The "lactone rule" is more of a naming convention than a strict rule, referring to how lactones are named based on their ring size and relationship to the parent carboxylic acid.
Here's a breakdown:
-
Lactones: Lactones are cyclic esters formed when a hydroxyl group (-OH) and a carboxylic acid group (-COOH) within the same molecule react.
-
Naming Convention: Lactones are generally named according to the precursor acid molecule (e.g., aceto, propio, butyro, valero, capro) with a "-lactone" suffix. A Greek letter prefix (alpha, beta, gamma, delta, etc.) indicates the size of the lactone ring – or, more precisely, the number of carbon atoms between the carbonyl carbon of the ester and the oxygen atom derived from the alcohol.
- α-lactone: 3-membered ring
- β-lactone: 4-membered ring
- γ-lactone: 5-membered ring
- δ-lactone: 6-membered ring
- ε-lactone: 7-membered ring
-
Relationship to Parent Acid: The Greek letter reflects the position of the carbon atom bearing the hydroxyl group relative to the carboxyl group in the open-chain precursor. For example, in γ-butyrolactone, the hydroxyl group was on the γ-carbon of butyric acid before cyclization.
-
Example: Consider 4-hydroxybutanoic acid. The -OH group is on the gamma (γ) carbon relative to the carboxylic acid. When it cyclizes, it forms γ-butyrolactone, a five-membered ring lactone. Butyric acid has four carbons; hence, "butyro-lactone."
In summary, the "lactone rule" provides a systematic way to name these cyclic esters based on the size of the ring and the relationship to the parent carboxylic acid, denoted by the Greek letter prefix.
