Carboxylic Acid

An organic compound with a carboxyl functional group is known as a carboxylic acid. They can be found in abundance in nature and are also synthesized by humans. Carboxylic acids deprotonate to produce a carboxylate anion with the general formula R-COO-, which can be used to make a number of useful salts including soaps. 

This article will study Synthesis of Carboxylic Acids, Oxidation of Carboxylic Acid, Hydrolysis of Carboxylic Acid  and How are Carboxylic Acids Formed in detail.

Structure of Carboxylic Acid

A carboxylic acid's general formula is R-COOH, where COOH denotes the carboxyl group and R denotes the rest of the molecule to which this group is attached. There is carbon in this carboxyl group that has a double bond with an oxygen atom and a single bond with a hydroxyl group.

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The primary method of preparation involves the oxidation of various forms of functional groups. Let's take a look at one of the most significant planning techniques.

Method of Preparation of Carboxylic Acid

Below are different methods given for Preparation of Carboxylic Acid

1. Using Primary Alcohols for Preparation of Carboxylic Acid

With the aid of oxidizing agents such as potassium permanganate (KMnO₄ for neutral, acidic, or alkaline media), chromium trioxide (CrO₃,  H₂SO₄ Jones reagent), and potassium dichromate (K₂Cr₂O₇^– acidic media), primary alcohols and aldehydes can undergo oxidation reactions to form corresponding carboxylic acids.

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2. Preparation from Aldehydes

The normal strong oxidizing agents can be used to prepare carboxylic acid. Mild oxidizing agents like Tollens reagents [Ag(NH₃)₂OH] and manganese dioxide can produce carboxylic acids (MnO₂).

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3. Preparation from Alkylbenzenes

The oxidation of alkylbenzenes can be used to make aromatic carboxylic acids. Aromatic carboxylic acid compounds can be formed by vigorous oxidation of alkylbenzene compounds with acidic or alkaline potassium permanganate or chromic acid. Regardless of the length of the side chain, the carboxyl group's full side chain is oxidized.

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The reaction's side products differ depending on whether the primary or secondary alkyl groups are present. The tertiary alkyl group, on the other hand, is unaffected. Furthermore, with the aid of these oxidizing agents, properly substituted alkenes can be oxidized to form carboxylic acids. The reactions under this preparation technique are shown in the illustration below.

4. Preparation from Nitriles

Amides are formed when nitriles are hydrolyzed. The amides are then subjected to a reaction in the presence of a catalyst, resulting in carboxylic acids. H⁺ or OH^- acts as a catalyst in this reaction. Furthermore, the use of mild reaction conditions aids in the termination of the amide stage reaction.

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5. Preparation of Amides

Amides are hydrolyzed in the presence of a catalyst, such as H⁺ or OH^–, to produce carboxylic acids.

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6. Preparation of Grignard Reagents

Grignard reagents reaction allows for carboxylic acid formation. The formation of carboxylic acid salts occurs when Grignard reagents react with crushed dry ice or solid carbon dioxide. Furthermore, the formation of corresponding carboxylic acids occurs when the salts of carboxylic acid are acidified with mineral acids.

As a result, alkyl halides can be used to make Grignard reagents and nitriles. The techniques aid in the conversion of alkyl halides into their carboxylic acids counterparts. The carboxylic acid that results often has one more carbon atom than the corresponding alkyl halides.

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7. From Acid Chlorides

Carboxylic acids are formed when acid chlorides are hydrolyzed with water. Furthermore, acid chlorides can be quickly hydrolyzed with an aqueous base to create carboxylate ions, which can then be acidified to produce the desired carboxylic acids. Anhydrides, on the other hand, is hydrolyzed with water to create the appropriate acid. 

• Acid chlorides are hydrolyzed with water to create carboxylic acids.

• As acid chlorides react with a base, they form carboxylic acid, which can be further acidified.

• Carboxylic acids are formed when acid anhydrides are hydrolyzed.

Physical Properties of Carboxylic Acid

• Owing to the presence of two electronegative oxygen atoms, carboxylic acid molecules are polar.

• Due to the interaction of the carbonyl group (C=O) and the hydroxyl group, they also participate in hydrogen bonding.

• These compounds form dimers when put in nonpolar solvents due to hydrogen bonding between the hydroxyl group of one carboxylic acid and the carbonyl group of the other.

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• The solubility of carboxyl functional group-containing compounds in water is proportional to their size. The higher the solubility, the smaller the compound (the shorter the R group).

• A carboxylic acid's boiling point is usually higher than that of water.

• These compounds are Bronsted-Lowry acids because they have the capacity to donate protons.

• They have a heavy sour odor in general. Their esters, on the other hand, have a sweet odor and are used in perfumes.

Carboxylic Acid Reactions

Given below is Chemical Reaction of Carboxylic Acid

1. Reaction with Metals

The reaction of carboxylic acids with metals like K, Na, Mg, and Ca results in the formation of salts. A proton will be released from the carboxyl group of the carboxylic acid where the metal substation will occur during the reaction phase. H₂  gas is produced as a result of the reaction.

2CH₃COOH+ 2Na → 2CH₃COONa+ H₂

2. Reaction with Alkalis

The reaction of carboxylic acids with alkalis results in the formation of salts and water.

CH₃COOH+NaOH → CH₃COONa+H₂O

3. Reaction with Carbonates and Bicarbonates

Carboxylic acids decompose with carbonates and bicarbonates to produce salts, vapor, and carbon dioxide gas.

This reaction can also be used to check for carboxyl groups. Carboxylic acids react with a saturated sodium bicarbonate solution to create effervescence, which is caused by the release of CO₂. With an aqueous NaHCO₃ solution, however, most phenols do not release effervescence. As a result, the reaction of bicarbonate with carboxylic acids aids in the identification of phenols and carboxylic acids.

2CH₃COOH+Na₂CO₃ → 2CH₃COONa+ H₂O+ CO₂ 

2CH₃COOH+NaHCO₃ → CH₃COONa +H₂O + CO₂

4. Formation of Acyl Chlorides

Acid chlorides are formed when carboxylic acids react with thionyl chloride (SOCl₂), phosphorus pentachloride (PCl₅), or phosphorus pentachloride (PCl₅).

RCOOH+SOCl₂ → RCOCl+ SO₂ + HCl

RCOOH + PCl₅ → RCOCl+SO₂ + HCl

3RCOOH+PCl₅ → 3RCOCl+H₃PO₃

5. Formation of Esters (Esterification)

When carboxylic acids and alcohols are heated in the presence of concentrated sulphuric acid or dry hydrochloric acid, fruity esters are formed.

The dehydrating agent in this reaction is concentrated sulphuric acid. An equilibrium reaction is exemplified by this reaction.

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6. Formation of Amide

Ammonium salts are formed by treating carboxylic acids with ammonia. Ammonium salts lose a water molecule when heated, resulting in the formation of amides.

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7. Decarboxylation 

Decarboxylation reaction occurs when soda lime (NaOH + CaO) is distilled with sodium salts of carboxylic acids, resulting in alkanes.

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8. Formation of Anhydrides

Acid anhydrides are formed when two molecules of carboxylic acid are heated with a dehydrating agent such as phosphorus pentoxide.

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9. Hell-Volhard Zelinsky Reaction

Carboxylic acids shape -substituted carboxylic acids when they react with a chlorine (Cl₂) or bromine (Br₂) molecule. In the presence of red phosphorus, the reaction takes place. The H.V.Z. reaction, or Hell-Volhard Zelinsky reaction, is the name given to this reaction.

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10. Electrophilic substitution reactions such as nitration, sulphonation, and halogenation occur in aromatic carboxylic acids. Carboxyl (-COOH) is an electron removing group. As a result, the reaction will take place in the meta-position. The carboxyl group becomes inactive. As a result, the reaction can only take place in extreme circumstances.

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11. Friedel Crafts Reactions

Since the carboxyl group is a heavy electron attracting group, Friedel-Crafts reactions will not occur. The benzene ring would be deactivated as a result. As a result, it will not be alkylated or acylated.

Did You Know?

1. Carboxylic acids are the building blocks of fatty acids, which are important for human health. Omega-6 and omega-3 fatty acids are two examples.

2. Soaps made with higher fatty acids are also common.

3. Many carboxylic acids are used in the manufacture of soft drinks and other food products.

4. Acetic acid is used as a coagulant in the production of rubber.

5. The chemical hexanedioic acid is used to make nylon-6,6.

6. Carboxylic acids are used in a variety of industries, including rubber, textiles, and leather.

7. The chelating agent ethylenediaminetetraacetic acid is commonly used.

8. These compounds are used in the production of many medicines. As a result, carboxylic acids play a crucial role in pharmaceuticals.

9. Compounds with the carboxyl functional group are used in the manufacture of many polymers.