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Short answer: Para-xylene (p-xylene, 1,4-dimethylbenzene, CAS 106-42-3) is the xylene isomer whose two methyl groups sit on opposite sides of the benzene ring. It is produced from naphtha and used chiefly as the feedstock for terephthalic acid, which goes into PET. In the lab it is a flammable liquid that freezes at about 13 °C, so it can solidify in a cool store.
Definition and scope
Xylene has the formula C6H4(CH3)2 and exists as three isomers: ortho (1,2-), meta (1,3-) and para (1,4-). Para-xylene is the symmetric one; its freezing point is far higher than the ortho isomer’s (see the table below), and it is the isomer oxidised to terephthalic acid. “Xylene” on a reagent label or a histology bottle usually means a mixed-isomer product, not pure para-xylene. For the three isomers’ boiling points and why the grade matters on the bench, see xylene boiling point: o-, m- and p-xylene properties.
How para-xylene is made and used
According to a 2023 review in Molecules (Lapa and Martins, 28(4):1922), p-xylene is produced via the fractional distillation of naphtha, and separating it from its isomers is still a challenge. The review describes the downstream step that explains most of its commercial importance: the AMOCO process, a liquid-phase air oxidation of p-xylene to terephthalic acid (TPA) using a cobalt, manganese and bromide catalyst in acetic acid, with conversion above 98% and about 95% selectivity to TPA. TPA is used mainly to make PET, which goes into fibres, films and plastic products.
Why separation from the other isomers is hard
The isomers are chemically similar and their boiling points are close (see the table below). The same review describes the separation from isomers as still a challenge.
Para-xylene vs o-xylene: NIOSH values
The table uses only values published in the NIOSH Pocket Guide entries for p-xylene and o-xylene (read 2026-11-08). The meta isomer is not tabulated here.
| Property | p-Xylene | o-Xylene |
|---|---|---|
| CAS number | 106-42-3 | 95-47-6 |
| Boiling point | 281 °F | 292 °F |
| Freezing point | 56 °F (about 13 °C) | -13 °F |
| Flash point | 81 °F | 90 °F |
| Explosive limits (LEL / UEL) | 1.1% / 7.0% | 0.9% / 6.7% |
| NIOSH REL (TWA / STEL) | 100 ppm / 150 ppm | 100 ppm / 150 ppm |
| OSHA PEL (TWA) | 100 ppm | 100 ppm |
| IDLH | 900 ppm | 900 ppm |
The practical difference is the freezing point: para-xylene freezes at about 13 °C, so a container kept in a cold room can contain solid material, whereas o-xylene stays liquid to well below 0 °C. PubChem lists the para-xylene melting point as 13.3 °C (CRC Handbook, 95th ed.) and its boiling point as 138.3 °C.
Where it matters for labs and supply decisions
p-Xylene has a NIOSH-listed flash point of 81 °F and a flammable range of 1.1% to 7.0% in air. The NIOSH recommended exposure limit is 100 ppm as a time-weighted average (435 mg/m3) with a 150 ppm short-term limit (655 mg/m3), and OSHA’s permissible exposure limit is also 100 ppm TWA. NIOSH lists the immediately-dangerous-to-life-or-health concentration at 900 ppm.
The symptoms NIOSH lists for p-xylene exposure are irritation of the eyes, skin, nose and throat; dizziness, drowsiness and incoordination; corneal vacuolization; nausea; and dermatitis. Its target organs are the eyes, skin, respiratory system, central nervous system, gastrointestinal tract, blood, liver and kidneys. Glove selection against xylenes depends on breakthrough data for the specific glove, covered in nitrile gloves chemical resistance. Labs that want to move away from xylene in histology can read xylene substitutes and what to verify before switching.
Common misunderstandings
- “Xylene” and “para-xylene” are not interchangeable. Para-xylene is one isomer; mixed xylene contains all three.
- The CAS number is not the PubChem CID. The p-xylene CAS number is 106-42-3; PubChem’s compound ID is 7809.
FAQ
What is para-xylene used for?
Mainly as the starting material for terephthalic acid, which is used mostly to make PET for fibres, films and plastic products (Lapa and Martins, 2023).
What is the difference between para-xylene and xylene?
Para-xylene is the 1,4-isomer. Xylene without a prefix usually means a mixture of the ortho, meta and para isomers.
What is the exposure limit for p-xylene?
NIOSH REL: 100 ppm TWA and 150 ppm STEL; OSHA PEL: 100 ppm TWA (NIOSH Pocket Guide, read 2026-11-08).
Why can para-xylene freeze in storage?
NIOSH lists its freezing point as 56 °F (about 13 °C), so it can solidify in a cool store.
Sources
- NIOSH Pocket Guide to Chemical Hazards, p-xylene: cdc.gov/niosh/npg/npgd0670.html (read 2026-11-08)
- NIOSH Pocket Guide to Chemical Hazards, o-xylene: cdc.gov/niosh/npg/npgd0668.html (read 2026-11-08)
- PubChem, p-xylene, CID 7809: pubchem.ncbi.nlm.nih.gov/compound/7809 (read 2026-11-08)
- Lapa and Martins, “p-Xylene Oxidation to Terephthalic Acid: New Trends,” Molecules 2023, 28(4):1922: mdpi.com/1420-3049/28/4/1922 (read 2026-11-08)
