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Xylene (also known as dimethylbenzene or xylol) is an aromatic hydrocarbon consisting of a benzene ring with two methyl (–CH₃) substituents, with the molecular formula C₆H₄(CH₃)₂ or equivalently C₈H₁₀. The term "xylene" actually refers to a family of three structural isomers that differ only in the relative positions of the two methyl groups on the ring.

The Three Isomers

The three isomers are designated using the ortho (o), meta (m), and para (p) prefixes — or equivalently in IUPAC nomenclature by numbering the ring positions:

Key Structural Differences

The only structural distinction between the isomers is the angular relationship of the two methyl groups relative to the benzene ring. Despite being constitutional isomers with identical molecular formulas, this positional difference gives each molecule slightly different physical properties and significantly different chemical reactivities. Notably, para-xylene's uniquely high melting point (+13.3°C) — compared to the other two isomers, which are liquid well below zero — is a direct consequence of its highly symmetric, linear geometry, which allows closer packing in the crystal lattice.

Symmetry and Geometry

• ortho-Xylene: The two methyls are adjacent, giving the molecule a C₂ᵥ symmetry with a slight asymmetry that makes it the most polar of the three isomers
• meta-Xylene: The methyls are separated by one ring carbon; the molecule has C₂ᵥ symmetry but with a different dipole orientation
• para-Xylene: The methyls are directly opposite, giving the molecule D₂ₕ symmetry — it is the most symmetric and geometrically linear, which explains its distinct crystallization behavior

All three isomers are colorless liquids at ambient temperature (except p-xylene, which can be a solid just below room temperature), with a characteristic sweet, aromatic odor.