The term
xylene or
xylol refers to a mixture of three
aromatic hydrocarbon isomers which is used as a
solvent in the
printing,
rubber, and
leather industries. Xylene is a clear, colorless, sweet-smelling liquid that is very flammable. It is usually refined from
crude oil in a process called
alkylation. Like other solvents, xylene is also used as an
inhalant drug for its intoxicating properties.
Chemical properties
Xylene is a generic term for a group of three aromatic hydrocarbon isomers, essentially
benzene derivatives,
which encompasses
ortho-,
meta-, and
para-
isomers of dimethyl benzene. The
o-,
m- and
p- designations specify to which
carbon atoms (of the
benzene ring) the two
methyl groups are attached. Counting the
carbon atoms from one of the ring carbons bonded to a methyl group, and counting towards the second ring carbon bonded to a methyl group, the
o- isomer has the
IUPAC name of 1,2-dimethylbenzene, the
m- isomer has the IUPAC name of 1,3-dimethylbenzene, and the
p- isomer has the IUPAC name of 1,4-dimethylbenzene. Solvent grade xylene usually contains a small percentage of
ethylbenzene as a contaminant.
The xylene isomers
The chemical properties differ slightly from isomer to isomer. The melting point is between (
m-xylene) and (
p-xylene). The boiling point for each isomer is around . The density is at around 0.87 g/mL (7.26
lb/U.S. gallon or 8.72 lb/
imp gallon) and thus is less dense than water. Xylene in air can be smelled at 0.08 to 3.7 parts of xylene per million parts of air (ppm) and can begin to be tasted in
water at 0.53 to 1.8 ppm.
The xylenes, as with many alkylbenzene compounds form complexes with various
halocarbons. The complexes of different isomers often have dramatically different properties from each other.
Applications
Xylene is used as a
solvent and in the
printing,
rubber, and
leather industries.
p-Xylene is used as a feedstock in the production of
terephthalic acid, which is a
monomer used in the production of
polymers. It is also used as a
cleaning agent for
steel and for
silicon wafers and
chips, a
pesticide , a
thinner for paint, and in
paints and
varnishes. It may be substituted for
toluene to thin lacquers where slower drying is desired. It is found in small amounts in
airplane fuel and
gasoline. In animal studies it is often swabbed on the ears of rabbits to facilitate blood flow and collection, although the area must subsequently be cleansed with alcohol to prevent inflammation. In histology, xylene is also used for clearing the tissues following dehydration in preparation for paraffin wax infiltration. It is also used after sections have been stained to make them hydrophobic so that a coverslip may be applied with a resin in solvent.
Related compounds
Xylenes are a starting material for the production of other chemicals. For instance
chlorination of both methyl groups gives three isomeric xylene dichlorides or 1,2-bis(chloromethyl)benzenes. With oxidizing agents, such as
potassium permanganate (KMnO
4), the methyl group can be oxidized to a
carboxylic acid. By oxidizing both methyl groups,
o-xylene forms
phthalic acid and
p-xylene forms
terephthalic acid.
In articles of commerce, the term
xylol refers to the solvent mixture of three xylene isomers, sometimes containing ethylbenzene. In the chemical trade, this mixture is referred to as
xylenes (plural). This substance must not be confused with the toxic and corrosive
xyleneol compounds, which are dimethyl phenol isomers.
Health effects
Xylene exhibits
neurological effects. High levels from exposure for acute (14 days or less) or chronic periods (more than 1 year) can cause headaches, lack of muscle coordination, dizziness, confusion, and balance . Exposure of people to high levels of xylene for short periods can also cause irritation of the skin, eyes, nose, and throat, difficulty in breathing and other problems with the lungs, delayed reaction time, memory difficulties, stomach discomfort, and possibly adverse effects on the liver and kidneys. It can cause unconsciousness and even death at very high levels (see
inhalants). Xylene or products containing Xylene should not be used indoors or around food. Xylene is not a controlled substance.
Studies of unborn animals indicate that high concentrations of xylene may cause increased numbers of deaths, and delayed growth and development. In many instances, these same concentrations also cause damage to the mothers. It is not yet known whether xylene harms the unborn fetus if the mother is exposed to low levels of xylene during pregnancy.
Besides
occupational exposure, the principal pathway of human contact is via
soil contamination from leaking
underground storage tanks containing petroleum products. Humans who come into contact with the soil or groundwater may become affected. Use of contaminated groundwater as a water supply could lead to adverse health effects.
See also
