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Acid Anhydrides – Definition, Structure, Properties & Uses

Sep 2, 2022

An anhydride is one chemical product formed by removing a water molecule from an acid. Now, what is Anhydride? An oxygen atom connects two acyl groups, which define a functional group. Anhydride means “without water.” You may describe it as the chemical created when water is removed from another chemical. Anhydride and water can combine to form a base or an acid. A mixed anhydride is generated from two different carboxylic acids and is defined as having different R groups in the generic structure depicted below. Let us now learn about Acid Anhydrides.

Definition and Meaning of Acid Anhydride

Non-metal oxides that react with water to form acidic solutions are called acid anhydrides. This is a molecule that can form acidic solutions in water. It is a functional group in organic chemistry consisting of two acyl groups joined by an oxygen atom. Non-metals that can react with water are strictly called anhydrides, while non-metals that cannot react with water are not considered anhydrides.


 According to organic chemistry, organic acid anhydrides have the functional group R(CO)O(CO)R’. An organic acid anhydride is often formed when one equivalent of water is removed from two equivalents of an organic acid in a dehydration reaction.

Structure of a General Acid Anhydride

Acid Anhydrides


   Here R and R‘ are any alkane groups.

Acetic Anhydride Formula

Acid Anhydrides formula


This is Acetic anhydride molecule. The R and R’ from above when replaced by methane (CH3) groups gives us the acetic anhydride formula. This molecule is one of the basic examples of acid anhydrides.

Synthesis of Acid Anhydride

Acid anhydrides are substances with two acyl groups (R-C=O) attached to the same oxygen atom. When a carboxylic acid combines with an acid chloride, this reaction often results in the anhydride in the presence of a base. Two carboxylic acids are heated at a high temperature of 800°C or higher to generate an organic acid anhydride. In this step, water molecules are removed from the reaction. Carboxylic acids can be used to react with and produce P2O5.


 The dehydration method shows the chemical reaction of acid anhydride synthesis.

Synthesis of acid anhydrides


Here R is any alkane group.

Physical properties of acid anhydrides

Let us consider acetic anhydride as a typical example of its physical properties.


Appearance – Acetic anhydride, also known as acetic anhydride, is a colourless liquid with the characteristic odour of vinegar (acetic acid). The odour is produced when acetic anhydride combines with water vapour in the air and moisture in the nose to reform acetic acid. This reaction with water is described in detail on another page. 

 Water solubility – Acetic anhydride combines with water to form acetic acid, so it cannot be said to be soluble in water. Aqueous solutions of acetic anhydride do not exist.


Boiling point – absolute ethanol boils at 140 °C. They are rather large polar molecules with inter-dipole and van der Waals dispersion forces. However, it does not form hydrogen bonds. This indicates a lower boiling point than a comparably sized carboxylic acid. For example, pentanoic acid is the closest acid in size and boils at 186 °C.

Reactivity of acid anhydrides

  •  Comparison of acid anhydrides and acid chlorides (acid chlorides). Perhaps you recently completed research on acyl chlorides and are working simultaneously on acid anhydrides as acyl chlorides. 
  • Rather than trying to learn everything about acid anhydrides from scratch, it’s much easier to think of them as some modified acid chloride. That’s the attitude I want to take throughout this section. 
  • A closer inspection of the colour coding in the figure helps compare the structures of the acid anhydrides and acid chlorides. 

ethanoic anhydride

The bottom red group is never broken in the ethanoic anhydride reaction. Like the chlorine atom in acyl chloride, it looks like a single atom in many ways. Acyl chlorides often react by replacing chlorine with another substance. Using ethanoyl chloride as an example, the first reaction would be: 

ethanoyl chloride

Gaseous hydrogen chloride is produced, which may later react with other components of the combination. An anhydride slows down the reaction, but the main difference is that acetic acid is formed as a by-product instead of hydrogen chloride.  

hydrogen chloride

This, like hydrogen chloride, can continue to interact with other environmental elements. Water, alcohols and phenols, ammonia and amines, and other substances participate in acyl chloride and anhydride reactions. In each of these specific situations, a highly electronegative element with an active lone pair (oxygen or nitrogen) exists.

Chemical properties of acid anhydride 

By changing the -OCOR group, the acid anhydride undergoes a nucleophilic substitution reaction. The higher electronegativity of the Cl atom compared to the -OCOR group makes it less reactive than the acid chloride.

acid chloride

Here R,R|, and R2 are any alkane groups. 

Formation of Carbonic acid

 Carbonic acid is produced when carbon dioxide and water react. The chemical formula of carbonic acid is given below. 

CO2(g) + H2O → H2CO3(aq)

Carbon dioxide can interact with water and release hydrogen to form acidic carbonic acid or H2CO3. Acid rain is the result of this process. Important in altering the pH of streams, rivers and oceans. The above equation shows that increasing the amount of carbon dioxide in the air produces life-threatening carbon dioxide. The main problem we are currently working on is this.

Formation of sulfuric acid: 

 Sulfur trioxide and water combine to form sulfuric acid. It will explain chemically.  Sulfuric acid is then formed when sulfur trioxide gas and water interact.

SO3(g) + H2O → H2SO4(aq)

 Sulfur trioxide is formed when sulfur dioxide combines with oxygen in the air. Acid rain is formed when it reacts with water when it rains. These have a very negative impact on the ecosystem.

Learn More About Acetic Anhydride

  • Acetic anhydride is one of the simplest examples of acid anhydrides. Chemically, the formula for acetic anhydride, also called acetic anhydride, is (CH3CO)2O. Often referred to as Ac2O, it is the most easily separated anhydride of carboxylic acids. It is a common reagent in the synthesis of organic compounds. It is a white liquid that reacts with moisture in the air to produce acetic acid and has a strong acetic acid odour.
  • Acetic acid can be dehydrated at 800 °C to produce acetic anhydride. Alternately, acetic anhydride and salt are produced when the acid chloride reacts with an acetic acid salt such as sodium acetate. Acetic anhydride formula can also be illustrated as:

acetic anhydride

Properties of Acetic anhydride

  • IUPAC name: Acetic anhydride 
  • Classification: Organic acid anhydride
  • Soluble in: Water, Benzene
  • Formula: C4H6O3
  • Density of acetic anhydride: 1.08 g/cm³
  • Acetic anhydride Molar mass: 102.09 g/mol
  • Boiling point: 139.5 °C

The acetic anhydride density is 1.072 g/ml. Water cannot absorb acetic anhydride. The acetic anhydride density is slightly higher than water’s, with a density of 0.999 g/mL.  The pKa value of acetic anhydride, which is less than zero and has reasonably high acidity, is -6.9. Lower than zero pKa values are found in strong acids. The acid is more potent and the lower the pKa value.Acetic anhydride molar mass is found to be 102.09 g/mol. The molar mass of water is approximately 18.015 g/mol. Therefore, the acetic anhydride molar mass is almost five times that of water. 

Structure and Properties of Acetic Anhydride

Like most acid anhydrides, acetic anhydride is a malleable molecule with a nonplanar shape. Compared to the dipole-dipole repulsion between the two carbonyl oxygens, coupling the pi system via the central oxygen provides relatively modest resonance stability. There are relatively few energy barriers to coupling rotation between ideal planar-free conformations.

Uses of Acetic anhydride

  • Acetic anhydride, as suggested by organic chemistry, is primarily used for acetylation leading to commercially important products. 
  •  Its main use is to convert cellulose to cellulose acetate. Cellulose acetate is used to manufacture cigarette filters and is a building block of photographic film and other coated materials. 
  •  It is used in this process, similar to how salicylic acid is converted to aspirin, also called acetylsalicylic acid. 
  •  It is also used as an activity modifier by autoclave impregnation and subsequent acetylation to produce strong and durable wood.
  • Acetic anhydride is the primary chemical used as an acetylation agent in the manufacture of illicit narcotics heroin and methaqualone.

Organic Acid Anhydrides

  • An acid anhydride that is an organic chemical is known as an organic acid anhydride. Substances with two acyl groups attached to the same oxygen atom are called acid anhydrides. Carboxylic anhydrides of the formula (RC(O)) 2 O are a typical type of organic acid anhydride in which the parent acid is a carboxylic acid. This form of symmetrical anhydride is denoted by replacing the word acid in the name of the parent carboxylic acid with the word anhydrous. Therefore, (CH3CO)2O is also called acetic anhydride. 
  • Mixed (or unsymmetrical) acid anhydrides are known in which there is a reaction between two different carboxylic acids, such as formic anhydride acetic acid. Other forms of organic acids, such as sulfonic or phosphonic acids, can also serve as the source of one or both acyl groups in the anhydride. Inorganic acids such as phosphoric acid serve as one source of acyl groups in anhydrides.

Key Points To Remember 

  • Acid anhydrides are widely used in organic chemistry. 
  • Many products are made with their help: pharmaceuticals, industrial chemicals, explosives, perfumes. 
  • Esters are made by acetylating alcohols.  
  • Synthesis of aspirin (acetylsalicylic acid). 
  •  Heroin is made by deacetylating morphine.


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