3.12 Carboxylic acids

Carboxylic acids are molecules with a -CO₂H group. They are named according to how many total carbon atoms are in the chain, but some of the names are different from those of the corresponding alkyl groups. The first 5 aliphatic carboxylic acids are formic, acetic, propionic, butyric, and valeric acids. They are also sometimes called methanoic, ethanoic, propanoic, butanoic, and pentanoic acids for consistency with the alkanes.

Longer chains have both common names and consistent names up to 10 carbons: caproic acid is hexanoic acid; oenanthic acid is heptanoic acid; caprylic acid is octanoic acid; pelargonic acid is nonanoic acid; capric acid is decanoic acid. Three of the common names are derived from the Latin word caper, meaning "goat", making it easy to confuse them. Whoever discovered these must have been working with materials extracted from goats!

Beyond 10 carbons, the systematic names are usually preferred: undecanoic acid, dodecanoic acid, tridecanoic acid, etc. However, the even numbered acids and a few odd numbered ones have common names all the way up to 35 carbons! I'm not going to make you memorize any of these, but it's good to be aware of at least lauric acid (12 carbons), myristic acid (14 carbons), palmitic acid (16 carbons), stearic acid (18 carbons), and arachidic acid (20 carbons).

Branched carboxylic acids are known such as isobutyric acid, isovaleric acid, etc. Pivalic acid is a specially named compound with the structure (H₃C)₃CCO₂H.

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