Xylene

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The three xylene isomers, each of which has two methyl groups attached to a benzene ring. The carbon atoms on the benzene ring are numbered from one through six, and each methyl group is indicated by a straight line.

The term xylene refers to a group of three benzene derivatives, each of which has two methyl functional groups attached to the benzene ring. The three members of the group are isomers and are called ortho-, meta-, and para-xylene (or o-, m-, and p-xylene). Each xylene is a colorless, sweet-smelling liquid that is highly flammable. They occur naturally in petroleum and coal tar and are formed during forest fires.

Contents

The xylene group ranks among the top 30 chemicals produced in the United States, measured in terms of volume. The xylenes are solvents and are used as cleaning agents. They are found in pesticides, paint thinners, paints, and varnishes, and they are used by the printing, rubber, and leather industries.

Nomenclature

The o-, m- and p- isomers specify the relative positions of attachment of the two methyl groups to carbon atoms on the main ring structure. A ring carbon atom directly bonded to a methyl group is considered as position one, and the other ring carbon atoms are then counted, while moving toward the point of attachment of the next methyl group. In this manner, the o- isomer takes the name 1,2-dimethylbenzene, according to the system recommended by the IUPAC (International Union of Pure and Applied Chemistry). Likewise, the m- isomer has the IUPAC name 1,3-dimethylbenzene, and the p- isomer has the IUPAC name 1,4-dimethylbenzene.

Notable characteristics

The xylenes have very similar properties, differing slightly from isomer to isomer. The boiling point for each isomer is around 140 °C (284.00 °F), and the melting point lies between −47.87 °C (−54.17 °F) (m-xylene) and 13.26 °C (55.87 °F) (p-xylene). The density is at around 0.87 kg/L (7.26 lb/U.S. gallon or 8.72 lb/imp gallon), indicating that each xylene 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.

With oxidizing agents such as potassium permanganate (KMnO4), the methyl group can be oxidized to a carboxylic acid. By oxidizing both methyl groups towards the acid, o-xylene forms phthalic acid, and p-xylene forms terephthalic acid.

The Xylene Group
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
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[1] o-Xylene[2] m-Xylene[3] p-Xylene[4]
EU Classification Harmful (Xn)
NFPA 704

NFPA 704.svg

3
2
0
 
Flash point 24 °C (75 °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)

Applications

The xylenes are used as solvents 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 to manufacture polymers. The xylenes are also used as cleaning agents for steel, silicon wafers, and integrated circuits. They are found in pesticides, paint thinners, paints, and varnishes. A xylene mixture may be substituted for toluene to thin lacquers where slower drying is desired. Xylene may be present in small amounts in airplane fuel and gasoline. In animal studies, a xylene (or xylene mixture) is often swabbed on the ears of rabbits to facilitate blood flow and collection, although the area must be subsequently cleansed with alcohol to prevent inflammation.

Health effects

Xylene affects the brain. High levels from exposure for short periods (14 days or less) or long periods (more than one year) can cause headaches, lack of muscle coordination, dizziness, confusion, and changes in one's sense of 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; problems with the lungs; delayed reaction time; memory difficulties; stomach discomfort; and possibly changes in the liver and kidneys. It can cause unconsciousness and even death at very high levels.

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.

Another common form of human exposure to xylene is in the use of certain types of pens, writing and drawing instruments, and art supplies.

See also

Notes

  1. Xylenes MSDS ScienceLab.com. Retrieved January 4, 2008.
  2. o-Xylene MSDS ScienceLab.com. Retrieved January 4, 2008.
  3. m-Xylene MSDS ScienceLab.com. Retrieved January 4, 2008.
  4. p-Xylene MSDS ScienceLab.com. Retrieved January 4, 2008.

References

  • McMurry, John. 2004. Organic Chemistry. 6th ed. Belmont, CA: Brooks/Cole. ISBN 0534420052
  • Morrison, Robert T., and Robert N. Boyd. 1992. Organic Chemistry. 6th ed. Englewood Cliffs, NJ: Prentice Hall. ISBN 0-13-643669-2
  • Solomons, T.W. Graham, and Craig B. Fryhle. 2004. Organic Chemistry. 8th ed. Hoboken, NJ: John Wiley. ISBN 0471417998

External links

All links retrieved July 31, 2013.

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