p-Xylene: Properties, Uses & Lab Handling Guide

p-Xylene is a colorless, flammable aromatic hydrocarbon and the para isomer of dimethylbenzene, widely valued in both industrial synthesis and laboratory research for its high purity, distinct physicochemical properties, and role as a key precursor to purified terephthalic acid (PTA).

Molecular Formula

C₈H₁₀

Molecular Weight

106.17 g/mol

Boiling Point

138.4 °C

Melting Point

13.2 °C

Density

0.861 g/mL at 20 °C

CAS Number

106-42-3

Flash Point

27 °C (closed cup)

SKU

P431331

Chemical Identity and Physical Properties

p-Xylene (para-xylene, CAS 106-42-3) is one of three structural isomers of xylene, distinguished by methyl substituents positioned symmetrically at the 1- and 4-positions of the benzene ring. Its molecular formula is C₈H₁₀ and molecular weight is 106.17 g/mol. The compound appears as a colorless, water-immiscible liquid with a characteristic sweet aromatic odor.

Key physical constants include a boiling point of 138.4 °C, a melting point of 13.2 °C, and a density of approximately 0.861 g/mL at 20 °C. Its relatively high melting point compared to the ortho and meta isomers reflects the symmetric molecular geometry, which allows for efficient crystal packing. p-Xylene is practically insoluble in water but miscible with most common organic solvents including ethanol, diethyl ether, and acetone.

The refractive index (n²⁰/D) is approximately 1.495, and the compound exhibits low viscosity typical of small aromatic hydrocarbons. Its vapor pressure at 20 °C is approximately 8.7 mmHg, contributing to its moderate volatility under ambient conditions.

Industrial and Research Applications

The dominant industrial application of p-xylene is as the direct feedstock for the production of purified terephthalic acid (PTA) via catalytic oxidation. PTA is subsequently polymerized with ethylene glycol to produce polyethylene terephthalate (PET), a polyester resin used extensively in beverage bottles, synthetic fibers (polyester textiles), and packaging films. This single downstream chain accounts for the vast majority of global p-xylene consumption.

In the research laboratory, p-xylene serves multiple roles. It is used as a high-boiling aromatic solvent for organic synthesis reactions that require temperatures above those accessible with lower-boiling solvents such as toluene. It is also employed as a reference standard and calibration compound in gas chromatography (GC) and mass spectrometry (MS) due to its well-characterized retention behavior and fragmentation pattern.

p-Xylene is also used as a starting material or intermediate in the synthesis of specialty chemicals, pharmaceuticals, and functional polymers, as well as in studies of aromatic hydrocarbon metabolism and toxicology. In polymer science, it is relevant as both a processing solvent and a model compound for studying pi-stacking and aromatic interactions.

Safe Handling, Storage, and Regulatory Information

p-Xylene is classified as a flammable liquid (Flash Point 27 °C, closed cup) and poses inhalation, skin contact, and eye irritation hazards. Under GHS/OSHA HazCom standards it carries the following hazard classifications: Flammable Liquid Category 3, Acute Toxicity (Inhalation) Category 4, Skin Irritant Category 2, and Aspiration Hazard Category 1. Prolonged or repeated exposure may cause central nervous system effects including headache, dizziness, and narcosis.

When handling p-xylene in the laboratory, researchers should work in a well-ventilated fume hood, wear appropriate chemical-resistant gloves (nitrile or neoprene), safety goggles, and a lab coat. Open flames and ignition sources must be eliminated in areas where p-xylene is used or stored. The vapor is heavier than air and can accumulate in low-lying areas, creating a fire and explosion risk.

Storage should be in tightly sealed containers in a cool, well-ventilated area away from oxidizers, heat, and direct sunlight. Compatibility with container materials should be verified; stainless steel, glass, and certain plastics are generally acceptable. Disposal must comply with local and national regulations governing flammable organic waste. In many jurisdictions p-xylene waste must be collected as halogen-free organic solvent waste for licensed incineration.

Analytical Specifications and Quality Considerations

For research applications, the purity of p-xylene is a critical parameter. High-purity grades (≥99%) are required for use as GC calibration standards or as solvents in sensitive spectroscopic measurements, where trace xylene isomers (ortho- and meta-xylene) could interfere with results. The Aladdin Scientific p-Xylene (SKU: P431331) is supplied to meet rigorous quality benchmarks, referenced against the Sigma quality standard 1.08684.

Typical analytical parameters evaluated for high-purity p-xylene include: purity by GC (%), water content by Karl Fischer titration, isomeric impurity profile (o-xylene, m-xylene, ethylbenzene), color (APHA scale), and refractive index. Researchers should consult the Certificate of Analysis (CoA) supplied with each batch for lot-specific data.

For NMR applications, deuterated solvents are usually preferred as the bulk solvent; however, p-xylene may serve as an internal reference or analyte. Its ¹H NMR spectrum (in CDCl₃) shows a characteristic singlet for the aromatic protons and a singlet for the methyl groups, providing a straightforward fingerprint for identity confirmation.

Frequently asked questions

What is the difference between o-xylene, m-xylene, and p-xylene?

The three xylene isomers differ only in the relative positions of the two methyl groups on the benzene ring: ortho (1,2-positions), meta (1,3-positions), and para (1,4-positions). This structural difference significantly affects their physical properties—notably the melting point—and their chemical reactivity, making them suitable for different industrial and research applications. p-Xylene's symmetrical structure gives it a higher melting point (13.2 °C) than its isomers and makes it the preferred feedstock for PTA synthesis.

Is p-xylene suitable for use as an HPLC or GC solvent?

p-Xylene is frequently used as a solvent or standard in GC analysis due to its well-defined retention time and fragmentation pattern. However, it is less commonly used as an HPLC mobile-phase solvent because of its strong UV absorption in the region used for detection and its incompatibility with aqueous reversed-phase systems. For GC use, high-purity grade (≥99% by GC) is recommended to minimize interference from isomeric impurities.

How should p-xylene spills be handled in the laboratory?

In the event of a p-xylene spill, personnel should immediately evacuate ignition sources and ensure adequate ventilation. Small spills can be absorbed using dry sand, vermiculite, or an appropriate chemical absorbent, and the absorbed material collected in a sealed, labeled container for disposal as flammable organic waste in accordance with local regulations. Skin or eye contact requires immediate flushing with copious water for at least 15 minutes, followed by medical attention if irritation persists. Refer to the current Safety Data Sheet (SDS) for comprehensive emergency response guidance.

What purity grade of p-xylene is appropriate for polymer synthesis research?

For polymer synthesis research, a purity of ≥99% (by GC) is generally recommended to ensure reproducible results and to prevent undesired side reactions caused by isomeric or other organic impurities. Researchers should review the batch-specific Certificate of Analysis (CoA) to confirm the impurity profile before use, paying particular attention to o-xylene, m-xylene, and ethylbenzene content.

References

1. Lide, D. R. (Ed.). CRC Handbook of Chemistry and Physics, 97th Edition. CRC Press, Boca Raton, FL.
2. ECHA. Substance Information: p-Xylene (CAS 106-42-3). European Chemicals Agency.
3. NIOSH. Pocket Guide to Chemical Hazards: Xylene. National Institute for Occupational Safety and Health, CDC.
4. Sigma-Aldrich / Merck. p-Xylene Safety Data Sheet, Sigma number 1.08684.

Products are supplied for research and development use only. Not for use in humans, animals, diagnosis, or therapy.