ArCOOH

ArCOOH refers to aryl carboxylic acids, a class of compounds in which a carboxyl group (CO2H) is directly attached to an aromatic ring. The Ar denotes an aryl group, most commonly a phenyl ring, with benzoic acid (C6H5CO2H) being the simplest and most widely studied example.

The carboxyl group in ArCOOH is highly characteristic, giving the molecule acidic properties. In water, benzoic acid has a pKa around 4.2, making it a weak acid. Substituents on the aryl ring modulate acidity: electron-withdrawing groups increase acidity by stabilizing the conjugate base, while electron-donating groups decrease acidity. The carboxyl group also makes ArCOOH a versatile starting point for further transformations, and the ring can influence reactivity through resonance and induction.

Common reactions of aryl carboxylic acids include esterification to form aryl esters (ArCOOR) and conversion to acid chlorides (ArCOCl) using reagents such as thionyl chloride. Reduction with strong hydride donors (for example LiAlH4) reduces ArCOOH to the corresponding primary alcohol (ArCH2OH). Decarboxylation can occur under certain heating conditions, yielding the arene (ArH) or shortened products in some contexts. Substituted aryl carboxylic acids serve as key intermediates in organic synthesis and materials chemistry.

Notable members and applications include benzoic acid and its derivatives, salicylic acid (2-hydroxybenzoic acid), p-hydroxybenzoic acid, and the dicarboxylic acids terephthalic and isophthalic acid. These compounds are used as preservatives, flavor and fragrance precursors, pharmaceutical intermediates, and, in the case of terephthalic and related acids, as monomers for polyesters such as PET. Safety data vary by compound, but ArCOOHs are generally irritants and should be handled with appropriate care and disposal.