Chemical Properties
Xylene occurs in the manufacture of different petroleum products and as an impurity in benzene and toluene. It is a colorless and flammable liquid. Commercial xylene is a mixture of three isomers, namely, ortho, meta, and para isomer. It is extensively used in different industries associated with paints, rubber, inks, resins, adhesives, paper coating, solvents, and emulsifi ers. Also, xylene is used as an important raw material in the manufacture of plasticizers, glass-reinforced polyesters, and alkyd resins.
General Description
A light colored to colorless liquid with a hydrocarbon odor. Flash point between 52-93°F. Less dense than water. Vapors are heavier than air. Vapors may irritate the eyes, nose, throat and respiratory tract. High vapor concentrations may cause central nervous system depression or damage. Liquid contact may irritate eyes and skin. Prolonged liquid contact mat result in defatting and drying of the skin. Avoid ingestion.
Reactivity Profile
Vigorous reactions, sometimes amounting to explosions, can result from the contact between these materials and strong oxidizing agents. They can react exothermically with bases and with diazo compounds. Substitution at the benzene nucleus occurs by halogenation (acid catalyst), nitration, sulfonation, and the Friedel-Crafts reaction.
Air & Water Reactions
Highly flammable. Water insoluble.
Health Hazard
Exposures to xylene cause toxicity and adverse health effects to animals and humans. Acute and chronic exposure to xylene induces adverse effects on the skin and respiratory system of animals and humans. Prolonged exposure to xylene demonstrated burning effect, drying, defatting of skin, eye irritation, lung congestion, CNS excitation, depression, mucosal hemorrhage, and mild liver damage
Health Hazard
Inhalation or contact with material may irritate or burn skin and eyes. Fire may produce irritating, corrosive and/or toxic gases. Vapors may cause dizziness or suffocation. Runoff from fire control or dilution water may cause pollution.
Physical properties
Xylene is benzene to which two methyl groups have been added to two carbon atoms in the benzene ring. The addition of two methyl groups gives three isomers of xylene labeled according to the relative positions of the methyl groups. Ortho-xylene has methyl groups on consecutive carbons in the ring, meta-xylene's metyl groups are separated by a single carbon bonded to hydrogen atoms, and para-xylene has the methyl groups on carbon atoms on opposite sides of the ring. The three xylene isomers are abbreviated using o-,m-, p- for ortho, meta, and para, respectively. Xylene is used both as a mixture, where it is referred to as xylenes or xylol, and as individual isomers. Because their boiling points are close, separation using distillation is difficult. Therefore isomers are separated using techniques such as recrystallization and adsorption. Xylenes are flammable, colorless liquids with a pleasant odor. Xylene was first isolated from coal tar in the mid-19th century. The name xylene comes from the Greek word for wood xulon because xylene was obtained from the distillation of wood in the absence of oxygen.
Definition
An organic
hydrocarbon present in the light-oil
fraction of crude oil. It is used extensively
as a solvent. There are three isomeric compounds
with this name and formula,
distinguished as 1,2-, 1,3-, and 1,4-dimethylbenzene
according to the positions
of the methyl groups on the benzene ring.
Production Methods
Xylene is produced by catalytic reforming, and, depending
on the feedstock, yields of >85% can be achieved.
Commercially, xylene is also recovered from coal tar, yielding
a typical mixture of about 10–20% ortho, 40–70%
meta, and 10–25% para isomer. Impurities include ethylbenzene,
benzene, toluene, phenol, thiophene, and pyridine
(53, 438).
Carcinogenicity
Mixed xylene and the individual xylene
isomers have tested negative in a wide variety
of genotoxic assays; they are considered to be
nonmutagenic. The IARC has determined that there is
inadequate evidence in humans and experimental
animals for the carcinogenicity of xylenes.
Environmental Fate
Xylene is an eight carbon aromatic hydrocarbon compound
that is a minor component of all gasoline. Xylene is
a colorless, flammable liquid that is about 14% lighter than
water. It has a sweet, pungent odor. Xylene has a molecular
weight of 106.7 g mol1. At 25 C, xylene has a solubility in
water of 106 mg l1, an estimated vapor pressure of
6.6–8.8mm Hg and a Henry’s law constants ranging from
5.18 103 to 7.18 103 atm-m3 mole1. The log octanol/
water partition coefficient is 3.12. Conversion factors for
xylene in air are as follows: 1 mgm3 ? 0.23 ppm;
1 ppm? 4.36 mgm3.
If released to air, the moderate vapor pressure predicts
xylene will exist solely as a vapor in the ambient atmosphere.
Vapor-phase xylene will be degraded in the atmosphere by
a reaction with photochemically produced hydroxyl radicals;
the half-life for this reaction in air is estimated to be 2 days.
Xylenes do not contain chromophores that absorb at wavelengths
>290 nm and therefore are not expected to be
susceptible to direct photolysis by sunlight.
If released into water, xylene would only moderately adsorb
to suspended solids and sediment based upon an estimated Koc
range of 36–365. Volatilization from water surfaces is expected
to be the dominant fate process based upon the estimated
Henry’s law constant. Estimated volatilization half-lives for
a model river and model lake are 1 h and 4 days, respectively.
Using a standard biochemical oxygen demand (BOD) dilution
technique and an activated sewage inoculum, a theoretical BOD
of 72% was observed over a 20-day incubation period for
a mixture of xylene isomers, suggesting that biodegradation is an important environmental fate process in water. Hydrolysis is
not expected to be an important environmental fate process
since this compound lacks functional groups that hydrolyze
under environmental conditions.
If released to soil, xylene is expected to have high to
moderate mobility in the subsurface based upon the range of
known Koc values. Volatilization of xylene from moist soil
surfaces is expected to be an important fate process. Xylene may
volatilize from dry soil surfaces based on the moderate vapor
pressure.
Using a measured log Kow of 3.12, the USEPA’s EPI Suite
computer program estimates both a bioconcentration factor
and a bioaccumulation factor of 110.7. These predicted bioaccumulation
and biomagnifications values are relatively low.
Xylene would therefore not be expected to be found in the
tissues of fish or wildlife as (1) xylene contains no persistent
functional groups (e.g., chlorine, bromine); (2) exposure
would be expected to be low based on a low half-life in the
environment; and (3) subsequent to exposure, trace levels of
xylene would be rapidly metabolized by the liver (similar to
what is seen with other organic compounds).
Purification Methods
Usual impurities are ethylbenzene, paraffins, traces of sulfur compounds and water. It is not practicable to separate the m-, and p-isomers of xylene by fractional distillation, although, with a sufficiently efficient still, o-xylene can be fractionally distilled from a mixture of isomers. Purify (and dry) by fractional distillation from LiAlH4, P2O5, CaH2 or sodium. This treatment can be preceded by shaking successively with conc H2SO4, water, aqueous 10% NaOH, water and mercury, and drying with CaCl2 for several days. Xylene can be purified by azeotropic distillation with 2-ethoxyethanol or 2-methoxyethanol, the distillate being washed with water to remove the alcohol, then dried and fractionally distilled. [Beilstein 5 H 360.]
Toxicity evaluation
The mechanism of toxicity is suspected to be similar to other
solvents that rapidly induce anesthesia-like effects, i.e.,
a ‘nonspecific narcosis’ due to disruption (solvation) of the
integrity of the cellular membranes of the central nervous
system (CNS). The effect is similar to the ‘high’ experienced
upon exposure to other hydrocarbon solvents.
As seen with exposure to other hydrocarbon solvents, upon
inhalation, xylene is moderately toxic and may cause irritation
of the respiratory tract and narcosis. Xylene appears to produce
reversible effects upon the liver, renal, and nervous systems.
The nervous system appears to be the most sensitive to the
effects of xylene. High level xylene exposures produced incoordination,
ataxia, unconsciousness and eventually, death.
Lower level acute exposures in man produce dizziness, exhilaration,
and confusion. Although the actual biochemical
mechanism of toxicity has not been discerned, the narcotic effects seen are most likely related to its physical solvent
properties.
