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.
(Image will be uploaded soon)
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
(Image will be uploaded soon)
Anhydrides are composed of two different acids called mixed anhydrides and are named by using the names of the individual acids.
(Image will be uploaded soon)
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 an acid anhydride, and what is its characteristic functional group?
An acid anhydride is a type of organic compound that contains the -C(O)O(O)C- functional group. This structure consists of two acyl groups (R-C=O) bonded to the same oxygen atom. They are considered derivatives of carboxylic acids and are formed by the removal of one molecule of water from two molecules of the parent acid. The general formula for a carboxylic acid anhydride is (RCO)₂O.
2. How are acid anhydrides named according to IUPAC nomenclature?
Acid anhydrides are named based on the parent carboxylic acid(s) from which they are derived. The rules differ for symmetrical and unsymmetrical types:
- Symmetrical Anhydrides: If both acyl groups are identical, you replace the word "acid" in the name of the parent carboxylic acid with the word "anhydride". For example, the anhydride derived from ethanoic acid is named ethanoic anhydride.
- Unsymmetrical (Mixed) Anhydrides: If the two acyl groups are different, the names of both parent carboxylic acids are listed alphabetically, followed by the word "anhydride". For example, the anhydride formed from ethanoic acid and propanoic acid is called ethanoic propanoic anhydride.
3. What are some common methods for preparing acid anhydrides in a laboratory?
There are several key methods for the synthesis of acid anhydrides:
- Dehydration of Carboxylic Acids: This involves heating two molecules of a carboxylic acid, typically in the presence of a strong dehydrating agent like phosphorus pentoxide (P₂O₅), to eliminate a molecule of water and form a symmetrical anhydride.
- Reaction of an Acyl Chloride with a Carboxylate Salt: An acyl chloride is reacted with the sodium salt of a carboxylic acid. This method is particularly useful for preparing unsymmetrical (mixed) anhydrides as it provides more control over the final product.
4. What are the main physical properties of a typical acid anhydride like ethanoic anhydride?
Using ethanoic anhydride as a typical example, its main physical properties are:
- Appearance: It is a colourless liquid with a sharp, pungent odour resembling vinegar (ethanoic acid).
- Boiling Point: It has a relatively high boiling point (140°C) due to its polarity and size, but it is lower than a comparable carboxylic acid because it cannot form hydrogen bonds with itself.
- Solubility: It does not dissolve in water but instead reacts with it (hydrolysis) to form two molecules of ethanoic acid.
5. What are some important industrial applications of acid anhydrides?
Acid anhydrides are crucial reagents in the chemical industry for various applications. Some of the most important examples include:
- Pharmaceuticals: Acetic anhydride is a key raw material in the large-scale synthesis of painkillers like aspirin (acetylsalicylic acid) and paracetamol.
- Polymers and Plastics: They are used to manufacture polymers like cellulose acetate, a material used for synthetic fibres and plastics.
- Acylating Agents: They are widely used to introduce an acyl group into other molecules, a fundamental step in producing dyes, perfumes, and other speciality chemicals.
6. Is carbon dioxide (CO₂) considered an acid anhydride?
Yes, carbon dioxide (CO₂) is a classic example of an inorganic acid anhydride. An acid anhydride is a non-metal oxide that reacts with water to form an acid. When carbon dioxide dissolves in water (H₂O), it reacts to form carbonic acid (H₂CO₃). This same principle applies to other non-metal oxides like SO₃, which is the anhydride of sulfuric acid.
7. Why are acid anhydrides generally preferred over acyl chlorides for acylation reactions in a laboratory setting?
Acid anhydrides are often chosen over the more reactive acyl chlorides for acylation reactions due to several practical advantages:
- Controlled Reactivity: Acid anhydrides are less reactive than acyl chlorides. This makes the reaction rate more manageable and helps to prevent unwanted side reactions.
- Safer Byproducts: The reaction with an acid anhydride produces a carboxylic acid as a byproduct, which is significantly less corrosive and easier to handle than the toxic hydrogen chloride (HCl) gas produced from an acyl chloride reaction.
- Easier Handling: Acid anhydrides are generally less volatile and less sensitive to atmospheric moisture than acyl chlorides, making them safer to store and handle.
8. How does the structure of an acid anhydride relate to its reactivity with nucleophiles like water?
The high reactivity of an acid anhydride is a direct result of its electronic structure. The central oxygen atom is bonded to two powerful electron-withdrawing carbonyl groups (-C=O). This pulls electron density away from the carbonyl carbons, making them highly electron-deficient (electrophilic). This makes them an attractive target for attack by nucleophiles (electron-rich species) like water or alcohols. When the nucleophile attacks, the bond to the central oxygen breaks, and the resulting carboxylate ion is a very stable leaving group, driving the reaction forward.
9. What is the key difference between a symmetrical and an unsymmetrical acid anhydride?
The primary difference between these two types of anhydrides lies in the acyl groups attached to the central oxygen atom.
- A symmetrical acid anhydride contains two identical acyl groups. It is typically formed from two molecules of the same carboxylic acid. A prime example is acetic anhydride, (CH₃CO)₂O.
- An unsymmetrical (or mixed) acid anhydride contains two different acyl groups. It is conceptually formed from two different carboxylic acids. An example is acetic propanoic anhydride.
10. How does the reaction of inorganic acid anhydrides with water explain environmental issues like acid rain?
The same chemical principle governing lab anhydrides explains acid rain. Pollutants like sulfur trioxide (SO₃) and nitrogen oxides, released from burning fossil fuels, are inorganic acid anhydrides. In the atmosphere, these non-metal oxides react with water (H₂O) droplets in clouds. For example, SO₃ reacts with H₂O to form highly corrosive sulfuric acid (H₂SO₄). This acid then falls to the earth as acid rain, causing significant damage to forests, lakes, and buildings.
