Carboxylic acid anhydrides

Carboxylic acid anhydrides are organic compounds formed from the chemical reaction between two carboxylic acids.

Carboxylic acid anhydrides are organic oxygenated compounds originated from an elimination or dehydration reaction between two carboxylic acids . Thus, when two carboxylic acids interact, they lose a molecule of water and form a carboxylic acid anhydride. Below is a chemical equation that represents the formation of an anhydride:

Carboxylic acid anhydride originated from dehydration between two carboxylic acids

Note: A carboxylic acid anhydride can be caused by the reaction between two same or different carboxylic acids.

Analyzing the above equation, we can observe that, for the formation of a carboxylic acid anhydride, the hydroxyl (circled in black) of one of the carboxylic acids interacts with the hydrogen (circulated in black) in the hydroxyl of the other acid, which results in the formation of water. The carbon where the hydroxyl was located joins the oxygen that lost the hydrogen, forming the anhydride.

In general, the recognition of a carboxylic acid anhydride is easy, since its functional group has three oxygen atoms related to two carbons. Look:

General formula of a carboxylic acid anhydride

♦ Nomenclature

The nomenclature , according to Iupac, of a carboxylic acid anhydride depends on the carboxylic acids that gave rise to it. If it originated from the same type of carboxylic acid, the nomenclature rule will be:

Anhydride + name of carboxylic acid

For example:

 Etanoic Anhydride

Analyzing the two parts of the anhydride, upper and lower, we noticed that both have the same amount of carbons, that is, it originated from the same carboxylic acid (oxy suffix) and this one had two carbons (prefix et) with simple connection between them ( infix an). Therefore, ethanolic anhydride.

 Butanoic Anhydride

Analyzing the two parts of the anhydride, upper and lower, we noticed that both have the same amount of carbons, that is, it originated from the same carboxylic acid (oico suffix) and this one had four carbons (prefix but) with simple connection between them ( infix an). Therefore, butanoic anhydride.

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When the anhydride originates from different carboxylic acids, the nomenclature rule will have a difference:

Anhydride + name of carboxylic acids in alphabetical order and separated by hyphen

For example:

 Butanoic-propanoic anhydride

Analyzing the structure, we noticed that the upper and lower parts have different amounts of carbons. The upper part was originated by a carboxylic acid with three carbon atoms (prop prefix) and single bonds (infix an); therefore, propanoic. The lower part was originated by a carboxylic acid with four carbon atoms (prefix but) and simple bonds (infix an); therefore, butanoic. When writing the name of the anhydride, the name butanoic will come first to obey the alphabetical order.

 Ethananoic-metanoic anhydride

Analyzing the structure, we noticed that the upper and lower parts have different amounts of carbons. The upper part was originated by a carboxylic acid with two carbon atoms (prefix et) and single bond (infix an); therefore, etanoic. The lower part was originated by a carboxylic acid with only one carbon atom (prefix met); therefore, metanoic. In writing the name of the anhydride, the ethaneic name will come first than the metanoic one to obey the alphabetical order.

♦ Physical and chemical characteristics

Another important factor in relation to anhydrides are their physical and chemical characteristics, namely:

  • They are less dense than water;
  • They usually have a strong and irritating smell;
  • They have polar molecules;
  • They make hydrogen bonds between their molecules;
  • They have good solubility in water;
  • They have a high reactivity when compared to carboxylic acids.

As examples of uses of carboxylic acid anhydrides , we can highlight:

  • Production of the phenolphthalein indicator (phthalic anhydride);
  • Manufacture of resins (phthalic anhydride);
  • Manufacture of plasticizers (phthalic anhydride);
  • Manufacture of dyes (ethanolic anhydride)
  • Raw material for the production of photographic films (ethanolic anhydride);
  • Raw material for the manufacture of textile fibers (ethanolic anhydride);
  • Widely used in organic syntheses.

 

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