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Acid Anhydrides: How They’re Formed, Applications, and Examples

Also, there is a class of acid anhydrides that is commonly found: carboxylic anhydrides, so named because the starting acid is a carboxylic acid . To name those of this type whose structure is symmetric, only one term substitution must be made.

Acid anhydrides

The term acid in the nomenclature of its original carboxylic acid should be replaced by the term anhydride, which means “without water”, without altering the rest of the name of the molecule formed. These compounds can also be generated starting from one or two acyl groups from other organic acids, such as phosphonic acid or sulfonic acid.

How are acid anhydrides formed?

Some acid anhydrides originate in a number of ways, either in the laboratory or in industry. An example of industrial synthesis is acetic anhydride, which is produced mainly by the carbonylation process of the methyl acetate molecule.

Another example of these syntheses is that of maleic anhydride, which is generated through the oxidation of the benzene molecule or the butane molecule.

However, in the synthesis of acid anhydrides in the laboratory, emphasis is placed on the dehydration of the corresponding acids, such as the production of ethanoic anhydride, in which two molecules of ethanoic acid are dehydrated to give rise to the mentioned compound.

Intramolecular dehydration can also occur; that is, within the same molecule of an acid with two carboxyl (or dicarboxylic) groups, but if the opposite occurs and an acid anhydride undergoes hydrolysis, the regeneration of the acids that originated it occurs.

However, these species are also generated when a reaction occurs between an acyl halide (whose general formula is (RCOX)) with a carboxylate molecule (whose general formula is R’COO-)). [2]

General Formula

The general formula for acid anhydrides is (RC (O)) 2 O, which is best seen in the image placed at the beginning of this article.

For example, for acetic anhydride (from acetic acid) the general formula is (CH 3 CO) 2 O, being written similarly for many other similar acid anhydrides.

As mentioned above, these compounds have almost the same name as their precursor acids, and the only thing that changes is the term acid for anhydride, since the same rules for numbering atoms and substituents must be followed to get their nomenclature right.

Applications

Acid anhydrides have many functions or applications depending on the field that is being studied because, having high reactivity, they can be reactive precursors or form part of many important reactions.

An example of this is industry, where acetic anhydride is produced in large quantities because it has the simplest structure that can be isolated. This anhydride is used as a reagent in important organic syntheses, such as acetate esters.

Industrial use

On the other hand, maleic anhydride shows a cyclic structure, being used in the production of coatings for industrial use and as a precursor of some resins through the copolymerization process with styrene molecules. In addition, this substance acts as a dienophile when the Diels-Alder reaction is carried out.

Similarly, there are compounds that have two molecules of acid anhydrides in their structure, such as ethylenetetracarboxylic dianhydride or benzoquinonetetracarboxylic dianhydride, which are used in the synthesis of certain compounds such as polyimides or some polyamides and polyesters.

In addition to these, there is a mixed anhydride called 3′-phosphoadenosine-5′-phosphosulfate, from phosphoric and sulfuric acids, which is the most common coenzyme in biological sulfate transfer reactions.

Examples of acid anhydrides

Acetic anhydride

Below is a list with the names of some acid anhydrides, to give some examples of these compounds so important in organic chemistry that they can form compounds of linear structure or multi-membered rings:

– Acetic anhydride.

– Propanoic anhydride.

– Benzoic anhydride.

– Maleic anhydride.

– Succinic anhydride.

– Phthalic anhydride.

– Naphthalene tetracarboxylic dianhydride.

– Ethylenetetracarboxylic dianhydride.

– Benzoquinonetetracarboxylic dianhydride.

Just as these anhydrides are formed with oxygen, there are other compounds where a sulfur atom can replace the oxygen in both the carbonyl group and the central oxygen, such as:

– Thioacetic anhydride (CH 3 C (S) 2 O)

There is even the case of two acyl molecules that form bonds with the same sulfur atom; These compounds are called thioanhydrides , namely:

– Acetic thioanhydride ((CH 3 C (O)) 2 S)

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