oic acid

Meaning: In IUPAC naming, -oic acid marks a carboxylic acid, R–COOH. Take the longest chain that includes the carboxyl carbon, drop the final -e of the alkane name, and add -oic acid. Examples: methanoic acid (formic), ethanoic acid (acetic), propanoic acid, butanoic acid, and benzoic acid. The carboxyl carbon is C-1, so it needs no locant, and substituents are numbered from it (3-methylbutanoic acid). When COOH is attached to a ring, the suffix becomes -carboxylic acid (cyclohexanecarboxylic acid).

On the MCAT: You convert between names and structures. A carboxylic acid outranks aldehydes, ketones, alcohols and amines, so those groups appear as prefixes (oxo, hydroxy, amino) in a name that ends in -oic acid. Related derivatives are named from the same stem: -oate for carboxylate salts and esters (ethyl ethanoate), -oyl chloride for acyl chlorides, -amide for amides. Passages also use the group's properties: pKa is roughly 3 to 5, so carboxylic acids are much more acidic than alcohols because the carboxylate is resonance-stabilized, and electron-withdrawing substituents raise the acidity.

Common confusion: Students mix up -oic acid, -oate and -al. The neutral acid ends in -oic acid, its deprotonated form or ester ends in -oate, and an aldehyde ends in -al. Also, C-1 is fixed at the carboxyl carbon, so you do not number from the end nearer a substituent.