Ethylbenzene: The Fourth Compound in Mixed Xylenes

The label “mixed xylenes” suggests a blend of the three xylene isomers and nothing else. In reality there is almost always a fourth compound riding along: ethylbenzene. It is a close relative that comes out of the refinery in the same stream, and it quietly changes what a drum of commercial xylene actually contains.

Ethylbenzene is a simple variation on the theme. Where xylene is a benzene ring carrying two separate methyl groups, ethylbenzene carries a single two-carbon ethyl group instead. That makes it an isomer of xylene in the loose sense, same formula, C8H10, different arrangement, and it behaves so similarly as a solvent that it dissolves into the mix without anyone needing to separate it out. Typical commercial mixed xylene contains something on the order of 15 to 20 percent ethylbenzene, though the exact figure varies by source and grade.

Its more interesting life is as a chemical, not a solvent. The large majority of ethylbenzene made in the world is not sold as solvent at all; it is a dedicated feedstock, produced on purpose from benzene and ethylene and then dehydrogenated into styrene, the building block of polystyrene plastic and many synthetic rubbers. So the same compound that tags along as an impurity in a solvent drum is, elsewhere, a deliberately manufactured link in the plastics chain.

For a user, the practical point is small but worth knowing. The ethylbenzene in mixed xylene adds no trouble for ordinary solvent work, since it acts much like the xylenes around it, but it is one reason a solvent grade and a chemical grade are not identical, and one reason a certificate of analysis lists ethylbenzene as its own line. The three isomers get the headlines, but the C8 aromatic cut is really a quartet.