Naming Acid Anhydrides
Definition of Acid Anhydrides:
Acid Anhydrides are organic chemical compounds whose structure has the general form:
where the symbols "R1" and "R2" usually represent carbon chains. |
[Symbols such "R","R' ", "R1", "R2", "R1" and "R2" and similar are often used to represent organic radicals in general, which are frequently (linear or branched) carbon chains that may or may not have addtional functional groups attached, but can also be hydrogen atoms or halogen atoms, e.g. chlorine (Cl) atoms as in acid chlorides, also known as acyl chlorides..] |
The simplest acid anhydrides to describe are those in which R1 = R2 = "an alkane less the hydrogen atom at the end of the chain", where the hydrogen atom at the end of the corresponding alkane is replaced by the carbon atom to which that "R" (so, R1 or R2 in the diagram above) is attached.
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The above definition concerns only the molecular structure of acid anhydrides, but raises and does not answer two obvious questions:
- Why are acid anhydrides called "acids" ?
and
- Why are acid anhydrides called "anhydride"s ?
It is useful to understand the answers to these questions, which can be explained by use of an example.
Example of a simple Acid Anhydride:
Ethanoic Anhydride
The prefix "eth-" usually indicates a chain of two carbon atoms (e.g. ethane, ethene, ethyne, ethanol, etc).
The simplest carboxoylic acid based on a chain of two carbon atoms is ethanoic acid, whose structure may be represented as:
 or more simply as:  .
By combining two molecules of a carboxylic acid in such a way as to remove or extract (from them) one molecule of water, an acid anhydride may be formed*.
*This statement should be understood theoretically rather than practically because acid anhydrides are not necessarily produced in this way. For example, although it is possible to produce ethanoic anhydride by dehydrating ethanoic acid, it is usually made via a more efficient though less obvious process (probably beyond the scope of UK A-Level Chemistry courses).
The reason for this description of acid anhydrides as the theoretical result of dehydration of carboxylic acids is to explain and so make it easier to remember that acid anhydrides may be called acids because they may (in some cases) be produced from carboxylic acids, and that they may be called anhydrides because because they may (in some cases) be produced by removing water (from acids): The word "anhydride" means "without water".
Names and Structures of simple Acid Anhydrides
A series of linear acid anhydrides is shown below with the molecular structure of each acid anhydride in the series drawn out in full (showing bond types but not accurate bond angles or, therefore, realistic molecular shapes).
Number carbons in each chain |
Name |
Simple Structure |
Other Names = Synonyns
(*Examples not complete lists) |
and simple formula
 |
showing bond types but not accurate bond angles, hence not shapes of molecules
|
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1 |
formic anhydride
(C2H2O3)
|
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|
2 |
ethanoic anhydride
(C4H6O3)
|
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- acetic anhydride
- acetic acid anhydride
- acetanhydride
- acetic oxide
- acetyl oxide
- acetyl anhydride
- acetyl acetate
- acetyl ether
|
3 |
propanoic anhydride
(C6H10O3)
|
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- propionic anhydride
- propanoic acid anhydride
- methylacetic anhydride
- propionyl oxide
|
4 |
butanoic anhydride
(C8H14O3)
|
 |
- butanoic acid anhydride
- butyric anhydride
- n-butyric anhydride
- butyric acid anhydride
- n-butyric acid anhydride
- butyryl oxide
|
5 |
pentanoic anhydride
(C10H18O3)
|
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- pentanoic acid anhydride
- valeric anhydride
- n-valeric anhydride
- pentanoyl pentanoate
|
6 |
hexanoic anhydride
(C12H22O3)
|
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- hexanoic acid anhydride
- caproic anhydride
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7 |
heptanoic anhydride
(C14H26O3)
|
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- n-heptanoic anhydride
- heptanoic acid anhydride
- n-heptanoic acid anhydride
- enanthic anhydride
- heptanoyl anhydride
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8 |
octanoic anhydride
(C16H30O3)
|
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Common Synonyms: |
- octanoic acid anhydride
- n-octanoic anhydride
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- caprylic anhydride
- n-caprylic anhydride
- octanoyl octanoate
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9 |
nonanoic anhydride
(C18H34O3)
|
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Common Synonyms: |
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10 |
decanoic anhydride
(C20H38O3)
|
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Common Synonyms: |
- decanoic acid anhydride
- n-decanoic anhydride
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- capric anhydride
- n-capric anhydride
- decanoyl decanoate
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Why stop at ten carbon atoms (in the chain) ? It doesn't ... there are more similar acid anhydrides. .... We stopped here due to page width ! |
Important Note: *The synonyms indicated for compounds listed above are just some examples of alternative names found online and believed to have been used to refer to the substance indicated. They have not all been verified and may include common (non-scientific) names, trade names specific to particular suppliers, and perhaps errors. The purpose of these lists is to give a general indication of the range of names by which acid anhydrides are known - both generally, and specifically.
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Note:
This is one of many pages about types of organic compounds in our Organic Chemistry Section. Similar and related pages describe hydrocarbons, alkanes, haloalkanes, alkenes, alkynes, cycloalkanes, aromatic hydrocarbons (arenes), alcohols, esters, ethers, aldehydes, ketones, carboxylic acids, acid chlorides, acid anhydrides, azo dyes, amines, amides, nitriles, amino acids (chemistry), peptides, proteins (chemistry), polypeptides and others. If you need further information ask your chemistry tutor.
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