carboxylic acid
/ kar-bok-SIL-ik /
Smell vinegar, and you are smelling a carboxylic acid: acetic acid. Taste the sourness of citrus or the tang of yogurt, and you are tasting them too. Carboxylic acids are the most familiar sour, acidic molecules in everyday chemistry, and the word "acid" in the name is honest — they really do donate protons to water.
A carboxylic acid is any molecule carrying the carboxyl group, written -COOH or -CO2H. Look closely at that group: a single carbon is double-bonded to one oxygen (the carbonyl, C=O) and single-bonded to a second oxygen that itself carries a hydrogen (the hydroxyl, O-H). So the carboxyl is a carbonyl and a hydroxyl fused onto the same carbon, and the two halves talk to each other electronically, which is exactly why this group behaves so differently from a plain alcohol or a plain ketone. The simplest examples are formic acid (HCOOH, from ant stings) and acetic acid (CH3COOH, vinegar).
Carboxylic acids are everywhere in biology and industry. Fatty acids are long-chain carboxylic acids that build fats and cell membranes; amino acids carry a carboxyl group that lets them link into proteins; citric, lactic, and acetic acids run through metabolism and food. The carboxyl group is also the launch pad for a whole family of derivatives — esters, amides, acid chlorides, anhydrides — so understanding it opens the door to most of acyl chemistry.
Acetic acid is CH3-COOH. The two oxygens differ in job: the C=O oxygen has no hydrogen, while the C-O-H oxygen carries the acidic proton that vinegar gives up to water (pKa about 4.76).
Acetic acid: the carboxyl group is a fused carbonyl plus hydroxyl on one carbon.
Do not confuse a carboxylic acid (one carbon bonded to both oxygens, -COOH) with an ester or an alcohol; only the -OH bound to a carbonyl carbon gives that strong acidity and dimer-forming behavior.