Terephthalic acid: chemical properties, production and applications

Important representatives of multi-basic aromatic carbonic compounds are phthalic acids, represented by some isomers - the ortho-isomer (directly, phthalic acid), the meta-isomer (isophthalic) and the para-isomer (terephthalic). All the substances in this group are extremely widely used in various industrial branches.

Terephthalic acid is a colorless, pure crystalline powder produced by the liquid-phase oxidation reaction of para-xylene in the presence of cobalt salts that act as catalysts. The interaction of this substance with various alcohols leads to the formation of chemical compounds of the ester group. The greatest practical application is dimethyl terephthalate.

Terephthalic acid is also used for the synthesis of polyethylene terephthalate (PET), a transparent, heat-resistant polymer produced by the polycondensation reaction of this substance with ethylene glycol. Then, plastic bottles, polyester fibers of the terylene group, more commonly known under the general name "lavsan", as well as various packaging containers for the food industry, radio components and various equipment are produced from it.

In terms of its chemical properties, terephthalic acid is similar to monobasic carboxylic compounds. When heated or under the influence of so-called dehydrating substances, it easily forms salts, esters of complex molecular structure, anhydrides, amides, both one and two carboxyl groups. Also, terephthalic acid enters the esterification reaction, as a result of which mono- and di-esters are formed. When this crystalline substance is heated to a temperature of at least two hundred degrees, decarboxylation is observed, forming compounds containing a smaller number of carboxyl groups. However, the reactions of electrophilic substitution into the deactivated ring begin to take place with great difficulty.

Phthalic acid in nature is found in the green pigment of plants and seed poppy boxes. Of its derivatives, the most important are the ether compounds dibutyl- and dimethyl phthalate, which are used as plasticizers for cellulose products, vinyl polymers and rubbers. Also, dimethyl-, diethyl- and dibutyl phthalates are important components of various repellents.

Isophthalic acid is actively used in the manufacture of unsaturated polyester resins. Because isophthalates are excellent plasticizers. But the greatest use of the group of benzenepolycarboxylic compounds is still terephthalic acid, which is used for the synthesis of such important substances for the industry as dimethyl terephthalate and polyethylene terephthalate. They are used for the manufacture of drive belts of various mechanisms, ropes, conveyor belts, fishing nets, sails, ship gear, trawls, gasoline and oil resistant hoses, zippers, strings of tennis rackets and much more. In addition, from the smooth textile threads, in the production of which this substance is used, knitted products, various fabrics (crepe, tweed, satin, etc.), curtain-tulle production, raincoat, umbrella and costume materials, shirts, stockings, baby clothes etc.

In conclusion, I would like to note that all acids of the benzene polycarboxylic series are extremely aggressive and toxic substances. Therefore, they are always accompanied by a Material Safety Data Sheet, which indicates all the properties and characteristics of this connection, as well as the procedure and rules for working with it.