Methods of Preparations of Carboxylic Acids: Have you heard about acetic acid and vinegar? If yes, then you must be well aware that they are carboxylic acids. Carboxylic acid is a component of amino acids, which are the building blocks of proteins, and is produced during metabolism. Coconut oil, butter, peanut oil, and citrus fruits are a few examples of food that contains carboxylic acid. In this article, we will study different methods of preparation of carboxylic acid and some of their uses.

What is Carboxylic Acid?

A carboxylic acid is an organic compound with a carboxyl functional group. They are abundant in nature and are also synthesised by humans.

The general formula for carboxylic acid is \({\rm{R}} – {\rm{COOH}}\), where \({\rm{COOH}}\) refers to the carboxyl group, and \({\rm{R}}\) refers to the rest of the molecule to which this group is attached. In this carboxyl group, the carbon atom has a double bond with an oxygen atom and a single bond with a hydroxyl group.

Some examples of the carboxylic acid are given below-

Formula	Trivial Name	IUPAC Name
\({\rm{H}} – {\rm{COOH}}\)	Formic acid	Methanoic acid
\({\rm{C}}{{\rm{H}}_3}{\rm{CH}} = {\rm{CH}} – {\rm{COOH}}\)	Crotonic acid	But-\(2\)-enoic acid
\({\rm{OH}} – {\rm{COOH}}\)	Carbonic acid	Carbonic acid
\({\rm{C}}{{\rm{H}}_3}{\left( {{\rm{C}}{{\rm{H}}_2}} \right)_2}{\rm{COOH}}\)	Butyric acid	Butanoic acid

Preparation Methods of Carboxylic Acids

The most common direct method for preparing carboxylic acids is oxidation. Carboxylic acids are formed by the direct oxidation of alcohols, ketones, aldehydes, alkenes, and alkylbenzene.

Oxidation of Primary Alcohols and Aldehydes

When primary alcohols are oxidised, they produce aldehydes. However, the reaction does not stop there, and the oxidation process continues to produce carboxylic acids as a final product.

For example:

\(\mathop {{\rm{C}}{{\rm{H}}_3}{\rm{C}}{{\rm{H}}_2}{\rm{CH}}\left( {{\rm{C}}{{\rm{H}}_3}} \right){\rm{C}}{{\rm{H}}_2}{\rm{OH}} + {\rm{KMn}}{{\rm{O}}_4}}\limits_{{\rm{2 – Methyl – 1 – butanol}}}  \to \mathop {{\rm{C}}{{\rm{H}}_3}{\rm{C}}{{\rm{H}}_2}{\rm{CH}}\left( {{\rm{C}}{{\rm{H}}_3}} \right){\rm{COOH}}}\limits_{{\rm{2 – Methyl – 1 – butanoic}}\,\,{\rm{acid}}} \)

Oxidation of Alkylbenzenes

The oxidation of alkylbenzenes results in the formation of aromatic carboxylic acids. Aromatic carboxylic acid compounds can be formed by oxidising alkylbenzene compounds vigorously with acidic or alkaline potassium permanganate or chromic acid. Regardless of side-chain length, the carboxyl group’s complete side chain is oxidised.

The reaction’s side products vary depending on whether the alkyl groups are primary or secondary. The tertiary alkyl group, on the other hand, is unlikely to be affected. Furthermore, with the help of these oxidising agents, properly substituted alkenes can undergo oxidation to form carboxylic acids.

In the previous example, \({\rm{t}}\)-butylbenzene lacks benzylic hydrogen and therefore does not undergo oxidation.

Oxidation of Alkenes

Carboxylic acids are formed as a result of the strong oxidation of alkenes with hot alkaline potassium permanganate or potassium dichromate.

For example-

Hydrolysis of Nitriles

Nitriles are hydrolyzed to form amides. The amides are then subjected to further reaction in the presence of a catalyst, resulting in the formation of carboxylic acids. This reaction is catalysed by \({{\rm{H}}^ + }\) or \({\rm{O}}{{\rm{H}}^ – }\). Furthermore, using mild reaction conditions helps to terminate the reaction in the amide stage.

The mechanism follows these steps, which are as follows:
i. The nitrile group’s nitrogen atom is protonated.

ii. A water molecule is drawn to the carbocation formed in the preceding step.

iii. The oxonium ion loses a proton to the nitrogen atom, resulting in the formation of an enol.

iv. The enol tautomerization results in the more stable keto form.

v. The acid protonates the oxygen atom of amide, resulting in the formation of a carbocation.

vi. A water molecule is attracted to the carbocation.

vii. The oxonium loses a proton.

viii. The amine group is protonated.

ix. An electron pair of one of the oxygen displaces the ammonium group from the molecule.

Carboxylic Acid Preparation from Grignard Reagents

Grignard synthesis has the advantage of increasing the length of the carbon chain, which is useful for broadening the range of materials available.

Carbon dioxide gas is bubbled into an ether solution of Grignard reagent, resulting in the addition of Grignard reagent to the \({\rm{C = O}}\) bond, resulting in the formation of magnesium salts of carboxylic acids, from which carboxylic acids are produced by treatment with mineral acids.

This method is useful for converting alkyl halides into carboxylic acids with one more carbon atom than the alkyl halides (ascending the series).

For example:

Hydrolysis of Acyl halides and Anhydrides

When acid chlorides are hydrolyzed with water, carboxylic acids are formed. Furthermore, acid chlorides can be easily hydrolyzed with an aqueous base to yield carboxylate ions, which can then be acidified to yield the corresponding carboxylic acids. In contrast, anhydrides are hydrolyzed with water to produce the corresponding acid.

\(\mathop {{\rm{R}} – {\rm{COCl}} + {{\rm{H}}2}{\rm{O}}}\limits{{\rm{Acid Chloride}}} \to \mathop {{\rm{R}} – {\rm{COOH}} + {\rm{C}}{{\rm{l}}^ – }}\limits_{{\rm{Carboxylic acid}}} \)

\(\mathop {{{\left( {{{\rm{C}}6}{{\rm{H}}_5}{\rm{COO}}} \right)}_2}{\rm{O}} + {{\rm{H}}_2}{\rm{O}}}\limits{{\rm{Benzoic}}\,{\rm{anhydride}}} \to \mathop {{{\rm{C}}6}{{\rm{H}}_5}{\rm{COOH}}}\limits{{\rm{ Benzoic acid }}} \)

Hydrolysis of Esters

Carboxylic acids are formed as a result of the acidic hydrolysis of esters. However, the hydrolysis by base produces carboxylate ions, which are then acidified to form the corresponding carboxylic acids. Furthermore, esters are hydrolyzed with mineral acids or alkalis to produce carboxylic acid.

Uses of Carboxylic Acids

Carboxylic acids are extremely useful in our daily lives. They play an important role in the production of many everyday items. The following are some of the most common applications:

• i. Soap production
• ii. It is extremely useful in the food industry. It’s also used in the production of soft drinks.
• iii. They are used in the production of vinegar.
• iv. It is also very useful in the pharmaceutical industry. It is a key ingredient in the manufacture of pharmaceuticals such as aspirin.
• v. It is also utilised in the rubber industry.
• vi. These acids are used to make dyes, perfumes, and other products.

Summary

This article gives information that carboxylic acid has the general formula \({\rm{R}} – {\rm{COOH}}\), where \({\rm{COOH}}\) denotes the carboxyl group and \({\rm{R}}\) denotes the remainder of the molecule to which this group is attached. The most common direct method for preparing carboxylic acids is oxidation. Carboxylic acids are formed by the direct oxidation of alcohols, ketones, aldehydes, alkenes, and alkylbenzene. We also studied some of the uses of carboxylic acids.

FAQs on Methods of Preparations of Carboxylic Acids

Q.1. What are carboxylic acids? Give any four methods of preparation?
Ans: An organic compound with a carboxyl functional group is known as a carboxylic acid. They are abundant in nature and are also synthesised by humans.

The general formula for carboxylic acid is \({\rm{R}} – {\rm{COOH}}\), where \({\rm{COOH}}\) refers to the carboxyl group, and \({\rm{R}}\) refers to the rest of the molecule to which this group is attached. This carboxyl group contains carbon that has a double bond with an oxygen atom and a single bond with a hydroxyl group.

Carboxylic acids are formed by the direct oxidation of-

i. Alcohols
ii. Aldehydes
iii. Alkenes
iv. alkylbenzene.

Q.2. What is the method of preparing carboxylic acids from alcohol or aldehydes?
Ans: When primary alcohols are oxidised, they produce aldehydes. However, the reaction does not stop there, and the oxidation process continues to produce carboxylic acids as a final product.

Q.3. Which of the following methods does give a carboxylic acid with homologation?
Ans: Using a homologation procedure of carboxylic acid, also known as the Arndt-Eistert reaction, an existing carboxylic acid can be elongated by one methylene group.

Q.4. How do you prepare carboxylic acids from nitriles?
Ans: Nitriles are hydrolyzed to form amides. The amides are then subjected to further reaction in the presence of a catalyst, resulting in the formation of carboxylic acids. This reaction is catalysed by \({{\rm{H}}^ + }\) or \({\rm{O}}{{\rm{H}}^ – }\). Furthermore, the use of mild reaction conditions aids in the termination of the reaction in the amide stage.

Q.5. Which is more acidic between carboxylic acid or phenol and Why?
Ans: Carboxylic acids dissociate in water to form the carboxylate ion and the hydronium ion. However, in the case of phenols, negative charge is less effectively delocalized over one oxygen atom and less electronegative carbon atoms in the phenoxide ion. As a result, the carboxylate ion has greater stability than the phenoxide ion. As a result, carboxylic acids are more acidic than phenols.

Q.6. What are the physical properties of carboxylic acids?
Ans: Some of the physical properties of carboxylic acid are as follows-
i. Because of the presence of two electronegative oxygen atoms, carboxylic acid molecules are polar.
ii. Because of the presence of the carbonyl group \(\left( {{\rm{ C = O}}} \right)\) and the hydroxyl group, they also participate in hydrogen bonding.
iii. In nonpolar solvents, hydrogen bonding between the hydroxyl groups of one carboxylic acid and the carbonyl group of the other forms dimers.
iv. The solubility of carboxyl functional group-containing compounds in water is proportional to their size. The smaller the compound, the greater its solubility (the shorter the \({\rm{R}}\) group).
v. The boiling point of a carboxylic acid is generally higher than that of water. Because these compounds have the ability to donate protons, they are classified as Bronsted-Lowry acids.

We hope this detailed article on the methods of preparation of carboxylic acids will be helpful in your preparation. If you have any doubts related to the article, or in general, about the Carboxylic acid, please reach out to us through the comments section, and we will get back to you as soon as possible.