O-xylene - also written as ortho-xylene or 1,2-dimethylbenzene - is a colorless, flammable aromatic hydrocarbon, and it's really defined by one job: it's the feedstock that gets oxidized into phthalic anhydride, full stop. It's one of three xylene isomers, sitting alongside meta-xylene and para-xylene, and the only thing separating them chemically is where the two methyl groups sit on the benzene ring. That small structural difference matters a lot industrially, because p-xylene feeds polyester production while o-xylene feeds an entirely different chain of products.
Identity and Basic Chemical Data
O-xylene carries CAS number 95-47-6, a molecular formula of C₈H₁₀, and a molecular weight around 106.17 g/mol. Structurally it's just benzene with two methyl groups attached to adjacent carbons - the "ortho" position is literally what gives the compound its name.
| Property | Value |
|---|---|
| CAS Number | 95-47-6 |
| Molecular Formula | C₈H₁₀ |
| Molecular Weight | ~106.17 g/mol |
| Appearance | Colorless liquid |
| Density | ~0.88 g/mL |
| Melting Point | ~-24°C to -25°C |
| Boiling Point | ~144°C |
| Solubility in Water | Very low (~0.02% at 20°C) |
| Solubility in Ethanol/Ether | High |
| Vapor Pressure | ~7 mmHg at 20°C |
| UN Number | 1307 |
Physical and Chemical Behavior
O-xylene stays liquid across a wide enough range that it's genuinely convenient to work with - melting around -24°C, boiling near 144°C, which means normal ambient and moderately hot processing temperatures never take it near a phase change. That stability is a big part of why it's easy to store and transport in standard tank equipment without special cold-chain handling.
The solubility profile explains its second life as a solvent. It barely dissolves in water - about 0.02% at room temperature - but mixes freely with ethanol and diethyl ether, which is exactly what you'd expect from a nonpolar aromatic hydrocarbon. That same property is what lets it dissolve resins and other organic compounds efficiently in solvent applications, separate from its main role as a reaction feedstock.
Hazard Classification and Handling
Under GHS, o-xylene is classed as a Category 3 flammable liquid, with hazard statements covering flammability (H226), aspiration risk (H304), skin and eye irritation (H315, H319), and respiratory irritation (H335). It ships under UN number 1307, which puts it firmly in dangerous-goods transport territory.
None of this is unusual for an industrial aromatic solvent - toluene and mixed xylenes carry a nearly identical hazard profile. But it does mean flammable-liquid storage practices, proper ventilation, and the right PPE for skin and vapor exposure aren't optional extras; they're baseline requirements for anyone handling it at volume.
Where O-Xylene Comes From
O-xylene doesn't get made on its own - it's split out of a mixed xylene stream produced by catalytic reforming or pyrolysis gasoline in petroleum refining. A typical mixed xylene cut runs roughly 20% ortho-, 70% meta-, and 10% para-xylene, and refiners bother separating out the ortho fraction specifically because it has one dominant downstream buyer waiting for it: phthalic anhydride producers.
Primary Industrial Uses
Phthalic Anhydride Production Drives the Market
The vast majority of o-xylene produced worldwide goes straight into phthalic anhydride manufacturing via catalytic oxidation, and this single use sets the tone for the whole market - pricing, demand cycles, everything. Phthalic anhydride itself feeds into plasticizers like DOP, unsaturated polyester resins, and alkyd coatings, so o-xylene demand tends to move with construction, PVC, and coatings industry activity rather than anything ortho-xylene-specific.
Smaller Uses Worth Knowing
Outside of phthalic anhydride, o-xylene shows up as an industrial solvent in select formulations, in some insect repellent and plasticizer products, and as a dye-manufacturing intermediate. It also has legitimate lab-scale roles - as a synthesis solvent and a starting material for compounds like o-xylyl bromide - though these applications are a rounding error next to the anhydride route in terms of total volume.
O-Xylene vs. M-Xylene vs. P-Xylene: Why the Isomer Matters
People searching "xylene" without specifying which isomer sometimes end up sourcing the wrong one, so it's worth spelling out: chemically these three are close cousins, but industrially they're barely interchangeable at all.
| Isomer | CAS Number | Boiling Point | Primary Industrial Use |
|---|---|---|---|
| Ortho-xylene (o-xylene) | 95-47-6 | ~144°C | Phthalic anhydride production |
| Meta-xylene (m-xylene) | 108-38-3 | ~139°C | Isophthalic acid, synthetic fragrance intermediates |
| Para-xylene (p-xylene) | 106-42-3 | ~138°C | Terephthalic acid → polyester (PET) fiber and resin |
If your process runs on polyester fiber or PET resin, you need p-xylene - o-xylene simply won't do that job. This isn't a minor labeling issue; the isomers aren't substitutable in these reactions, and sourcing generic "mixed xylene" when a process actually calls for one specific isomer is one of the more common mistakes buyers make in this category.
Choosing a Reliable O-Xylene Supplier
Because o-xylene quality feeds directly into phthalic anhydride yield downstream, it's generally smarter to evaluate a supplier on documentation and consistency rather than chase the lowest quote. A supplier worth working with should hand over a current Certificate of Analysis that actually matches the batch being shipped - not a generic spec sheet - along with a compliant Safety Data Sheet and the correct HS code (2902.41) for customs clearance. If the relationship is meant to be ongoing rather than a one-off spot buy, ask about their track record on volume consistency too; that's usually a better predictor of a good long-term supplier relationship than price alone.
Frequently Asked Questions
What is O-xylene mainly used for?
Its dominant industrial use is as feedstock for phthalic anhydride, which downstream becomes plasticizers, polyester resins, and alkyd coatings.
Is O-xylene the same as regular xylene?
No. "Xylene" often means a mixed-isomer industrial product containing ortho-, meta-, and para-xylene together, while o-xylene refers specifically to the isolated ortho isomer, with its own distinct downstream chemistry.
Is O-xylene dangerous to handle?
It's a flammable liquid with irritant and inhalation hazards, broadly similar to other industrial aromatic solvents like toluene - standard flammable-liquid storage, ventilation, and PPE cover it, nothing exotic.
What purity level is standard for industrial O-xylene?
Industrial-grade material is commonly supplied at 98% assay or higher, while lab-grade anhydrous material is usually specified separately, often around 97% with tighter moisture and residue limits.
Can O-xylene be substituted with P-xylene or M-xylene?
Not in most industrial processes. Each isomer feeds a different chemistry entirely - p-xylene for polyester, o-xylene for phthalic anhydride, m-xylene for isophthalic acid - so swapping one for another usually means redesigning the process, not a simple substitution.
Summary
Everything about o-xylene's commercial value traces back to one reaction: oxidation into phthalic anhydride, which then feeds plasticizer, resin, and coatings production around the world. For anyone buying it, that means the real work isn't confirming the textbook o-xylene properties - it's checking purity, moisture content, transport compliance, and whether the supplier can actually deliver the same quality shipment after shipment.
