Xylene is technically not one, but three isomers of dimethylbenzene, and can refer to any combination of the three. Xylene isomers are colourless and flammable liquids with many industrial applications. These isomers have the same chemical formula, and while they also all have a central benzene ring with two methyl groups attached as substituents, their chemical structures are actually slightly different.
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What Is Xylene Found In?
Xylene refers to either one of the isomers of dimethylbenzene, or to a mixture of the isomeric compounds. It’s naturally found in crude oil at a small percentage of around 0.5-1%, depending on the source of the crude oil. It is also found in very small quantities in gasoline and aircraft fuels.
Commercially, xylene can be produced through the catalytic reforming process as well as by the carbonisation of coal during the manufacture of coke fuel. The industrial production of xylenes, on the other hand, is accomplished by adding methyl groups to toluene and benzene. Laboratory-grade xylenes used in commercial applications have 40-65% of m-Xylene and up to 20% each of o-Xylene, p-Xylene, and ethylbenzene.
What Are The Properties Of Xylene?
Depending on the type of isomer and the isomer mixture, the physical and chemical properties of xylene vary. For instance, its melting point can range from −47.87 °C for m-Xylene to 13.26 °C for p-Xylene. Here are some basic properties of the different isomers:
- Density and Phase: 0.88 g/mL, liquid
- Boiling Point: 144 °C
- Viscosity: 0.812 cP at 20 °C
- Flashpoint: 17 °C
- Density and Phase: 0.86 g/mL, liquid
- Boiling Point: 139 °C
- Viscosity: 0.62 cP at 20 °C
- Flashpoint: 25 °C
1, 4-Dimethylbenzene (p-Xylene)
- Density and Phase: 0.86 g/mL, liquid
- Boiling Point: 138 °C
- Viscosity: 0.34 cP at 30 °C
- Flashpoint: 25 °C
All of these isomers are polar, making them practically insoluble in water. They’re also flammable because they have relatively low flashpoints. The flashpoint of any volatile liquid is the lowest possible temperature wherein its vapour can be ignited by an ignition source, like a flame from a candle.
What Is Xylene’s Chemical Formula?
The chemical formula of xylene is (CH3)2C6H4, and this is the same for all the three isomers. Each isomer molecule has a benzene ring and two methyl groups attached to it. The main difference between the different chemical structures is the locations of the methyl groups, as illustrated in the diagram below:
The designations ortho- (o-), meta- (m-) and para- (p-) for xylene describe the location of the two methyl groups, i.e. to which carbon atoms of the benzene ring they are attached.
What Are The Applications Of Xylene?
From being used as precursors in the production of polyethylene terephthalate (PET) plastics to laboratory solvents for removing synthetic immersion oils from microscope objectives, xylenes have a wide range of industrial and commercial applications:
- Polymer precursors
Plastic bottles are commonly made from extruded or moulded polyethylene terephthalate (PET). The plastic polymers and polyester fabrics are derived from terephthalic acid and dimethyl terephthalate monomers. The isomer 1,4-dimethylbenzene, a.k.a. p-Xylene is essential to the production of these monomers as it’s used as a precursor to the manufacture of polymers. In fact, approximately 98% of all p-Xylene production is used in this way.
About half of all the xylenes produced are used in the industrial production of plastics and polymer fabrics. For example, 1,2-dimethylbenzene, a.k.a. o-Xylene is used as a precursor to phthalic anhydride, which is another important chemical used in the large-scale production of plastics because it decreases the viscosity and increases the flexibility of the polymers.
- Commercial solvent
Xylenes are also used as solvents, and are most effective in this application when the isomers are mixed together. When used as solvent, a small amount of ethylbenzene is also added into the mixture. This kind of solvent is colourless, highly flammable, and smells sweet.
Xylene solvent is used in various industries such as printing, rubber manufacturing, and leather processing. In this way, it’s an important component of ink, rubber, and different types of adhesives. As a nonpolar mixture, xylene solvent is also excellent at dissolving nonpolar substances such as paints and varnishes. This is why it’s commonly used as a thinning agent. If you need to slowly dry paint, xylene solvent can even be used as a substitute for toluene. This makes it very useful in art conservation and restoration works, where it’s used to test the solubility of paints used on artworks.
- Cleaning agent
Xylene is used as a cleaning agent in a range of fields, from dentistry and electronics to the petroleum industry. It’s very useful as a cleaning agent for steel and electronic components, like integrated circuits and silicon wafers, because it doesn’t damage the surface that it’s used on. Xylene also has an application in dentistry, where it’s used to dissolve gutta percha, a material for treating root canals.
Meanwhile, the petroleum industry uses xylene as a component of paraffin solvents for tubings that are clogged by paraffin wax. Its effectiveness in dissolving wax also makes it a useful ingredient in cosmetic products for removing excessive ear wax or cerumen.
Is Xylene Corrosive?
While xylenes are effective solvents with excellent dissolving capacities, they’re not in fact corrosive. Corrosion is a chemical reaction that causes the breakdown or formation of new compounds through oxidative processes. Corrosive substances cause corrosion or oxidation to occur in other substances, including metals and organic compounds or biological tissues.
Corrosive substances can be an element, like oxygen in O3 (i.e. ozone form), or a compound, particularly acids, bases, and ionic solutions. Although xylenes can dissolve nonpolar organic compounds, they typically don’t react chemically with other substances in normal conditions. Therefore, xylenes aren’t considered corrosive. On the other hand, they can be converted to acids, like monocarboxylic acids (toluic acids), and dicarboxylic acids (phthalic acids) when oxidised, which are corrosive substances.
However, just because it isn’t corrosive, that doesn’t mean xylene is non-hazardous. Like other hydrocarbons, xylenes are toxic and have several health hazards. If you inhale its vapours in small amounts, you may feel dizzy, nauseated, drowsy, and could develop a headache. If you’re exposed to large amounts of vapours, serious symptoms include sleepiness, muscle motor miscoordination, irregular heartbeat, and loss of consciousness. If no medical attention is given, exposure to xylene vapours could even be fatal.
Additionally, liquid xylene can cause a painful burning sensation on the skin, and can damage the eyes if it makes contact. The vapours are also mildly irritating to the mucous membranes, eyes, lungs, and skin. No evidence is found indicating that xylenes are carcinogenic, despite the presence of the benzene group. So, while it isn’t a corrosive substance by definition, always take precaution when handling xylene.
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