dioic acid
Summary
A dioic acid is a dicarboxylic acid, named by adding "-dioic acid" to the alkane name, as in butanedioic acid, HOOC–CH₂–CH₂–COOH. Both carboxyl carbons are part of the chain count. Testable point: pKa1 is lower than pKa2, and a nearby second COOH makes the first proton more acidic.
Detail
Meaning: In IUPAC naming, a dioic acid has two COOH groups, one at each end of the longest chain. The suffix goes on the full alkane name, so the e stays: ethanedioic, propanedioic, butanedioic, pentanedioic, hexanedioic. Both carboxyl carbons are C1 and the last carbon, so no locants are needed. Common names are oxalic (2 C), malonic (3 C), succinic (4 C), glutaric (5 C) and adipic (6 C).
Each molecule has two acidic protons. The first is lost more easily (pKa1 < pKa2) because the second COOH is electron-withdrawing by induction and stabilizes the first conjugate base. The second proton must leave from an anion, which is harder. The inductive effect weakens as the chain between the groups gets longer.
On the MCAT: Expect questions that ask you to name or draw a structure, rank acidity, or read a titration curve with two buffer regions and two equivalence points. Heating succinic or glutaric acid can give a cyclic anhydride, because five- and six-membered rings form readily. A dioic acid with a diamine gives a polyamide (nylon), and with a diol it gives a polyester.
Common confusion: The carbons in the COOH groups count toward the chain. Butanedioic acid has 4 carbons in total, not 4 plus 2. Also, "di" refers to the two acid groups, not to two separate molecules or to a diol.
Sources
- Organic Chemistry (Clayden)
- Organic Chemistry (Wade)
- Organic Chemistry (Klein)
Written for the MCAT and checked before it was published. For study reference only. Spotted an error? Let us know.