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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. It is produced as a by-product from coal carbonisation derived from coke ovens, extracted from crude benzole from gas, and dehydrocyclodimerization and methylation is also used to produce xylene isomers. It can also be created by methylating toluene and benzene to obtain a mixture of xylene isomers, by the use of a catalyst of fluorine containing crystalline aluminosilicate. It is also manufactured from reformate. Like other solvents, xylene is also used as an inhalant drug for its intoxicating properties.

Contents

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 −47.87 °C (−54.17 °F) (m-xylene) and 13.26 °C (55.87 °F) (p-xylene). The boiling point for each isomer is around 140 °C (284.00 °F). 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.

Xylene Isomers
General
Common name Xylenes o-Xylene m-Xylene p-Xylene
Systematic name Dimethylbenzenes 1,2-Dimethylbenzene 1,3-Dimethylbenzene 1,4-Dimethylbenzene
Other names Xylols o-Xylol;
Orthoxylene
m-Xylol;
Metaxylene
p-Xylol;
Paraxylene
Molecular formula C8H10, C6H4(CH3)2 or C6H4C2H6
SMILES Cc1c(C)cccc1 Cc1cc(C)ccc1 Cc1ccc(C)cc1
Molar mass 106.16 g/mol
Appearance clear, colorless liquid
CAS number [1330-20-7] [95-47-6] [108-38-3] [106-42-3]
Properties
Density and phase 0.864 g/mL, liquid 0.88 g/mL, liquid 0.86 g/mL, liquid 0.86 g/mL, liquid
Solubility in water practically insoluble
Soluble in non-polar solvents such as aromatic hydrocarbons
Melting point -47.4°C (-53.3°F; 226 K) −25°C (-13°F; 248 K) −48°C (-54.4°F; 225 K) 13°C (55.4°F; 286 K)
Boiling point 138.5°C (281.3°F; 412 K) 144°C (291.2°F; 417 K) 139°C (282.2°F; 412 K) 138°C (280.4°F; 411 K)
Viscosity .812 cP at 20 °C (68 °F) .62 cP at 20 °C (68 °F) .34 cP at 30 °C (86 °F)
Hazards
MSDS Xylenes o-Xylene m-Xylene p-Xylene
EU Classification Harmful (Xn)
NFPA 704
NFPA 704.svg
Flash point 30 °C (86 °F) 17 °C (63 °F) 25 °C (77 °F) 25 °C (77 °F)
R/S statement R10, R20/21, R38: (S2), S25
RTECS number ZE2450000 ZE2275000 ZE2625000
Supplementary data page
Structure & properties n, εr, etc.
Thermodynamic data Phase behaviour
Solid, liquid, gas
Spectral data UV, IR, NMR, MS
Related compounds
Related aromatic
hydrocarbons
toluene, mesitylene, benzene, ethylbenzene
Related compounds xylenols - types of phenols
Except where noted otherwise, data are given for
materials in their standard state (at 25°C, 100 kPa)
Infobox disclaimer and references

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.[1]

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 [1], 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 (KMnO4), 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[citation needed] . 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.[citation needed] 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

[2]

References

  1. ^ Clark J. E.; Luthy, R. V. (1955). "Separation of Xylenes". Ind. Eng. Chem. 47 (2): 250–253. doi:10.1021/ie50542a028. 
  2. ^ Lauwerys R, Haufroid V, Hoet P, Lison D. Toxicologie industrielle et intoxications professionnelles. 5th Ed. 2007 Elsevier, Masson. pp. 553-556.

External links


1911 encyclopedia

Up to date as of January 14, 2010

From LoveToKnow 1911

XYLENE, or Dimethyl Benzene, C6H4(CH3)2. Three isomeric hydrocarbons of this formula exist; they occur in the light oil fraction of the coal: tar distillate, but they cannot be separated by fractional distillation owing to the closeness of their boiling points. The mixture can be separated by shaking with sulphuric acid, whereupon the ortho and meta forms are converted into soluble sulphonic acids, the para form being soluble only in concentrated acid; the ortho and meta acids may be separated by crystallization of their salts or sulphonamides. Ortho-xylene is obtained from ortho-bromtoluene, methyl iodide and sodium as a colourless mobile liquid boiling at 142°, melting at - 28°, and having a specific gravity of 0.8932 at o°. Oxidation by potassium permanganate gives phthalic acid; whilst chromic acid gives carbon dioxide and water. Metaor iso-xylene, the most important isomer, may be prepared by nucleus-synthetic reactions, or by distilling mesitylenic acid, C 6 H 3 (CH 3) 2 CO 2 H, an oxidation product of mesitylene, C 6 H 3 (CH 3) 3, which is produced on the condensation of acetone, with lime; this reaction is very important, for it orientates meta-compounds. It boils at 139°, melts at - 54°, and has a specific gravity of o 8812. Para-xylene is obtained when camphor is distilled with zinc chloride, but it is best prepared from para-brom-toluene or dibrombenzene, methyl iodide and sodium. Dilute nitric acid oxidizes it first to para-toluic acid and then to terephthalic acid. It boils at 138°, melts at 15°, and has a specific gravity of o 8801 at 0°.


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