the acts of 1922 and 1930 (pars. 27 and 1651) depend upon distillation range (boiling points) for classification and assessment of duty (see pp. [119] and [124]).

Table 56.—Tar acids: Commercial and chemical names, boiling points and average percentage present in coal tar

Commercial nameChemical nameBoiling
point ° C		Average
percent in
coal tar
PhenolPhenol181.50.7
Orthocresol2-methyl phenol190.8.4
Metacresol3-methyl phenol202.8.4
Paracresol4-methyl phenol201.8.3
2-3 Xylenol2-3-dimethyl phenol218.0⎫.2
2-4 Xylenol2-4-dimethyl phenol211.5⎪
2-5 Xylenol2-5-dimethyl phenol211.5⎬
2-6 Xylenol2-6-dimethyl phenol212.0⎪
3-4 Xylenol3-4-dimethyl phenol225.0⎪
3-5 Xylenol3-5-dimethyl phenol220.0⎭
Ortho ethylphenol2-ethyl phenol206.5⎫.5
Meta ethylphenol3-ethyl phenol217.0⎪
Para ethylphenol4-ethyl phenol218.5⎪
s-methyl ethylphenol3-methyl-5-ethyl phenol232.5⎬
Iso pseudocumenol2-3-5-trimethyl phenol233.0⎪
Mesitol2-4-6-trimethyl phenol219.5⎪
Pseudocumenol2-4-5-trimethyl phenol234.0⎭

Source: Ellis, Chemistry of Synthetic Resins.

Since the quantities of tar acids present in coal tar are small (see table [56]), it is usually uneconomical to distill coal tar completely unless the creosote oil and pitch can be marketed profitably. Beginning in 1936, production of tar acids in the United States was increased by the practice of topping. Topping is the recovery in tar distillation of the light fractions only, leaving a residual thin enough to flow through the pipe lines to supply fuel to open hearth and other type furnaces. These light fractions contain the naphthalene and tar acids. The practice permits recovery of these products from tar to be used as fuel, thus providing a new alternative intermediate between the two older practices of either complete distillation or using the undistilled tar as fuel.

In the United States, consumption of most of the tar acids greatly exceeds the quantities extracted from tar, necessitating large production of synthetic phenol and importation of large quantities of the cresols and xylenols. The calculated amount of these tar acids present in the tar produced in this country vastly exceeds present day requirements. Table [57] shows the approximate amounts of the several tar acids contained in the coal tar produced and distilled in 1936. These estimates are based on a 1936 production of coal tar of 560,385,578 gallons and a distillation of 292,140,249 gallons. The calculation is made by using the percentage of tar acids in tar shown in table [56] and converting the gallons to pounds in accordance with the specific weights of the pure tar acids. Actual production of all tar acids in the United States in 1936 was about 29 million pounds.

Table 57.—Tar acids available in coal tar produced and distilled in 1936

Tar acidAvailable in
tar produced
in 19361		Available in
tar distilled
in 19362
1,000 pounds1,000 pounds
Phenol34,91218,200
Orthocresol19,27710,050
Metacresol19,27710,050
Paracresol14,2907,450
Xylenols10,2005,316
Others25,20013,290