Terephthalic acid

Terephthalic acid is one isomer of the three phthalic acids. It finds important use as a commodity chemical, principally as a starting compound for the manufacture of polyester (specifically PET), used in clothing and to make plastic bottles. It is also known as 1,4-benzenedicarboxylic acid, and it has the chemical formula C6H4(COOH)2. It has recently become an important component in the development of hybrid framework materials.

History
Phthalic acid (the ortho isomer of terephthalic acid) was obtained by French chemist Auguste Laurent in 1836 by oxidizing naphthalene tetrachloride, and, believing the resulting substance to be a naphthalene derivative, he named it naphthalenic acid. Swiss chemist Jean Charles Galissard de Marignac determined its formula and showed Laurent’s supposition to be incorrect, upon which Laurent gave it its present name.

Properties
It is almost insoluble in water, alcohol and ether; it sublimes rather than melting when heated. This insolubility makes it relatively awkward to work with, and up until around 1970 much crude terephthalic acid was converted to the dimethyl ester for purification.

Production
Terephthalic acid can be formed in the laboratory by oxidizing para-diderivatives of benzene, or best by oxidizing caraway oil, a mixture of cymene and cuminol, with chromic acid.



On an industrial scale, terephthalic acid is produced, similar to benzoic acid, by oxidation of p-xylene by oxygen from air. This is done using acetic acid as solvent, in the presence of a catalyst such as cobalt-manganese, using a bromide promoter. Alternatively, it can be made via the Henkel process, which involves the rearrangement of phthalic acid to terephthalic acid via the corresponding potassium salts. The terephthalic acid is and dimethyl terephthalate, in turn, often used as a monomer component in the production of polymers, principally polyethylene terephthalate (polyester or PET). World production in 1970 was around 1.75 million tonnes.[2]. By 2006, global PTA demand had substantially exceeded 30 million tonnes.