Xylene

[4] Xylenes are an important petrochemical produced by catalytic reforming and also by coal carbonisation in the manufacture of coke fuel.

[5] Xylene was first isolated and named in 1850 by the French chemist Auguste Cahours (1813–1891), having been discovered as a constituent of wood tar.

[3][7] Commercial or laboratory-grade xylene produced usually contains about 40–65% of m-xylene and up to 20% each of o-xylene, p-xylene and ethylbenzene.

[8][9][10] The ratio of isomers can be shifted to favor the highly valued p-xylene via the patented UOP-Isomar process[11] or by transalkylation of xylene with itself or trimethylbenzene.

The demand for isophthalic acid is relatively modest, so m-xylene is rarely sought (and hence the utility of its conversion to the o- and p-isomers).

[15] In thinning paints and varnishes, it can be substituted for toluene where slower drying is desired, and thus is used by conservators of art objects in solubility testing.

Xylene is used in the laboratory to make baths with dry ice to cool reaction vessels,[17] and as a solvent to remove synthetic immersion oil from the microscope objective in light microscopy.

Being benzylic and hence weakened, the C–H bonds of the methyl groups are susceptible to free-radical reactions, including halogenation to the corresponding xylene dichlorides (bis(chloromethyl)benzenes), while mono-bromination yields xylyl bromide, a tear gas agent.

[3] The main effect of inhaling xylene vapor is depression of the central nervous system (CNS), with symptoms such as headache, dizziness, nausea and vomiting.

At an exposure between 200 and 500 ppm, symptoms can include feeling "high", dizziness, weakness, irritability, vomiting, and slowed reaction time.

[20][21] The side effects of exposure to low concentrations of xylene (< 200 ppm) are reversible and do not cause permanent damage.

[22][clarification needed] A condition called chronic solvent-induced encephalopathy, commonly known as "organic-solvent syndrome" has been associated with xylene exposure.

The use of impervious gloves and masks, along with respirators where appropriate, is recommended to avoid occupational health issues from xylene exposure.