Important representatives of aromatic polybasic carboxylic compounds are phthalic acids represented by some isomers - the ortho-isomer (directly, phthalic acid), the meta-isomer (isophthalic) and the para-isomer (terephthalic). All substances of this group are extremely widely used in various industrial sectors.
Terephthalic acid is a colorless pure crystalline powder obtained in the process of liquid-phase oxidation of para-xylene in the presence of cobalt salts acting as catalysts. The interaction of this substance with various alcohols leads to the formation of chemical compounds of the ether group. Dimethyl terephthalate has the greatest practical application.
Terephthalic acid is also used for the synthesis of polyethylene terephthalate (PET) - a transparent heat-resistant polymer obtained by the polycondensation reaction of this substance with ethylene glycol. Then they produce plastic bottles, polyester fibers of the terylene group, better known under the general name "lavsan", as well as various packaging containers for the food industry, radio components and various equipment.
In its chemical properties, terephthalic acid is similar to monobasic carboxylic compounds. When heated or under the influence of the so-called water-taking substances, it easily forms salts, esters of a complex molecular structure, anhydrides, amides, both in one and in two carboxyl groups. Terephthalic acid also enters into an esterification reaction, as a result of which mono- and diesters are formed. When this crystalline substance is heated to a temperature of not less than two hundred degrees, decarboxylation is observed with the formation of compounds containing a smaller number of carboxyl groups. However, electrophilic substitution reactions in a deactivated ring begin to proceed with great difficulty.
Phthalic acid in nature is found in the green pigment of plants and seed poppy seeds. Of its derivatives, ether compounds dibutyl and dimethyl phthalate, which are used as plasticizers for cellulosic products, vinyl polymers and rubbers, are of the greatest importance. Dimethyl, diethyl and dibutyl phthalates are also important components of various repellents.
Isophthalic acid is actively used in the manufacture of unsaturated polyester resins. Because isophthalates are excellent plasticizers. But nevertheless, terephthalic acid, which is used for the synthesis of substances important for the industry, such as dimethyl terephthalate and polyethylene terephthalate, has the greatest application from the group of benzene-polycarboxylic compounds. They are used for the manufacture of drive belts of various mechanisms, ropes, conveyor belts, fishing nets, sails, ship gear, trawls, gas and oil-resistant hoses, zippers, strings of tennis rackets and much more. In addition, knitted products, a variety of fabrics (crepe, tweed, satin, etc.), curtain and tulle products, cloak, umbrella and costume materials, shirts, stockings, children's clothes are made from smooth textile yarns in the production of which this substance is used. etc.
In conclusion, I would like to note that all the acids of the benzene polycarboxylic series are extremely aggressive and toxic substances. Therefore, a material safety data sheet is always attached to them, where all the properties and characteristics of this compound are indicated, as well as the procedure and rules for working with it.