Question

What are some ways acetic anhydride can be produced?

Answer 1

Upon removal of a water molecule from an acid molecule, an anhydride is formed. Some inorganic anhydrides are $S{{O}_{3}}$ derived from ${{H}_{2}}S{{O}_{4}}$ or CaO derived from $Ca{{(OH)}_{2}}$. One of the most important organic acid anhydrides is acetic anhydride.

Complete step by step answer: In an organic acid anhydride, there are two acyl groups (R-C=O) bonded to the same oxygen atom. Its generic structure is

The simplest anhydride of a carboxylic acid which can be isolated is acetic anhydride. It is also known as ethanoic anhydride. It has the molecular formula ${{(C{{H}_{3}}CO)}_{2}}O$ but is commonly abbreviated as $A{{c}_{2}}O$.
Its structure is as follows

Acetic anhydrides can be prepared using the following methods:
- One of the most common methods of preparing acetic anhydride is by carbonylation (addition of carbon monoxide) of methyl acetate in the presence of rhodium chloride and lithium iodide as catalysts.
$C{{H}_{3}}COOC{{H}_{3}}+CO\xrightarrow[LiI]{RhC{{l}_{3}}{{({{H}_{2}}O)}_{N}}}{{(C{{H}_{3}}CO)}_{2}}O$
These conversions are conducted in anhydrous conditions as acetic anhydride is unstable in water.
This process of production of acetic anhydride is also known as Tennessee Eastman acetic anhydride process.
- Acetic anhydride can also be made by reacting ethenone, a ketene, with acetic acid. This reaction takes place at a low pressure of around 0.05-0.2 bar and temperature between 45-55${}^\circ C$.
${{H}_{2}}C=C=O+C{{H}_{3}}COOH\xrightarrow[45-55{}^\circ c]{0.05-0.2bar}{{(C{{H}_{3}}CO)}_{2}}$
This method was developed by Wacker Chemie in 1922 due to the increase in demand for acetic anhydride which is used during the production of cellulose acetate.
Acetic anhydride is low cost and inexpensive and hence is usually purchased when used in laboratories instead of being prepared.

Note: When an acid anhydride is named, one should note that simply renaming the word "acid" to "anhydride" of the parent carboxylic acid would suffice in a symmetrical acid anhydride. For example, acetic anhydride.
Whereas in an unsymmetrical acid anhydride, both of the carboxylic acids reacted must be named before adding the word anhydride. For example, benzoic propanoic anhydride.