Quick answer: o-Xylene (C₈H₁₀) is a benzene ring with two methyl groups attached to adjacent carbons - the 1,2 positions, which is what "ortho" means. That adjacent placement is why it's also written as 1,2-dimethylbenzene, and it's the single structural detail responsible for how this isomer behaves differently from meta- and para-xylene.
Ortho-Xylene (o-Xylene), identified by CAS 95-47-6, is an aromatic hydrocarbon used as a raw material for producing various industrial chemicals. Tianjin Gnee Biotech Co., Ltd. provides an overview of its fundamental chemistry, key properties, and industrial uses to help buyers understand its applications.
The Molecular Structure
o-Xylene's structure is straightforward: a six-carbon aromatic ring with two methyl (-CH₃) substituents on neighboring carbon atoms. In shorthand, that's C₆H₄(CH₃)₂, and it's commonly written with the SMILES notation Cc1ccccc1C. The ring itself is planar, as aromatic rings are, but the two methyl groups sitting right next to each other break the molecule's symmetry - and that asymmetry is the whole story behind why o-xylene's physical properties differ from its two sister isomers.
Why the Structure Matters: Explaining o-Xylene's Properties
This is the part most property tables skip over: o-xylene's boiling point (about 144°C) is the highest of the three xylene isomers, while its melting point (around -24°C) is actually the lowest - and both facts trace directly back to structure.
The boiling point comes down to polarity. Because the two methyl groups sit adjacent rather than opposite each other, their bond dipoles don't cancel out the way they do in para-xylene, where the methyl groups sit directly across the ring in a fully symmetric arrangement. That leaves o-xylene with a measurable net dipole moment, and the close proximity of the two methyl groups also increases the contact area between neighboring molecules, strengthening the London dispersion forces on top of the dipole interaction. Both effects add up to stronger intermolecular attraction in the liquid phase, which is why it takes more energy - a higher boiling point - to turn o-xylene into vapor compared with m- or p-xylene.
Melting point runs on a completely different mechanism: crystal packing. Para-xylene's symmetric structure lets its molecules stack neatly and efficiently in a solid crystal lattice, which is why p-xylene has a far higher melting point (around 13°C) than the other two isomers despite being the least polar. o-Xylene's asymmetric shape packs far less efficiently into a solid lattice, so it takes much less energy to break that structure apart - hence the low melting point, well below 0°C.
Full Property Profile
| Property | Value |
|---|---|
| CAS Number | 95-47-6 |
| Molecular Formula | C₈H₁₀ |
| Structural Formula | C₆H₄(CH₃)₂, 1,2-substitution |
| Molecular Weight | ~106.17 g/mol |
| Appearance | Colorless liquid |
| Density | ~0.88 g/mL |
| Melting Point | ~-24°C |
| Boiling Point | ~144°C (highest of the three xylene isomers) |
| Solubility in Water | Very low (~0.02% at 20°C) |
| UN Number | 1307 |
| GHS Classification | Flammable liquid (Cat. 3), irritant, aspiration hazard |
How Structure Connects to Industrial Uses
The same adjacent-methyl structure that shapes o-xylene's physical properties also determines its main industrial role: catalytic oxidation of the two methyl groups converts o-xylene into phthalic anhydride, the building block behind plasticizers, polyester resins, and alkyd coatings. This reaction depends specifically on the ortho arrangement - the two methyl groups being close enough together to both oxidize and cyclize into the five-membered anhydride ring - which is exactly why m-xylene and p-xylene aren't used for this process; their methyl groups simply aren't positioned to form the same ring structure.
Because o-xylene has the highest boiling point of the three isomers, it's also conveniently the easiest one to separate from a mixed xylene stream by straightforward distillation - p-xylene and m-xylene sit closer together and typically require crystallization or adsorption methods instead.
Frequently Asked Questions
What does the "ortho" in o-xylene actually describe?
It describes the position of the two methyl groups on the benzene ring - adjacent carbons, specifically the 1 and 2 positions.
Why does o-xylene have a higher boiling point than m-xylene or p-xylene?
Its asymmetric structure creates a net dipole moment and brings the methyl groups close enough to increase intermolecular contact, both of which strengthen the forces holding liquid molecules together.
Why does o-xylene have a lower melting point than p-xylene despite similar boiling points?
Melting point depends on how efficiently molecules pack into a solid crystal, and o-xylene's asymmetric shape packs far less efficiently than p-xylene's symmetric one.
Does the structure affect which industrial process o-xylene is used for?
Yes - the adjacent methyl groups are specifically what allow oxidation into phthalic anhydride, a reaction m-xylene and p-xylene can't undergo the same way.
Structure determines performance - a shipment that's off-spec on purity or isomer content won't behave the way the chemistry above predicts, no matter how correct the theory is. At Tianjin Gnee Biotech Co., Ltd., every batch of o-xylene we supply is held to strict quality standards, because consistency at this level is what advanced chemical manufacturing actually depends on.
