Xylene

From Free net encyclopedia

The term xylenes refers to a group of 3 benzene derivatives which encompasses ortho-, meta-, and para- isomers of dimethyl benzene. The o-, m- and p- isomers specify to which carbon atoms (of the main 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 p- isomer has the IUPAC name of 1,4-dimethylbenzene.

Image:Iupac-cyclic.png

It is a colorless, sweet-smelling liquid that is very flammable. It occurs naturally in petroleum and coal tar and is formed during forest fires. The chemical properties differ slightly from isomer to isomer. The melting point is between −47.87 °C (m-xylene) and 13.26 °C (p-xylene). The boiling point is for each isomer at around 140 °C. The density is at around 0.87 kg/L and thus lighter 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.

Chemical industries produce xylene from petroleum. It is one of the top 30 chemicals produced in the United States in terms of volume. 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, a pesticide [1], a thinner for paint, and in paints and varnishes. It is found in small amounts in airplane fuel and gasoline. 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, whereas p-xylene forms terephthalic acid.

Isomers of Xylene
General
Common name	o-xylene	m-xylene	p-xylene
Systematic name	1,2-dimethylbenzene	1,3-dimethylbenzene	1,4-dimethylbenzene
Other names	o-xylol	m-xylol	p-xylol
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	[95-47-6]	[108-38-3]	[106-42-3]
CAS number for mixture of xylenes [1330-20-7]
Properties
Density and phase	0.88 g/cm³, liquid	0.86 g/cm³, liquid	0.86 g/cm³, liquid
Solubility in water	practically insoluble
Soluble in non-polar solvents such as aromatic hydrocarbons
Melting point	−25 °C (248 K)	−48 °C (225 K)	13 °C (286 K)
Boiling point	144 °C (417 K)	139 °C (412 K)	138 °C (411 K)
Viscosity	? cP at 25 °C	? cP at 25 °C	? cP at 25 °C
Hazards
MSDS	External MSDS	?	?
EU Classification	Harmful (Xn)
Flash point	32 °C	27 °C	27 °C
R/S statement	Template:R10, Template:R20/21, Template:R38: Template:S2, Template:S25
RTECS number	ZE2450000	ZE2275000	ZE2625000
Supplementary data page
Structure & properties	n, ε_r, 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

[edit]

Structure

Background: Hydrocarbons can be divided into two main groups, aliphatics and aromatics. Aliphatics are further subdivided into alkanes, alkenes, alkynes and cycloaliphatics. Cycloaliphatics are further subdivided into cycloalkanes, cycloalkenes and cycloalkynes. The difference between them is that cycloalkanes have all single bonds, cycloalkenes have one or more double bonds and cycloalkynes have one or more triple bonds. When speaking of hybridization I will speak of it relative to most of the molecule, considering that most of the carbons in the cycloaliphatic molecule have single bonds to the rest of the carbons (sp3 hybridized). sp3 hybridization tends to create a 109 degree angle of separation between the carbon-carbon single bond. No single cycloaliphatic actually reaches a full 109 degree angle of separation (this can be proved mathematically), and most have to twist in order to avoid stress. This means that most cycloaliphatics (except cyclopropane) are twisted. In turn aromatic hydrocarbons, which are mostly cyclic, have alternating double bond-single bonds. This allows all of the carbons in the main chains to be sp2 hybridized which make an angle of 120 degrees with each other. Benzene is the only aromatic compound which has 6 carbons in the main ring. From mathematics we know that the corners of a hexagon have an angle of 120 degrees.

Xylenes, like normal benzene have a planar structure which makes them very stable. They are soluble in alcohol and ether, but nearly insoluble in water. Xylenes are often mixed with ethylbenzene when preparing industrial and commercial solutions of the isomers. Each mix is colorless at room temperature. para-Xylene forms crystals at high temperatures.

[edit]

Health effects

Xylene affects the brain. High levels from exposure for short periods (14 days or less) or long periods (more than 1 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 (often seen similar to huffing).

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 if xylene harms the unborn child if the mother is exposed to low levels of xylene during pregnancy.

[edit]