Acid anhydride: Preparation from Carboxylic Acids

An acid anhydride is the derivative of carboxylic acid, containing two acyl groups attached to the same oxygen atom. It is obtained by the elimination of water molecules from the two molecules of carboxylic acid. It can also be obtained by the reaction of a carboxylic acid with acid halide in the presence of a base. 

Chemistry of acid anhydride

An acid anhydride is a chemical compound that is formed by the elimination of a water molecule from two carboxylic acids. 

Acid anhydride contains the functional group given below:

Fig 10.10 

Nomenclature 

The names of the acid anhydride are derived from the parent carboxylic acids by replacing the word ‘acid’ with the word ‘anhydride’. 

Fig 10.11 

Physical properties of acid anhydride 

  • Lower anhydrides are liquids at room temperature whereas higher anhydries are solids. 
  • Their boling points are close to the aldehyde or ketones but are higher than their parent carboxylic acid. 
  • They are soluble in organic solvents. 

Preparation of acid anhydride 

By the reaction of acid halide with a carboxylic acid 

Carboxylic acid is react with an acid chloride in the presence of pyridine to produce acid anhydride. This method is used for the preparation of symmetrical as well as unsymmetrical anhydride. 

Fig 10.1 

Mechanism 

Step 1: In the first step pyridine abstract a proton from the carboxylic acid to form a carboxylate anion. 

Step 2: The negatively charged carboxylate anion attacks the electrophilic carbon of acid halide to form a tetrahedral intermediate. 

Step 3: In the last step, acid anhydride is formed by the elimination of chloride. 

Fig 10.2 

 

Sodium salts of carboxylic acid can also be used for the preparation of anhydride. 

Fig 10.3

By dehydration of acid 

Anhydride can be prepared by dehydration of two molecules of carboxylic acid. This reaction takes place at a lower temperature in the presence of dehydrating agents such as p2o5 and concentrated  H2SO4. 

Fig 10.4 

 

Mechanism 

Step 1: the oxygen atom of the carbonyl group is protonated to make carbonyl carbon a good electrophile.

Step 2: In the second step, nucleophile adds to the carbonyl carbon. It breaks the pi bond between oxygen and carbon.  

 

Step 3: The proton is transferred from one atom to another atom to make a good leaving group. 

Step 4: In this step, a water molecule leaves. 

Step 5: in the last step, deprotonation occurs and anhydride is formed. 

Fig 10.5 

by the addition of carboxylic acid to ketene 

An anhydride can also be prepared by the addition of carboxylic acid to ketene. 

Fig 10.6 

Mechanism 

Step 1: In the first step, the oxygen atom of ketene is protonated. It makes a good electrophile. 

Step 2: The acetate ion acts as a nucleophile and attacks the electrophilic carbon. 

Step 3: The proton is transferred from one atom to another atom leads to the formation of anhydride.  

Fig 10.7

preparation of cyclic anhydride 

Some dicarboxylic acids produce cyclic anhydride on heating. This method is useful only when the anhydride formation leads to a five or six-membered ring. 

formation of succinic anhydride 

Succinic anhydride can be prepared by the dehydration of succinic acid. This dehydration occurs by heating succinic acid. 

Fig 10.8

formation of phthalic anhydride 

Phthalic anhydride can be prepared by the dehydration of phthalic acid. It is an important industrial chemical. It is commonly used for the production of certain dyes. 

Fig 10.9 

 

Adipic acid does not react by this method to form anhydride because they form seven membered ring. It is converted into more stable five membered ring(cyclopentanone). 

Reactivity of acid anhydride 

An acid anhydride is the second most reactive derivative of carboxylic acid. They are reactive towards nucleophiles. They react more slowly as compared to acid chloride because of the presence of more electronegative atoms such as chlorine. It increases the electrophilicity of carbonyl carbon more strongly. On an industrial scale, acid anhydride is more used than acetyl chloride because of the following reasons:

  • It is cheaper than acetyl chloride 
  • It does not produce corrosive hydrogen chloride. 

Application of acid anhydride 

Acid anhydride has a wide variety of applications. For example:

  • They are used in the manufacturing of medicine.

For example, acetic anhydride is used in the manufacturing of cellulose acetate which on heating with salicylic acid produces acetylsalicylic acid (aspirin)

  • They produce carboxylic acid by hydrolysis. 
  • They are used in the synthesis of carboxylic acid derivatives. 

For example, they synthesize ester and amide by the reaction with alcohol and amines respectively. 

  • Maleic anhydride is used as the precursor to various resins.
  • They are used to form aromatic ketones by the acylation of aromatic compounds. 

Concepts Berg

How is anhydride formed from acetic acid?

Anhydride can be prepared from two molecules of acetic acid followed by dehydration. This dehydration takes place in the presence of dehydrating agents such as p2o5 and h2so4

How acid anhydride is produced?

An acid anhydride is produced by the reaction of the acid chloride with a carboxylic acid. This reaction occurs in the presence of pyridine. 

Why is CrO3 called an acidic anhydride?

CrO3 is an anhydride of h2cro4. Anhydride which can form acid in the solution is known as the acidic anhydride. Cro3 is called an acidic anhydride because it is capable of forming chromic acid in the solution. 

What happens when carboxylic acid is heated?

When two molecules of carboxylic acid are heated a molecule of water is eliminated. It results in the formation of anhydride. 

How is butanoic acid converted to an anhydride?

When two molecules of butanoic acid are heated in the presence of dehydrating agents such as p2o5 dehydration takes place. In this way, an anhydride is formed. 

What is the anhydride form of HCl?

Anhydride is a compound that can form by the elimination of a water molecule from another compound. HCl cannot form anhydride because it doesn’t contain any oxygen molecule. 

References 

https://www.khanacademy.org/test-prep/mcat/chemical-processes/carboxylic-acids/a/carboxylic-acid-reactions-overview

https://www.quora.com/What-is-the-anhydride-form-of-HCl

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