-oic Also -dioic.
A carboxyl group or its derivative.
Though the ending is strictly only a variant of -ic, it has been formalized by chemists to be the standard ending for organic acids in which the carboxyl group, —COOH, replaces the methyl group, —CH3. Some examples from a large group: valproic acid (a name indirectly derived from the plant called valerian), a synthetic crystalline compound used in the treatment of epilepsy; caproic acid (Latin caper, capr-, goat, because of its smell), a liquid fatty acid present in milk fat and coconut and palm oils; benzoic acid (from gum benzoin), a white crystalline substance used as a food preservative.
Many such names are formal chemical alternative names for compounds with common names, as ethanoic acid is the formal name of acetic acid. When two carboxyl groups are attached to one molecule, the ending becomes -dioic (Greek dis, twice), as in ethanedioic acid, the systematic name for oxalic acid, or hexanedioic acid, common name adipic acid, a crystalline fatty acid used especially in the manufacture of nylon. In complex systematic names, the form can appear as the final element in a hyphenated sequence, as in 11-oxo-5α-cholan-24-oic acid.
Words whose meanings lie outside chemistry contain the ending as an accident of spelling in which -ic follows a stem ending in o: echoic, heroic, paranoic.