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Acid Anhydride: Introduction, Preparation, Classification, Nomenclature of Acid Anhydride, Practice Problems & Frequently Asked Questions

Acid Anhydride: Introduction, Preparation, Classification, Nomenclature of Acid Anhydride, Practice Problems & Frequently Asked Questions

A bottle of aspirin is probably one of the first medications you turn to when you have a terrible headache.

Do you know that one of the ingredients used to create aspirin is acetic anhydride?

Like acetic anhydride, many other anhydrides are there and used in organic chemistry. These anhydrides are dehydrates of corresponding carboxylic acid. These are very useful for adding the alkyl carboxylate group to organic molecules. They are more reactive than their corresponding carboxylic acid and have a very unique history and set of characteristics in addition to their many uses.

A form of acid anhydride, acetic anhydride is an organic chemical. Acetic anhydride has a chemical formula C4H6O3. If you hear scientists refer to acetic anhydride as ethanoic anhydride, don't be worried. Acetic anhydride is sometimes known as ethanoic anhydride.

Table of Contents:

  • Introduction of Acid Anhydride
  • Preparation of Acid Anhydride
  • Classification of Acid Anhydride
  • Nomenclature of Acid Anhydride
  • Practice Problems
  • Frequently Asked Questions(FAQs)

Introduction of Acid Anhydride:

Alcohols, phenols, ethers, amines, and halides are prominent families of organic compounds composed of alkyl and/or aryl groups coupled to hydroxyl, alkoxy, amino, and halo substituents, respectively.

Carboxylic acids are chemical compounds containing the functional group -COOH that are employed in a wide range of products, including synthetic hormones and pharmaceuticals. Carboxylic acids have a hydroxyl group (-OH) connected to an acyl group (-C=O). The hydroxyl group is replaced by substituents such as -NH2, -X, -C ≡ N,–OOCR to form functional derivatives.

Several carboxylic acid derivatives are produced by changing the -OH group in the carboxylic acid's structure. One of the derivatives is acid anhydride which is created when a –OOCR group is added in place of the -OH group.

Preparation of Acid Anhydride:

As anhydride literally translates to "without water," Thus, the combination of two molecules of carboxylic acid followed by the removal of one molecule of water is known as an acid anhydride. Anhydrides are even regarded as activated forms of carboxylic acids, but they are less reactive than acid halides. By means of an inductive effect, the anhydride group also removes electron density from the carbonyl carbon, and the carboxylate anion behaves well as a good leaving group.

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  1. Laboratory Preparation:

i) Dehydrating carboxylic acid in the presence of strong dehydrating agents like phosphorous pentoxide.

2CH3COOH + P4O10 → (CH3CO)O + H2P4O11

ii) By heating the acyl chloride with the sodium salt of the acid.

CH3C(O)Cl + HCO2Na → HCO2COCH3 + NaCl

  1. Industrial Method of manufacture

i) Manufacture of acetic anhydride.

Acetic vapours along with pyrolysis catalyst are passed through heated coils where they are decomposed to ketene and water. After separating water and unpyrolysed acetic acid, the ketene is scrubbed with acetic acid. The two combine to form acetic anhydride.

CH3COOH → H2O + CH2CO (ketene)

↓ CH3COOH

(CH3CO)O

ii) Manufacture of Phthalic anhydride

Phthalic anhydride is manufactured by oxidizing naphthalene or o-xylene with molecular oxygen in presence of vanadium pentoxide catalyst.

Classification of Acid Anhydride:

Anhydrides are classified into two categories namely

  1. Symmetrical anhydrides or simple anhydrides
  2. Unsymmetrical anhydrides or mixed anhydrides

Symmetrical anhydrides or simple anhydrides:

A carboxylic acid anhydride with the general structural formula shown below is a symmetrical anhydride or simple anhydride.

R1=R2= hydrogen atoms, aryl groups, alkyl groups

Examples:

Unsymmetrical anhydrides or mixed anhydrides:

The following general structural formula describes a mixed anhydride or unsymmetrical anhydride, which is a carboxylic acid anhydride.

R1≠ R2= hydrogen atoms, aryl groups, and alkyl groups.

Examples:

Cyclic anhydride:

A ring-closing acid anhydride created by removing water from the space between two carboxylic groups in the same molecule.

Example:

Nomenclature of Acid Anhydride:

When two carboxylic acids join and lose water, the functional group is referred to as an acid anhydride (anhydride implies without water). Similar to carboxylic acids, symmetrical acid anhydrides are referred to by the suffix -anhydride rather than the ending -acid. Both the IUPAC and the Common nomenclature hold true in this regard.

For unsymmetrical acid anhydrides should be named using the alkanoic general approach, and the names should then be arranged alphabetically. As a result, identify each component before arranging it alphabetically, then add spaces and the term anhydride at the end.

The naming of symmetrical anhydrides:

Example 1:

The molecule contains an acid anhydride functional group and it comes under symmetrical anhydride. The structure of hydrocarbon is alkane so the primary prefix is -an. The longest hydrocarbon chain contains one carbon. The word root is ‘Meth’.

So the acid anhydride group is the secondary suffix with the suffix of ‘oic anhydride’.

So the compound IUPAC's name is Methanoic anhydride or Formic anhydride.

Example 2:

The molecule contains an acid anhydride functional group and it comes under symmetrical anhydride. The structure of hydrocarbon is alkane so the primary prefix is -an. The longest hydrocarbon chain contains two carbons. The word root is ‘Eth’.

So the acid anhydride group is the secondary suffix with the suffix of ‘oic anhydride’.

So the compound IUPAC's name is Ethanoic anhydride or Acetic anhydride.

Example 3:

The molecule contains an acid anhydride functional group and it comes under symmetrical anhydride. The structure of hydrocarbon is alkane so the primary prefix is -an. The longest hydrocarbon chain contains three carbons. The word root is ‘Prop’.

So the acid anhydride group is the secondary suffix with the suffix of ‘oic anhydride’.

So the compound IUPAC's name is Propanoic anhydride or Propionic anhydride.

The naming of Unsymmetrical anhydrides:

Example 1:

The molecule contains an acid anhydride functional group and it comes under Unsymmetrical anhydride. The structure of hydrocarbon is alkane so the primary prefix is -an. The longest hydrocarbon chain contains three and four carbons. The word root is ‘Prop’ and ‘But’.

So the acid anhydride group is the secondary suffix with the suffix of ‘oic anhydride’.

For unsymmetrical acid anhydrides should be named using the alkanoic general approach, and the names should then be arranged alphabetically. As a result, identify each component before arranging it alphabetically, then add spaces and the term anhydride at the end.

So the compound IUPAC's name is Butanoic Propanoic anhydride.

Example 2:

The molecule contains an acid anhydride functional group and it comes under Unsymmetrical anhydride. The structure of hydrocarbon is alkane so the primary prefix is -an. The longest hydrocarbon chain contains three and two carbons. The word root is ‘Prop’ and ‘Eth’.

So the acid anhydride group is the secondary suffix with the suffix of ‘oic anhydride’.

So the compound IUPAC's name is Ethanoic Propanoic anhydride.

Example 3:

The molecule contains an acid anhydride functional group and it comes under Unsymmetrical anhydride. The structure of hydrocarbon is alkane so the primary prefix is -an. The longest hydrocarbon chain contains two and one carbons. The word root is ‘Eth’ and ‘Meth’.

So the acid anhydride group is the secondary suffix with the suffix of ‘oic anhydride’.

So the compound IUPAC's name is Ethanoic Methanoic anhydride

Naming of cyclic anhydrides:

Dioic anhydrides are the name given to cyclic anhydrides that are produced from dicarboxylic acids.

Example 1:

The molecule contains an acid anhydride functional group and it comes under cyclic anhydride. The structure of hydrocarbon is alkane so the primary prefix is -an. The longest hydrocarbon chain contains five carbons. The word root is ‘Pent’.

So for the cyclic anhydride group the secondary suffix is ‘dioic anhydride’.

So the compound IUPAC's name is Pentanedioic anhydride.

Practice Problems:

Q1. What is the following compound’s IUPAC name?

Solution: The molecule contains an acid anhydride functional group and it comes under cyclic anhydride. The structure of hydrocarbon is alkane so the primary prefix is -an. The longest hydrocarbon chain contains four carbons. The word root is ‘But’.

So for the cyclic anhydride group the secondary suffix is ‘dioic anhydride’.

So the compound IUPAC's name is Butanedioic anhydride.

Q2. What is the following compound’s IUPAC name?

Solution: The molecule contains an acid anhydride functional group and it comes under symmetrical anhydride. The structure of hydrocarbon is alkane so the primary prefix is -an. The longest hydrocarbon chain contains four carbons. The word root is ‘But’.

So the acid anhydride group is the secondary suffix with the suffix of ‘oic anhydride’.

So the compound IUPAC's name is Butanoic anhydride.

Q3. Acetic anhydride is a liquid which is

(A) Colourless
(B) Red Colour
(C) Blue colour
(D) Violet colour

Answer: (A)

Solution: Acetic anhydride, commonly referred to as ethanoic anhydride, is a colourless liquid having a distinctive vinegar odour (acetic acid). When acetic anhydride reacts with airborne water vapour and nasal moisture to reform acetic acid, an odour is created.

Q4. Acid anhydrides are the derivatives of

(A) Carboxylic acids
(B) Alcohols
(C) Ethers
(D) None of the above

Answer: (A)

Solution: A functional derivative of the carboxylic acid is acid anhydride. So acetic acid's functional derivative is acetic anhydride.

Frequently Asked Questions(FAQs):

Q1. Which functional group has more priority Ester or Acid anhydride?
Answer:
One functional group is used as the major functional group and the other functional group is used as a substituent when a molecule has more than one functional group. An acid anhydride is given greater priority than ester when comparing the two.

Q2. Which carboxylic acid derivative is the most reactive?
Answer:
The most reactive carboxylic acid derivatives are acyl chlorides. The carbonyl carbon becomes more electrophilic as a result of the electronegative chlorine atom in the C-Cl bond drawing electrons in that direction. This strengthens a nucleophilic approach. Additionally, the chloride ion (Cl-), a great leaving group, quickens the process. Because of their reactivity, acyl chlorides are an essential synthesis intermediate that may be quickly transformed into esters and amides.

Q3. Why is ethanoic anhydride polar in nature?
Answer:
Because ethanoic anhydride is a large polar molecule, its boiling point is 413 K. It is susceptible to both dipole-dipole force of attraction and van der Waals dispersion force. However, because it lacks hydrogen bonding, its boiling point is lower than that of other carboxylic acids like pentanoic acid

(459 K).

Q4. Explain the applications of acetic anhydride.
Answer:
Acetic anhydride is mostly employed for acetylations that result in products with commercial importance, as suggested by its organic chemistry. Its main usage is the conversion of cellulose to cellulose acetate, which is used to make cigarette filters and is a component of photographic film and other coated materials. Similar to how salicylic acid is converted into aspirin (acetylsalicylic acid), it is used in this process. In order to create strong, long-lasting timber, it is also utilised as an active modifying agent through autoclave impregnation and subsequent acetylation.

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