Acid Anhydride

What is an Acid Anhydride?

Acid Anhydride is the molecule capable of forming acidic solutions in water. An acid anhydride is defined as a non-metal oxide capable of creating an acidic solution when reacted with water. In organic chemistry, an Anhydride is a functional group having two acyl groups combined together by an Oxygen atom. Only the non-metals capable of reacting with water are called Acid anhydrides, and non-metals that do not react with water don't come under acid anhydrides.

By this, we can say that all acid anhydrides are non-metals, and all non-metals are not acid Anhydride. For example, carbon monoxide is not an acid anhydride, even though it is an oxide of carbon due to the reason it does not react with water.


Acid Anhydride Structure

Ethanoic acid is like a carboxylic acid and the acid anhydride structure can be represented as follows.

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Carboxylic Anhydride is a common type of organic acid anhydride, where the parent acid is a carboxylic acid, with the acid anhydride formula being (RC(O))2O. These types of symmetrical acid anhydrides are named by replacing the word "acid" in the name of the parent carboxylic acid with the word "anhydride."

Therefore, (CH3CO)2O is known as acetic Anhydride. Mixed or unsymmetrical acid anhydrides, like acetic, formic Anhydride, are known, when the reaction occurs between two different carboxylic acids.


Properties of Acid Anhydrides

Physical properties

Let us take ethanoic anhydride as typical.

Appearance

Ethanoic anhydride is a liquid with no colour and smells strongly of vinegar (ethanoic acid).

The smell is due to the ethanoic anhydride reacting with water vapour in the air (and moisture in our nose) again to produce ethanoic acid.

Solubility in Water

Ethanoic anhydride cannot be said to dissolve in water because it reacts with it to form ethanoic acid. Like aqueous solution, there is no such thing as an ethanoic anhydride.

Boiling Point

Ethanoic anhydride's boiling point is at 140°C. This is only because it is fairly a big polar molecule, and so has both dipole-dipole attractions and van der Waals dispersion forces.

However, it doesn't form hydrogen bonds. It means that the boiling point is not as high as that of a carboxylic acid of a similar size. For example, pentanoic acid (one of the most similarly sized acids) boiling point is at 186°C.


Chemical Properties

Acid anhydrides are less reactive compared to acid chlorides. They are preferred to acyl chlorides for carrying out acylation reactions in alcohols, phenols, and amines.

Acid anhydrides are slowly hydrolysed by water.

(CH3CO)2O + H2O → 2CH3COOH

Acid anhydrides react with alcohols and phenols to produce esters.

(CH3CO)2 + C2H5OH → CH3COOC2H5 + CH3COOH

When reduced by lithium aluminum hydride, acid anhydrides contain primary alcohols. Two different types of primary alcohols are produced in the case of mixed anhydrides.

Acid anhydride also reacts to the formation of aromatic ketones with aromatic hydrocarbons in the presence of anhydrous aluminum chloride.


Reactions of Acid Anhydrides

Acid anhydrides are the reactive acyl group sources, and the reactions and uses of acid anhydrides resemble those of acyl halides.

A few of the reactions of Acid Anhydrides are explained below.


Formation of Carbonic Acid

When carbon dioxide reacts with water, it produces sulphuric acid. The chemical equation is given below.

CO2(g) + H2O → H2CO3(aq)

Carbon Dioxide is a non-metal that reacts with water and forms H2CO3, Carbonic acid, which is acid as it has a hydrogen to donate. This reaction is responsible for acid rain. In changing the pH levels of rivers, streams, and oceans, it plays a vital role. It can be observed in the equation given above that if there is more carbon dioxide in the air, more carbonic acid is produced, which in turn becomes harmful to life. And, this is the major problem that we are facing today.


Formation of Sulphuric Acid

Sulphur trioxide reacts with water and forms Sulphuric acid. It is explained chemically as below.

SO3(g) + H2O → H2SO4(aq)

Then, the Sulphur trioxide gas reacts with water, which results in the Sulphuric acid formation. Sulphur dioxide present in the air reacts with oxygen and produces sulphur trioxide. That is how acid rain occurs when it reacts with water during rain, and they have very harmful effects on the environment also.


Naming Acid Anhydrides

Naming acid anhydrides is quite easy. Just by taking the name of the parent acid, and replacing the word "acid" by "anhydride", we are done. "Anhydride" means simply "without water".

So, ethanoic acid forms ethanoic anhydride, whereas propanoic acid forms propanoic anhydride, and the flow goes on.

For A level purposes of the UK, the only one we are expected to find is ethanoic anhydride.


Nomenclature of Acid Anhydride

The word 'acid' is changed to 'anhydride' in both the common name and in the IUPAC name as well.

Some examples of Acid Anhydride nomenclature are,

  • Ethanoic Acid → Ethanoic Anhydride

  • Propanoic Acid → Propanoic Anhydride

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  • Anhydrides are composed of two different acids called mixed anhydrides and are named by using the names of the individual acids.

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Symmetrical Anhydrides - These change the word ‘acid’ of the carboxylic acid to the word ‘anhydride.’

Mixed Anhydrides - These are alphabetizing the names for both acids and replacing the word ‘acid’ with the word ‘anhydride.’

FAQ (Frequently Asked Questions)

1. Mention Some Uses of Acid Anhydrides?

Acetic anhydrides are used for a several of applications where the formation of esters is required, for example, the painkiller manufacturer, polymer cellulose acetate, and aspirin (acetylsalicylic acid), which is used in many applications including synthetic fabrics, consumer items, synthesis of heroin by deacetylation of morphine and more.

Maleic anhydride is used mostly in manufacturing synthetic resins like unsaturated polyester resins for use in fibre reinforced plastics.

In addition to the above-mentioned ones, a few of the uses are listed below.

  • They are used widely as acylating agents to acylate alcohols and phenols

  • They can also use for the detection and estimation of NH2 and OH groups

  • They are used in the preparation of dyes, synthesis of aspirin (Acetylsalicylic acid)

  • They are used in the manufacturing industries like pharmaceuticals, explosives, perfumes, and industrial chemicals

  • They can also be used as protection groups.

2. How Can We Produce An Acid Anhydride Starting With Hydrocarbon?

Let us start with the hydrocarbon ethane. If the hydrocarbon ethane reacts with H3O+, it produces ethanol.

Now oxidize ethanol to acetic acid using KMnO4 and start heating. Then, crank up the temperature to 350°C and add some P2O5 to pull out the water.

That’s it. The acetic anhydride is thus formed.

But, make sure to seal up the container tightly to avoid the water present in the air from reacting with any of our synthetic products.