Naming Acid Anhydrides

Definition of Acid Anhydrides:

The above definition concerns only the molecular structure of acid anhydrides, but raises and does not answer two obvious questions:

1. Why are acid anhydrides called 'acids' ?
   and
2. Why are acid anhydrides called 'anhydride's ?

It is useful to be aware of the answers to these questions, which can be explained using 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 carboxylic 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).

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.

* 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 and trade names specific to particular suppliers. The purpose of these lists is to give a general indication of the range of names by which these acid anhydrides are, or have been, known.

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.