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phthalic acid[′thal·ik ′as·əd]
(benzenedicarboxylic acid), C6H4(COOH)2, any one of three dicarboxylic acids of the aromatic series: ortho-phthalic acid (or simply phthalic acid), meta-phthalic acid (or isophthalic acid), and para-phthalic acid (or terephthalic acid). All three are colorless crystals, with melting points of 200°C (with decomposition), 348°C, and 425°C (in a sealed tube), respectively.
ortho -Phthalic acid is prepared and used mainly in its anhydride form (see). Of practical importance are the esters of ortho-phthalic acid C6H4(COOR)2, which are liquids with high boiling points; they are used as plasticizers for polyvinyl chloride, polystyrene, and many other polymers (for example, dibutyl phthalate and dioctyl phthalate) and serve as manometric liquids and insect repellents, such as dimethyl phthalate. The dinitrile of ortho-phthalic acid (phthalonitrile) is used in the production of phthalocyanine dyes and polyphthalocyanines. Also of practical importance are the cyclic imide (phthalimide) and dichloranhydride (phthalyl chloride) of ortho-phthalic acid, as well as phthalylsulfathiazole (Ftalazol). Polyester resins based on ortho-phthalic acid and polyhydric alcohols, such as glycerin and pentaerythritol, are widely used; these are called glyptal and penta resins, respectively.
meta-Phthalic acid and para-phthalic acid are prepared by the oxidation of meta-xylene and para-xylene, respectively. They serve as the basis for the commercial production of aromatic polyamides, such as poly-m-phenyleneisophthalamide, and in the production of polyarylates. The interaction of para-phthalic acid or its dimethyl ester with ethylene glycol yields polyethylene terephthalate, which is used in the manufacture of polyester fibers.
V. N. FROSIN