Acid Anhydride - Explained Detailly
Chemistry as a subject seems difficult unless you start reading it. The term Acid Anhydride might look complicated, but you can guess and say it has something to do with the acid. So it is nothing but a molecule that is capable of forming acid solution in the water. Was that difficult to understand?
The team of experts at Vedantu believe in simplifying the learning experience for the students. So, they come up with short articles, on different topics which are important for the exams. In this article, we shall be learning all about -
Introduction to Acid Anhydride
What is an Acid Anhydride?
Acid Anhydride Structure
Properties of Acid Anhydrides
Reactions of Acid Anhydrides
Naming Acid Anhydrides
Nomenclature of Acid Anhydride
Frequently asked questions
What is an Acid Anhydride?
Acid Anhydride is a 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
1. 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.
2. 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.’
FAQs on Acid Anhydride
1. What is the process of finding out the Anhydride of any compound?
To find out the Anhydride students need to remember one simple rule that is oxidation state remains the same in the different compounds of the same element. Let us take an example to prove it further -
If we take H3PO4 then the anhydride will be P2O5, as the oxidation state of phosphorus in H3PO4 is plus 5 and it also plus 5 in P2O5
Again, the anhydride of HClO4 where oxidation state of chlorine is plus 7 then the result is Cl207 with the same oxidation state of chlorine.
Thus, you should know the oxidation state of different compounds, and finding anhydride will be simple.
2. What are the methods of preparing Acid Anhydride?
Acid Anhydride can be prepared through the following steps:
Carboxylic Acid - taking two molecules of acetic acid and heating it in the presence of Phosphorous pentachloride, leaving the water molecules out we will get the final result
Acid Chloride and Carboxylic acid or their salts - taking acid chlorides reacted with carboxylic acid leaving HCl we will get the result
The reaction of Acetic acid with ketone will also give Acid Anhydride.
Try writing down the steps with the chemical formulas and build the chemical bonds.
3. What are some of the applications of Acid Anhydride?
It is a clear liquid with a vinegar-like odor that is a little pungent. It has various applications like-
Uses in the pharmaceutical industry to make various painkillers, aspirin, acetaminophen
The largest quantity is used as raw material for cellulose fibres and plastics
It is used in the making of perfumes, dyes, etc
It is also an essential material in the making of explosives
4. What are the harmful effects of Acid Anhydride?
Acid anhydride can have multiple harmful effects like -
May enter the body through inhalation or contact of the skin, can cause several breathing problems, nausea, dizziness
It is a highly Corrosive chemical that may cause a burn of the skin or eyes. And may also cause permanent damage to the eyes
A high concentration of the element can cause severe damage to the lungs and may cause its failure
It may also cause minor problems like sore throat, sneezing, coughing, short breaths, etc
It has been found that it has carcinogenic elements in it that may cause cancers
It is hazardous to the reproductive organ
The chemical is listed on the hazardous substance list.
5. What are the possible measures through which exposure to the Acid Anhydride can be avoided?
Acid anhydride have various hazardous effects on the human body, to prevent the effects steps that could be taken are:
Conduct the operation of the acid in an enclosed manner
Make use of exhaust or ventilation at the site of chemical release
Use of respirator by the operators on the site of the chemical reaction
Make use of protective body covers and aprons
Proper handling and storage of the acid
Regular and thorough washing after exposure to the acid
Educate and train the people handling the work of chemical reactions or have exposure to the acid.
These few precautions could save the lives of people who are working under exposure to Acid anhydride.
6. 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.
7. How Can We Produce An Acid Anhydride Starting With Hydrocarbon?
Let us start with 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.
