Methods of Preparation of Carboxylic Acids

Carbonyl groups are organic compounds that contain a carbon-oxygen double bond (>C=O) and are one of the most important functional groups in organic chemistry. The carbonyl group is bonded to a carbon and a hydrogen atom in aldehydes, while it is bonded to two carbon atoms in ketones. Carboxylic acids and their derivatives are carbonyl compounds in which the carbonyl group is bonded to oxygen.

Carboxylic Acids

Carboxylic acids are carbon compounds with a carboxyl functional group, –COOH. The carboxyl group is made up of a carbonyl group that is joined to a hydroxyl group, hence the name carboxyl. 

There are aliphatic (RCOOH) or aromatic (ArCOOH) carboxylic acids which depend on the group attached to the carboxylic carbon that is alkyl or aryl. Nature contains a large number of carboxylic acids. Some higher aliphatic carboxylic acid (C12 – C18) esters of glycerol are found in natural fats as fatty acids. Carboxylic acids are the precursors to a wide range of important organic compounds such as anhydrides, esters, acid chlorides, amides, and so on.

Examples of Carboxylic acids are – 

• HCOOH                           –           Formic acid
• CH₃COOH                       –           Acetic acid
• CH₃CH₂COOH                 –          Propionic acid
• CH₃CH₂CH₂COOH          –           Butanoic acid
• HOOC-COOH                  –          Oxalic acid
• HOOC -CH₂ -COOH       –          Malonic acid
• HOOC -(CH₂ )₄ -COOH  –          Adipic acid
• C₆H₅-COOH                    –          Benzoic acid
• C₆H5-CH₂-COOH            –          Phenylacetic acid

Methods of Preparation of Carboxylic Acids

The following are some important carboxylic acid preparation methods.

• From primary alcohols and aldehydes:

Primary alcohols are easily oxidized to carboxylic acids by common oxidizing agents such as potassium permanganate (KMnO₄ ) in neutral, acidic, or alkaline medium, or potassium dichromate (K₂Cr₂O₇ ) and Jones reagent that is chromium trioxide (CrO₃ ) in acidic medium.

Using mild oxidants, carboxylic acids can also be produced from aldehydes. When exposed to common oxidizing agents such as nitric acid, potassium permanganate, potassium dichromate, and others, aldehydes are easily converted to carboxylic acids.

• From alkylbenzenes:

Aromatic carboxylic acids can be prepared by oxidizing alkylbenzenes with chromic acid or acidic or alkaline potassium permanganate. Regardless of side-chain length, the entire side chain is oxidized to the carboxyl group. This method oxidizes primary and secondary alkyl groups while leaving tertiary groups alone. These oxidizing reagents also oxidize suitably substituted alkenes to carboxylic acids.

• From nitriles and amides:

In the presence of H⁺ or OH^– as a catalyst, nitriles are hydrolyzed to amides and then to acids. In this reaction, mild reaction conditions are preferred so that reaction can be stopped at the amide stage.

• From Grignard reagents:

Grignard reagents react with carbon dioxide (dry ice) in presence of dry ether to produce carboxylic acid salts, which are then acidified with a mineral acid to produce the corresponding carboxylic acids.

• From acyl halides and anhydrides:

When acid chlorides are hydrolyzed with water, they produce carboxylic acids. When acid chlorides are hydrolyzed with an aqueous base, they produce carboxylate ions, which when acidified produce the corresponding carboxylic acids. Anhydrides, on the other hand, is hydrolyzed with water to produce the corresponding acid.

• From esters:

Acidic hydrolysis of esters yields carboxylic acids directly, whereas basic hydrolysis yields carboxylates, which when acidified yield the corresponding carboxylic acids.

• Oxidation of Alkyl Benzenes:

Oxidation of alkylbenzenes with K₂Cr₂O₇ or KMnO₄ in acidic medium alkyl benzene is converted to benzoic acids. The oxidation takes place until all the hydrogen atoms of the alkyl group are oxidized.

• Oxidation of Alkenes :

Alkenes are easily oxidized in carboxylic acids when it is treated with the solutions of KMnO₄ ( potassium permanganate) or K ₂Cr₂O₇ (potassium dichromate.) 

• Hydrolysis of Trihalogen Derivatives:

Trihalogen derivative means 3 halogen atoms are connected to the same carbon atom. when Trihalogen derivatives undergo hydrolysis it produces carboxylic acids as:

• Heating Gem Dicarboxylic Acids:

When gem dicarboxylic acid is heating carbon dioxide is released and carboxylic acid is formed.

• Oxidation of Methyl Ketone:

The carboxylic acid can be made from methyl ketone-

• Koch Reaction:

When alkene is treated in steam with CO (carbon monoxide) under the pressure at 300-400°C, the carboxylic acid is formed. This reaction is also called Koch Reaction.

Sample Questions

Question 1: Write the IUPAC name of the following carboxylic acids-

• HCOOH 
• CH₃COOH 
• C₆H₅-COOH 
• HOOC -CH₂-COOH 

Answer:

• HCOOH                  –          Methanoic acid
• CH₃COOH             –          Ethanoic acid
• C₆H₅-COOH         –         Benzenecarboxylic acid
• HOOC -CH₂-COOH      –   Propanedioic acid

Question 2: How acetophenone can be converted into benzoic acid?

Answer:

Acetophenone can be converted in benzoic acid by oxidation of acetophenone in the presence of acidic or alkaline potassium permanganate.

Question 3: Which one is more acidic in the following compounds:

• Formic acid (HCOOH) or benzoic acid (C₆H₅-COOH),
• CH₃COOH or CH₂FCOOH.

Answer:

• Formic acid is more acidic as compared to benzoic acid because negative charge on O  is more delocalized in HCOOH than benzoic acid.
• CH₂FCOOH is more acidic than CH₃COOH because electronegative F atom is causing electron withdrawal from COOH and helps in release of H⁺.

Question 4: How to transform Butanol to butanoic acid?

Answer:

By oxidation of Butanol in presence of ammoniacal AgNO₃ (Tollens reagent) produces  butanoic acid.

Question 5: What is Grignard reagent, how Grignard reagent is used to produce carboxylic acid?

Answer:

A Grignard reagent is an organomagnesium halide with the formula RMgX, where X is a halogen (-Cl, -Br, or -I) and R is an alkyl or aryl group. Grignard reagents react with carbon dioxide (dry ice) in presence of dry ether to produce carboxylic acid salts, which are then acidified with mineral acid to produce the corresponding carboxylic acids.