Problem 55 - A Complete Resource Book in Chemistry for JEE Main

Question

Lactic acid on oxidation by alkaline potassium permanganate gives (a) propionic acid (b) cinnamic acid (c) pyruvic acid (d) tartaric acid

Step-by-Step Solution

Answer

The product is (c) pyruvic acid.

Step 1: Understanding the Reaction

Oxidation reactions with alkaline potassium permanganate typically result in the transformation of alcohol groups into carboxylic acids or ketones. For lactic acid, which has the structure CH₃-CH(OH)-COOH, the focus will be on the oxidation of the secondary alcohol group (-OH on the middle carbon).

Step 2: Known Outcome of Oxidation

In organic chemistry, the oxidation of lactic acid by an oxidizing agent like alkaline potassium permanganate generally converts the secondary alcohol group to a ketone. Thus, the CH(OH) group of lactic acid will oxidize to a carbonyl group (C=O), forming pyruvic acid (CH₃-CO-COOH).

Step 3: Identify the Correct Option

Based on the oxidation process, lactic acid, upon oxidation, converts to pyruvic acid. The other options like propionic acid, cinnamic acid, and tartaric acid do not match the structural transformation of lactic acid through this oxidation.

Key Concepts

Oxidation Reactions

Oxidation reactions in organic chemistry involve the increase of oxygen in a molecule or the removal of hydrogen. A hallmark of these reactions is the transformation of alcohols into more oxidized compounds such as ketones or carboxylic acids. In the case of lactic acid, which contains an alcohol group (-OH), oxidation often targets this functional group. The significance of oxidation reactions is widespread in chemistry, facilitating the breakdown of complex molecules and the synthesis of new ones.

• Alcohols transform into carbonyl compounds or carboxylic acids.
• Secondary alcohols oxidize to ketones.

Through the use of oxidizing agents like potassium permanganate, these transformations occur efficiently, leading to significant changes in molecular structures.

Lactic Acid

Lactic acid, chemically known as 2-hydroxypropanoic acid, has the structure CH₃-CH(OH)-COOH. It is a carboxylic acid that also contains an alcohol functional group. This combination allows various chemical transformations to take place. In nature, lactic acid plays a role in fermentation and is produced by muscles during intense exercise. The compound's structure offers a unique opportunity for targeted chemical reactions, especially oxidation processes.

• Consists of a secondary alcohol group.
• Readily oxidizes under certain conditions.

This ability to undergo chemical reactions makes lactic acid a precious compound both in biological systems and synthetic applications.

Pyruvic Acid

Pyruvic acid, with the chemical formula CH₃-CO-COOH, is the product of the oxidation of lactic acid. When lactic acid undergoes oxidation, the alcohol group is converted into a carbonyl group, transitioning from an -OH to a =O. This transformation results in pyruvic acid, a ketone and carboxylic acid derivative. Pyruvic acid is an essential intermediate in cellular respiration and energy production. Its structure and reactivity make it a key molecule for metabolism and chemical synthesis.

• Formed from the oxidation of secondary alcohols.
• Central to metabolic pathways.

Understanding the conversion to pyruvic acid provides insight into both chemical transformations and biological energy utilization.

Potassium Permanganate

Potassium permanganate, KMnO₄, is a powerful oxidizing agent commonly used in organic chemistry. It is known for converting various organic compounds into more oxidized forms and is especially effective in breaking down carbon-carbon bonds. Its strength lies in successfully transforming alcohols into ketones or carboxylic acids under alkaline conditions.

• Strong oxidizing properties.
• Typically used with alcohols and alkenes.

In the oxidation of lactic acid to pyruvic acid, potassium permanganate plays a crucial role by facilitating the removal of hydrogen atoms and the increase in oxidation state of carbon atoms.

Alcohols and Carboxylic Acids

In organic chemistry, alcohols are compounds that contain one or more hydroxyl (-OH) groups attached to a carbon atom. These functional groups are reactive and can transform into other groups through oxidation reactions. Carboxylic acids, on the other hand, feature a carbonyl group (C=O) and a hydroxyl group attached to the same carbon, creating the functional group -COOH. The relationship between alcohols and carboxylic acids is crucial in oxidation reactions.

• Alcohols can oxidize to form carboxylic acids.
• Serve as versatile building blocks in synthesis.

Through reactions with oxidizing agents like potassium permanganate, alcohols readily undergo structural changes, leading to the formation of carboxylic acids or ketones. Understanding these transformations explains the behavior and reactivity of organic molecules.