How to Prepare Carboxylic Acids ?

Introduction

Carboxylic Acids are also termed hydrocarbon oxidation products. Carboxylic acids are hydrocarbon derivatives that are formed by the replacement of 1 or more hydrogen atoms by carboxylic radicals.

Organic compounds with carboxyl groups are known as carboxylic acids. They are a crucial functional group in chemistry. They are named from the carbonyl (C=O) as well as hydroxyl (-OH) groups that they are composed of. They have distinct characteristics from the carbonyl and hydroxyl groups. They are divided into monocarboxylic acid, dicarboxylic acid, and tricarboxylic acid depending on the no. of carboxyl groups.

Structure of Carboxylic Acids

• According to recent spectroscopic and diffraction research, the acid group (-COOH) is planar. Namely the carbon atom of the (-COOH) group, as well as the 2 O-atoms, are sp² hybridised, whereas the alkyl carbon atoms are sp³ hybridised.

• Because the R group has 1 unpaired sp³ hybridized orbital, it aligns axially with the sp² hybridized orbital of carbon, generating the sigma- bond.

• The C-atom has undergone sp² hybridization. The last pz orbital is perpendicular to the plane formed by the 3 sp² orbitals. One sp² orbital aligns with the R group's sp³ orbital. The final 2 overlap with the sp² orbitals of 2 oxygen atoms, resulting in the formation of 2 -sigma bonds.

• 1 oxygen undergoes sp² hybridization. The final pz orbital is parallel to the plane. 1 sp² hybridized orbital aligns with the sp² hybridized orbital of carbon, resulting in the formation of a sigma- bond. The other 2 sp² hybridized orbitals feature lone e^- pairs.

• The 2^nd oxygen is likewise coplanar, with 1 sp² orbital hybridized overlapping with the hydrogen's s orbital, making a sigma- bond.

• All 3pz hybridized orbitals located perpendicular to the plane might overlap laterally in the entire orbital image, generating pi-bonds. As a result, these orbitals are delocalized.

Methods of Preparation of Carboxylic Acids

Some major carboxylic acid preparation techniques are as follows.

Preparation of Carboxylic Acid from The Grignard Reagent

In the existence of the dry ether to form carboxylic acid salts, which are subsequently acidified with a mineral acid to form carboxylic acids, Grignard reagents react with carbon dioxide (dry ice).

Preparation of Carboxylic Acid from Alcohol

Potassium dichromate as well as Jones reagent in acidic media or common oxidising agents like potassium permanganate in neutral, acidic, or alkaline media, quickly convert primary alcohols to carboxylic acids.

Preparation of Carboxylic Acid from Aldehyde

Carboxylic acids can also be synthesised from aldehydes (-CHO) using moderate oxidants. Aldehydes are rapidly transformed to carboxylic acids when subjected to common oxidising agents.

Preparation of Carboxylic Acid from Ester

Basic hydrolysis produces carboxylates, which are acidified to produce the equivalent carboxylic acids while acidic hydrolysis of esters immediately produces carboxylic acids.

Preparation of Carboxylic Acid from Acyl Halides and Anhydrides

Whenever acid chlorides are hydrolysed with water, carboxylic acids are formed. When acid chlorides are hydrolysed with an aqueous base, carboxylate ions are formed, which when acidified yield the respective carboxylic acids. In contrast, anhydrides are hydrolysed with H_2 O to yield the equivalent acid.

Preparation from Nitriles and Amides

Nitriles are hydrolysed to amides and then to acids in the existence of H⁺ or OH^- as a catalyst. Mild reaction circumstances are preferable in this reaction so that the process can be terminated at the amide step.

Preparation from Alkylbenzene

Alkylbenzenes can be oxidised, with alkaline or acidic potassium permanganate or chromic acid to produce aromatic carboxylic acids. The whole side chain is oxidised to the carboxyl group, irrespective of side chain length. Primary and secondary alkyl groups are oxidised but tertiary groups are not. These oxidising reagents can also convert appropriately substituted alkenes to carboxylic acids.

Conversion of Carboxylic Acid to Alcohol

The transformation of carboxylic acid to alcohol is a straightforward procedure. Carboxylic acid is reduced utilizing reagents like dry ether or lithium tetrahydridoaluminate. The -COOH group in carboxylic acid is converted to -CH_2 OH, a main alcoholic group.

Uses of Carboxylic Acids

• Organic acids are employed in the food industry to make, sodium salts, vinegar and other soft beverages, whereas sodium benzoate is used to preserve goods.

• Acetic is utilised in the chemical industry to make fragrances, rayons, and dyes, among other things.

• Acetic acids are utilised in the rubber manufacturing sector to create rubber.

• Carboxylic acids, such as stearic acid, are the most significant acids used in the production of detergents and soaps.

• In the nylon industry, hexanedioic acid is employed during the production process.

• Benzoic acid esters are utilised to manufacture body mist, perfume, and other cosmetics.

• Acetylsalicylic acid is utilised in the pharmaceutical industry to make phenacetin, aspirin and other drugs.

Conclusion

Carboxylic acids are classified as hydrocarbon derivatives. The existence of replaceable H-atoms in the COOH group gives carboxylic acid its acidic nature. According to recent spectroscopic and diffraction research, the acid group (-COOH) is planar. Water dissolves carboxylic acids. A carboxylic acid's boiling temperature is much greater than that of an equivalent molecular mass alkane or alcohol. These are categorised as aliphatic or aromatic complexes based on the kind of group connected to COOH (alkyl or aryl).

FAQS

1. Acids have greater boiling temperatures than their equivalent alcohols. Give the reasoning.

Because of the existence of intermolecular hydrogen bonding between the 2 molecules of acid, the boiling temperature of acids is higher. Because of dimer production, the effective molecular mass becomes just twice that of the usual mass, raising the boiling temperature. Furthermore, because acid has a more polar -OH group than alcohol, the hydrogen bonding in acid is stronger, raising the boiling temperature of acids.

2. Why are carboxylic acids referred to as fatty acids?

Because higher carboxylic acids are formed through the breakdown of fats and oils, carboxylic acids are referred to as fatty acids.

3. What is the result of treating an ester with LiAlH_4 and then acid hydrolysing it?

$$\mathrm{Ph-COO-CH_2-Ph+LiAlH_4→Ph-CH_2-OH+Ph-CH_2-OH}$$

4.Which of the following is more acidic?

• CH₃ COOH or CH₂ FCOOH,

• Formic acid or Benzoic acid

Because the electronegative F atom causes electron removal from COOH and aids in the release of H⁺, CH₂ FCOOH is more acidic than CH₃ COOH.

Formic acid is more acidic than benzoic acid because the -ve charge on O in HCOOH is more delocalized than in benzoic acid.

5. State Carboxylic Acid Physical Properties.

• The C₁-C₃ carboxylic acids (aliphatic) are pungent-smelling, caustic liquids, with no colour. Carboxylic acids C₄-C₉ are greasy liquids having a rancid butter odour. Higher acids are odourless waxy solids with substantially lower densities than water.

• The boiling temperature of a carboxylic acid is much greater than that of an equivalent molecular mass alkane or alcohol.

• Carboxylic acids are soluble in water given the potential of hydrogen bond synthesis with H₂ O molecules.