orthophthalic

Orthophthalic, or ortho-phthalic, refers to compounds derived from 1,2-benzenedicarboxylic acid, commonly known as orthophthalic acid, and its anhydride. The defining feature is that the two carboxyl groups are adjacent on the benzene ring (ortho relationship). The corresponding anhydride is orthophthalic anhydride, the anhydride form of the same diacid. These substances are distinguished from the meta- and para- isomers, isophthalic and terephthalic, which have carboxyl groups separated by one or more carbon atoms.

Orthophthalic acid has the formula C8H6O4, while orthophthalic anhydride has the formula C8H4O3. The diacid is a white crystalline solid and readily dehydrates to the anhydride upon heating; the anhydride can be hydrolyzed back to the diacid. Both forms are used primarily as intermediates in polymer chemistry and resin production.

Industrial relevance lies mainly in phthalic anhydride, derived from orthophthalic acid, which is a key precursor in the manufacture of alkyd resins and unsaturated polyesters. These resins are employed in coatings, paints, and composite materials, often via esterification with glycols or alcohols to form networked polymers. Some specialty orthophthalic derivatives are used as monomers or crosslinking agents in niche resin systems.

Safety considerations include typical handling precautions for aromatic diacids and anhydrides: they can be irritants to skin, eyes, and the respiratory tract and should be handled with appropriate personal protective equipment. Storage typically involves dry, well-ventilated conditions to avoid moisture uptake and hydrolysis.