4.2 Naming of Carboxylic Acids and Lactones
Carboxylic acids can be named using either atom numbers, or a system based on Greek letters. The carboxyl carbon does not have any hydrogens to substitute, so lettering begins with the next carbon over, called the alpha carbon. Additional carbon atoms are called beta, gamma, delta, etc. The amino acids that make up proteins are alpha-amino acids. One example is alanine CH3CH(NH2)CO2H. By its structure we see that alanine is the same as alpha-amino propionic acid or 2-amino propanoic acid in the IUPAC system. Another example is gamma hydroxy butyric acid or GHB, whose structure is HOCH2CH2CH2CO2H, which we can see is the same as 4-hydroxy butanoic acid.
Aldehydes sometimes also use Greek letters in their names, for example alpha methyl cinnamaldehyde C6H5CH=C(CH3)CHO.
Lactones are cyclic compounds that can be thought of as a hydroxy- carboxylic acid esterified to itself. The most common lactones are gamma (γ) lactones, where the ring closes on the gamma carbon, forming a 5-membered ring (four carbons and one oxygen), and delta (δ) lactones, where the ring forms on the delta carbon, making a six membered ring. Occasionally you will also see omega (ω) lactones, which just means the oxygen atom connects to the last carbon atom, irrespective of chain length.
γ-decalactone |
δ-decalactone |
ω-decalactone |
| All three of these lactones have the same number of carbon atoms. | ||