Nomenclature
Carboxylic acids take priority over most other functional groups in IUPAC nomenclature. The -COOH carbon is almost always C1 of the parent chain, and the suffix is -oic acid (replacing the final “-e” of the parent alkane name).
IUPAC Rules
- Find the longest carbon chain containing the COOH.
- The COOH carbon is always C1 (since it is terminal and gets priority).
- Replace “-e” with “-oic acid”.
- Add locants for substituents relative to C1.
Examples:
- HCOOH = methanoic acid (common: formic acid).
- CH₃COOH = ethanoic acid (common: acetic acid).
- CH₃CH₂COOH = propanoic acid (common: propionic acid).
- CH₃CH₂CH₂COOH = butanoic acid (common: butyric acid).
Common Names You Should Know
A handful of carboxylic acids have common names the MCAT uses regularly:
| IUPAC | Common name | Structure |
|---|---|---|
| Methanoic acid | Formic acid | HCOOH |
| Ethanoic acid | Acetic acid | CH₃COOH |
| Propanoic acid | Propionic acid | CH₃CH₂COOH |
| Butanoic acid | Butyric acid | CH₃(CH₂)₂COOH (rancid butter smell) |
| Pentanoic acid | Valeric acid | CH₃(CH₂)₃COOH |
| Hexanoic acid | Caproic acid | CH₃(CH₂)₄COOH |
| Benzoic acid | Benzoic acid | C₆H₅COOH (retained in IUPAC) |
| Ethanedioic acid | Oxalic acid | HOOC-COOH |
| Propanedioic acid | Malonic acid | HOOC-CH₂-COOH |
| Butanedioic acid | Succinic acid | HOOC-(CH₂)₂-COOH |
| Pentanedioic acid | Glutaric acid | HOOC-(CH₂)₃-COOH |
| Hexanedioic acid | Adipic acid | HOOC-(CH₂)₄-COOH |
Common names dominate for biologically important carboxylic acids: acetic acid (vinegar), lactic acid (milk), citric acid (citrus fruits), oxalic acid (spinach), succinic acid (Krebs cycle). Each has a biological story.
Dicarboxylic Acids
Dicarboxylic acids use the suffix -dioic acid with numbering giving the lowest locants to both COOH groups:
- HOOC-COOH = ethanedioic acid (oxalic acid).
- HOOC-CH=CH-COOH = but-2-enedioic acid. Cis = maleic acid; trans = fumaric acid.
Fumaric acid and maleic acid are cis/trans isomers. Fumarate is a Krebs cycle intermediate; maleate is an industrial precursor. The two differ dramatically in physical properties (maleic acid has higher solubility and lower melting point due to weaker packing).
Carboxylic Acid vs Higher-Priority Groups
Carboxylic acids are highest priority in the standard IUPAC list (besides a few rare functional groups like sulfonic acids). Lower-priority groups become prefixes:
- An alcohol + a carboxylic acid: the acid takes the -oic acid suffix; the alcohol becomes “hydroxy-”.
- A ketone + a carboxylic acid: the acid takes the suffix; the ketone becomes “oxo-”.
Example: HOOC-CH(OH)-CH₃ = 2-hydroxypropanoic acid (common: lactic acid). Example: HOOC-CH₂-CO-CH₃ = 3-oxobutanoic acid (common: acetoacetic acid).
Biological Fatty Acid Nomenclature
Fatty acids (long-chain carboxylic acids) are named by chain length:
- Capric acid = decanoic acid (C10).
- Lauric acid = dodecanoic acid (C12).
- Myristic acid = tetradecanoic acid (C14).
- Palmitic acid = hexadecanoic acid (C16).
- Stearic acid = octadecanoic acid (C18).
- Oleic acid = (9Z)-octadec-9-enoic acid (C18, one cis double bond at 9).
- Linoleic acid = (9Z,12Z)-octadeca-9,12-dienoic acid (C18, two cis double bonds).
The “C:n” shorthand (like C18:1 for oleic acid) specifies chain length:number of double bonds. This is the standard notation in lipid biochemistry.