Phthalic acid

The phthalic acid (C₆H₄(CO₂H)₂) is a colorless substance, soluble in water and alcohol. It has a phenyl system with two carboxy groups in position 1 and 2. Its pKa1=2.89 and pKa2=5.50. Its name derives from naphthalene, from which it was formerly generated by oxidation. Today the synthesis is based on the oxidation of o-xylene.

Phthalic acid is produced by the catalytic oxidation of naphthalene or ortho-xylene directly to phthalic anhydride and subsequent hydrolysis of the anhydride.

History

Phthalic acid was first obtained by the French chemist Auguste Laurent in 1836 by oxidation of naphthalene tetrachloride. Believing the substance that resulted was a derivative of naphthalene, he named it 'naphthalic acid'. After the Swiss chemist Jean Charles Galissard de Marignac determined its correct formula, Laurent gave it its present name. Manufacturing methods in the XIX century were by oxidation of naphthalene tetrachloride with nitric acid, or, better, oxidation of the hydrocarbon with fuming sulfuric acid, using mercury or mercury(II) sulfate as a catalyst.

Uses

Although phthalic acid is of modest commercial importance, it is closely related to the phthalic anhydride derivative, which is a large-scale produced chemical, as it is an important starting compound in many organic syntheses, most notably of dyes from the triphenylmethyl group. Thus, by its reaction with phenol in the presence of zinc chloride as Lewis acid and hygroscopic reagent, phenolphthalein is obtained, with resorcinol fluorescein, etc.

Phthalic acid esters with long-chain aliphatic alcohols are used to modify the mechanical properties of plastics such as polyvinyl chloride (PVC). There are suspicions that they are neurotoxic and could be carcinogenic.

Isophthalic acid is used in the synthesis of polyester resins; while terephthalic acid is, among other applications, the base of polyethylene terephthalate (PET), a plastic used, for example, in the manufacture of beverage containers. These isomers are also generated from the corresponding xylenes.

The monopotassium salt: potassium hydrogen phthalate is a standard acid in analytical chemistry, used as a primary standard to standardize bases.

Isomers

Phthalic acid has two isomers, isophthalic acid with the carboxy groups in position 1 and 3 and terephthalic acid with the groups in position 1 and 4 on the benzene ring.

phthalic acid	isoftalic acid	tereftalic acid
(acid) orthose-phtalic)	(acid) goal-phtalic)	(acid) for-phtalic)