Ксилолы

Описание

Xylene is used as a solvent. In this application, the mixture of isomers is often referred to as xylenes or xylol. Solvent xylene often contains a small percentage of ethylbenzene. Like the individual isomers, the mixture is colourless, sweet smelling, and highly flammable. Application of xylene is extensive and includes, but is not limited to, printing, rubber, and leather industries.
Similarly, it is used as a cleaning agent for steel and silicon wafers. In the petroleum industry, xylene is also a frequent component of paraffin solvents, used when the tubing becomes clogged with paraffin wax. Xylene is incompatible with strong oxidisers and is known to cause fires and explosions. There are three forms of xylene in which the methyl groups vary on the benzene ring: (i) meta-xylene, (ii) ortho-xylene, and (iii) para-xylene. These forms are referred to as isomers. Xylene is a colourless, sweet-smelling liquid. Xylene occurs naturally in petroleum and coal tar. Chemical industries produce xylene from petroleum. It is also used as a cleaning agent and a thinner for paint and in paints, in glues, in printing inks, and in varnishes. Xylene evaporates quickly from the soil and surface water into the air.

Химические свойства

Also known as dimethylbenzene, C6H4(CH3)2 is an isomeric mixture of 0- m-, and p-xylene. It is a clear liquid with various grades having different boiling points, that is insoluble in water and soluble in alcohol and ether,and used in aviation gasoline, coatings, lacquers, rubber cements, organic synthesis, and polyester resin manufacture.

Физические свойства

Xylene is benzene to which two methyl groups have been added to two carbon atoms in the benzene ring. The addition of two methyl groups gives three isomers of xylene labeled according to the relative positions of the methyl groups. Ortho-xylene has methyl groups on consecutive carbons in the ring, meta-xylene's metyl groups are separated by a single carbon bonded to hydrogen atoms, and para-xylene has the methyl groups on carbon atoms on opposite sides of the ring. The three xylene isomers are abbreviated using o-,m-, p- for ortho, meta, and para, respectively. Xylene is used both as a mixture, where it is referred to as xylenes or xylol, and as individual isomers. Because their boiling points are close, separation using distillation is difficult. Therefore isomers are separated using techniques such as recrystallization and adsorption. Xylenes are flammable, colorless liquids with a pleasant odor. Xylene was first isolated from coal tar in the mid-19th century. The name xylene comes from the Greek word for wood xulon because xylene was obtained from the distillation of wood in the absence of oxygen.

Использование

Xylene is used as a chemical feedstock in the chemical industry. Xylenes can undergooxidation where the side methyl groups are oxidized to give a carboxyl group (COOH)yielding a carboxylic acid. The particular acid produced depends on the isomer oxidized. Wheno-xylene is oxidized phthalic acid is produced, and when p-xylene is oxidized terephthalic acidresults. Terephthalic acid is one of the main feedstocks in making polyesters.Terephthalic acid reacts with ethylene glycol to form the ester polyethylene terephthalate(PET). PET is one of the most common plastics used as food and beverage containers. PETcontainers contain the recycling symbol with a number 1. PET is marketed using a numberof commercial names; the most generic of these is polyester. It is also the material known asDacron. Mylar is PET in the form of thin films. Although all three isomers of xylene are usedas chemical feedstocks, the greatest demand is for para-xylene to produce terephthalic acid.The smallest demand is for meta-xylene. Approximately 30 million tons of xylenes are usedannually worldwide.

Определение

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.

Методы производства

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).

Общее описание

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.

Реакции воздуха и воды

Highly flammable. Water insoluble.

Профиль реактивности

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.

Угроза здоровью

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

Профиль безопасности

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.

Канцерогенность

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.

Методы очистки

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.]