--------------------+-----------------------+---------+---------- | | | Average Commercial name | Chemical name | Boiling |percent in | |point ° C| coal tar --------------------+-----------------------+---------+---------- Phenol |Phenol | 181.5| 0.7 Orthocresol |2-methyl phenol | 190.8| .4 Metacresol |3-methyl phenol | 202.8| .4 Paracresol |4-methyl phenol | 201.8| .3 2-3 Xylenol |2-3-dimethyl phenol | 218.0| ⎫ 2-4 Xylenol |2-4-dimethyl phenol | 211.5| ⎪ 2-5 Xylenol |2-5-dimethyl phenol | 211.5| ⎬ .2 2-6 Xylenol |2-6-dimethyl phenol | 212.0| ⎪ 3-4 Xylenol |3-4-dimethyl phenol | 225.0| ⎪ 3-5 Xylenol |3-5-dimethyl phenol | 220.0| ⎭ Ortho ethylphenol |2-ethyl phenol | 206.5| ⎫ Meta ethylphenol |3-ethyl phenol | 217.0| ⎪ Para ethylphenol |4-ethyl phenol | 218.5| ⎪ s-methyl ethylphenol|3-methyl-5-ethyl phenol| 232.5| ⎬ .5 Iso pseudocumenol |2-3-5-trimethyl phenol | 233.0| ⎪ Mesitol |2-4-6-trimethyl phenol | 219.5| ⎪ Pseudocumenol |2-4-5-trimethyl phenol | 234.0| ⎭ --------------------+-----------------------+---------+---------- 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.