Anhydrides are organic compounds formed through a dehydration reaction, which involves removing a water molecule from one or more carboxylic acid molecules. The resulting structure is characterized by two acyl groups (R-CO) connected by a single oxygen atom (R-CO-O-CO-R’). Anhydride nomenclature is straightforward because the name is always derived directly from the parent acid or acids used to form it. This guide provides clear, structured rules for naming these compounds.
The Rules for Simple Anhydrides
Simple anhydrides, also known as symmetrical anhydrides, are formed from two identical molecules of the same carboxylic acid. The formation involves the intermolecular removal of water between these two acid molecules. This results in an anhydride where the two acyl groups on either side of the central oxygen bridge are chemically identical.
The naming convention follows a simple substitution rule: identify the parent carboxylic acid, remove the word “acid,” and substitute it with the term “anhydride.” For instance, if the starting material is acetic acid, the resulting compound is correctly named acetic anhydride.
Similarly, if the parent compound is butanoic acid, the resulting symmetrical anhydride is named butanoic anhydride. This structure is a common laboratory reagent, often used to introduce the acyl group into other molecules during synthesis reactions.
Naming Mixed Anhydrides
Naming mixed anhydrides, also referred to as unsymmetrical anhydrides, requires more attention because they are derived from two chemically distinct carboxylic acids. The structure features two different acyl groups attached to the central oxygen atom (R-CO-O-CO-R’). This difference necessitates including both parent acid names in the final designation.
The core rule is that the names of both parent acids must be cited. After identifying the two source acids, such as acetic acid and propanoic acid, the word “acid” is dropped from both names. This leaves the component names “acetic” and “propanoic” to be combined to form the anhydride’s full name.
The two resulting names must then be placed in strict alphabetical order, regardless of the size or complexity of the parent acid chain. This alphabetical requirement ensures consistency in chemical communication.
For an anhydride formed from acetic acid and propanoic acid, the correct alphabetical arrangement places “acetic” before “propanoic.” Therefore, the compound is correctly named acetic propanoic anhydride. This systematic approach eliminates structural ambiguity and is applied universally across all combinations of two different carboxylic acid precursors.
Naming Ring-Shaped Anhydrides
Ring-shaped anhydrides, or cyclic anhydrides, are typically formed through an internal reaction within a single molecule, rather than between two separate molecules. They are derived from dicarboxylic acids, which contain two separate carboxyl functional groups. The proximity of these groups allows for an intramolecular reaction.
The dehydration reaction is intramolecular, meaning water removal occurs between the two carboxyl groups on the same molecule. This process usually results in the formation of a stable five- or six-membered ring structure, which often exhibits greater reactivity compared to their linear counterparts.
Despite the unique ring structure, the naming convention remains closely aligned with the rules for simple anhydrides. The name is derived directly from the parent dicarboxylic acid by dropping the word “acid” and appending the term “anhydride.”
A common example is succinic anhydride, which is derived from succinic acid and forms a five-membered ring upon dehydration. Another example is maleic anhydride, formed from maleic acid. This straightforward adaptation of the nomenclature highlights the structural relationship between the acid and its derived anhydride.