terephthalic

Terephthalic is an adjective used in chemistry to refer to compounds derived from terephthalic acid, the common reference being terephthalic acid itself. The chemical name is benzene-1,4-dicarboxylic acid, and its formula is C6H4(COOH)2. It is typically encountered as a white, crystalline solid with a high melting point and very low solubility in water. In industry, the term also appears in reference to the various terephthalate esters formed from this diacid.

Properties and structure: Terephthalic acid contains two carboxyl groups located on opposite (para) positions of the benzene ring. This symmetry gives it distinct physical and chemical properties, including limited water solubility and stability under normal conditions. It is more commonly handled in finely purified, anhydrous forms for polymer production.

Production: The principal industrial route is the catalytic air oxidation of para-xylene to terephthalic acid, typically using transition metal catalysts that promote oxidation in the presence of oxygen. The crude product is then purified, often by crystallization and filtration, to achieve the high purity required for polymer synthesis. An alternative route involves transesterification of dimethyl terephthalate, followed by purification steps to yield terephthalic acid.

Uses: The major use of terephthalic acid is as a building block for polyesters, most notably polyethylene terephthalate (PET), which is used in polyester fibers and bottles, as well as in other resins such as polybutylene terephthalate (PBT) and various alkyd and unsaturated polyester resins. Its esters, the terephthalates, are also employed in specialty plastics and coatings.

Safety: Terephthalic acid is a chemical irritant and should be handled with standard industrial hygiene practices. It is not highly toxic, but exposure should be minimized and appropriate controls used in production and handling.

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