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."
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, which 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 the reaction can be stopped at the amide stage.
From Grignard Reagents
Grignard reagents react with carbon dioxide (dry ice) in the 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, are 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 an acidic medium converts alkylbenzene 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 they are treated with the solutions of KMnO₄ (potassium permanganate) or K₂Cr₂O₇ (potassium dichromate.)
Hydrolysis of Trihalogen Derivatives
"Trihalogen derivative" means 3 halogen atoms are connected to the same carbon atom. When trihalogen derivatives undergo hydrolysis, they produce carboxylic acids such as
Heating Gem Dicarboxylic Acids
When gem dicarboxylic acid is heated, 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 an alkene is treated in steam with CO (carbon monoxide) under pressure at 300-400°C, the carboxylic acid is formed. This reaction is also called the Koch reaction.
Sample Questions
Question 1: Write the IUPAC name of the following carboxylic acids-
- HCOOH
- CH3COOH
- C6H5-COOH
- HOOC-CH₂-COOH
- HCOOH - Methanoic acid
- CH3COOH - Ethanoic acid
- C6H5-COOH - Benzenecarboxylic acid
- HOOC -CH2-COOH - Propanedioic acid
Question 2: How can acetophenone 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.
- 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 a Grignard reagent, and how is a Grignard reagent used to produce a 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.
Unsolved Problems
- How are carboxylic acids prepared from nitriles?
- Describe the preparation of carboxylic acids using Grignard reagents.
What is the role of carbon dioxide in this reaction? - Differentiate between acidic and basic hydrolysis.
- Why is oxidation of aldehydes to carboxylic acids easier than oxidation of alcohols?
- How are carboxylic acids prepared from the following? (a) Acid chlorides (b) Acid anhydrides