78

Carboxylic Acid

It is an organic compound that contains a carboxyl functional group. It is widely found in nature and can also be prepared by humans. Its chemical formula is RCOOH. Carboxylic acid when deprotonates produces anion (R-COO-), which is used in the formation of many salts including sops.

Structure of Carboxylic Acid

The general chemical formula is R-COOH wherein R is the part of the molecule to which the carboxyl group is attached. The carbon atom of the carboxyl group is bonded to the oxygen atom by a double bond and bonded to the hydroxyl group by a single bond.

So, it contains a hydroxyl group bonded to a carbonyl carbon. As oxygen is electronegative, the functions group can undergo ionization to release a proton.

Some of the carboxylic acids derived from alkanes are methanoic acid (HCOOH), ethanoic acid (CH₃COOH), propanoic acid (C₂H₅COOH) and butanoic acid (C₃H₇COOH).

Physical properties of carboxylic acids

• Its molecules are polar due to the presence of electronegative oxygen atoms.
• Its molecules can form hydrogen bonds owing to the presence of the carbonyl group (C=O) and the hydroxyl group (OH).
• In non-polar solvents, they form dimers due to the formation of a hydrogen bond between two carboxylic acids.
• Their solubility depends on the size of their molecules, e.g., the shorter the R group, the higher will be the solubility.
• Its boiling point is higher than the boiling point of water.
• They are referred to as Bronsted-Lowry acids owing to their ability to donate protons.
• They have a sour smell. However, their esters smell pleasant so they are used in perfumes.

Chemical properties of carboxylic acids

• The alpha-carbon of carboxylic acid easily undergoes halogenation through the Hell-Volhard-Zelinsky reaction.
• They can be converted into amines through the Schmidt reaction.
• They undergo reduction to form alcohols when they are treated with hydrogen for a hydrogenation reaction.
• They form esters when react with alcohols.

Carboxylic Acid Reactions

1. With Metals:

When react with metals like K, Na, Mg and Ca, they form salts. The carboxyl group releases a proton and the metal substation occurs during the reaction phase and hydrogen gas is released due to this reaction, which is shown below:

2CH₃COOH + 2Na → 2CH₃COONa+ + H₂

2. With Alkalis

They form salts and water when react with alkalies as shown below:

CH₃COOH + NaOH → CH₃COONa + H₂O

3. Reaction with Carbonates and Bicarbonates

When they react with carbonates and bicarbonates, they form salts, vapor and carbon dioxide gas. This reaction can be used to detect the carboxyl groups. For example, carboxylic acids create effervescence when react with a saturated bicarbonate solution. However, when phenols react with aqueous sodium carbonate they don’t produce effervescence. So, this reaction helps identify phenols and carboxylic acids. The reactions take place as follows:

2CH₃COOH + Na₂CO₃ → 2CH₃COONa+ + H₂O + CO₂

2CH₃COOH + NaHCO₃ → CH3COONa + H₂O + CO₂

4. Formation of Acid Chlorides

Carboxylic acids form their respective acid chlorides when they react with thionyl chloride (SOCl2) and phosphorus pentachloride (PCl5).

RCOOH + SOCl₂ → RClOCl + SO₂ + HCl

RCOOH + PCl₅ → RCOCl + SO₂ + HCl

5. Ester Formation

They form esters with a fruity smell when they are heated with alcohols in the presence of concentrated sulphuric acid or dry hydrochloric acid. For example, the formation of Ethyl acetate (acetate ester) takes place as follows:

CH3COOH + C2H5OH →ß CH3COOC2H5 + H2O

6. Amide Formation

They form ammonium salts when they react with ammonia. Further, when ammonium salts are heated they lose water to form amides. The reaction is shown below:

CH₃COOH + NH₃ → CH₃COONH₄ + heat → CH₃CONH₂ + H₂O

7. Hell-Volhard Zelinsky Reaction

In this reaction, halogenation of a carboxylic acid occurs at the alpha carbon. They form alpha-substituted carboxylic acids whey react with chlorine or bromine molecule. This reaction occurs in the presence of red phosphorus as shown below:

CH₃COOH → ClCH₂COOH → Cl₂CHCOOH → Cl₃CCOOH

8. Electrophilic Substitution Reactions

Aromatic carboxylic acids take part in various electrophilic substitution reactions like nitration, sulphonation and halogenation. It occurs at the meta-position as the carboxyl group is an electron-withdrawing group. The reaction is shown below:

C₆H₅COOH + HNO₃ → C₆H₄ (COOH) NO₂ + H₂O

C₆H₅COOH + H₂SO₄ → C₆H₄ (COOH) SO₃H + H₂O

9. Friedel Crafts Reactions

Aromatic carboxylic acids do not undergo friedel-carfts reactions as the carboxyl group has a tendency to attract electrons. So, due to the deactivation of the benzene ring, it cannot undergo alkylation and acylation.

10. Decarboxylation:

Sodium salts of carboxylic acids when used for the distillation of soda-lime results in a decarboxylation reaction to form alkanes.

CH₃COONa + NaOH → CH₄ + Na₂CO₃

11. Anhydrides Formation

Two carboxylic acid molecules when heated with a dehydrating agent like phosphorus pentoxide lead to the formation of acid anhydrides.

2RCOOH + heat → RCO-O-RCO + H₂O

Uses of Carboxylic Acids

• It is used as a disinfectant.
• Formic acid is used as a reducing agent in the treatment of textiles.
• Acetic acid is used in the preparation of esters and cellulose plastics.
• Acetic acid is also used as a precursor for the formation of salicylic acid ester, which is used to produce aspirin.
• Stearic acid and palmitic acid are required in the production of soaps, candles, cosmetics, protective coatings, and more.
• Steric acid also aids rubber production.
• Acrylic acid by acting as an ester aids in the formation of polymers or acrylates. Likewise, methacrylic acid undergoes polymerization and forms Lucite.
• Oleic acid, which is also a carboxylic acid type, helps in the production of soaps and detergents and is also used in textiles.

---