Xylene

Description

Xylene is a common organic solvent used in industry. It is a mixture of three isomers of dimethylbenzene (1,2-dimethylbenzene, 1,3-dimethylbenzene, and 1,4-dimethylbenzene). Xylene is primarily used as a solvent in organic chemistry, including but not limited to the printing, rubber, and leather industries. It is also used as a cleaner for steel and silicon wafers. In the petroleum industry, xylene is also a common component of paraffin solvent, used to remove clogged pipes.

Chemical properties

Xylene is a mixture of isomers that appears as a clear liquid with various grades and boiling points. It is insoluble in water but soluble in alcohol and ether. It is a flammable, colorless liquid with a pleasant odor.

The Uses of Xylene

Xylene is widely used in industries such as paints, rubber, inks, resins, adhesives, paper coatings, solvents, and emulsifiers. Xylene is also a key raw material in the production of plasticizers, glass fiber-reinforced polyesters, and alkyd resins. In the chemical industry, it is used as a chemical raw material and can undergo oxidation reactions, converting side chain methyl groups into carboxyl groups (COOH), which in turn produce carboxylic acids.

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).

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.

Air & Water Reactions

Highly flammable. Water insoluble.

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.

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

Safety Profile

Moderately toxic by intraperitoneal and subcutaneous routes. LWdly toxic by ingestion and inhalation. An experimental teratogen. Human systemic effects by inhalation: olfactory changes, conjunctiva irritation, and pulmonary changes. Experimental reproductive effects. Mutation data reported. A human eye irritant, An experimental skin and severe eye irritant. Some temporary corneal effects are noted, as well as some conjunctival irritation by instillation (adding drops to the eyes one drop at a time). Irritation can start @ 200 ppm. A very dangerous fire hazard when exposed to heat or flame; can react with oxidzing materials. To fight fire, use foam, CO2, dry chemical. When heated to decomposition it emits acrid smoke and irritating fumes. See also other xylene entries.

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 mol
1. At 25 C, xylene has a solubility in water of 106 mg l
1, an estimated vapor pressure of 6.6–8.8mm Hg and a Henry’s law constants ranging from 5.18  10
3 to 7.18  10
3 atm-m3 mole
1. The log octanol/ water partition coefficient is 3.12. Conversion factors for xylene in air are as follows: 1 mgm
3 ? 0.23 ppm; 1 ppm? 4.36 mgm
3.
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.