Problem 145 - Arihant Objective Approach to Organic Chemistry for IIT JEE and NEET all Medical Entrances Part 4

Question

Oxidation of naphthalene by acidic \(\mathrm{KMnO}_{4}\) gives: IP (PMT) 20s0 (a) benzoic acid (b) benzaldehyde (c) toluene (d) phthalic acid

Step-by-Step Solution

Answer

The oxidation of naphthalene by acidic \(\mathrm{KMnO}_{4}\) gives phthalic acid.

Step 1: Identify the Reagents and Conditions

In this exercise, we are asked to consider the reaction of naphthalene with acidic \(\mathrm{KMnO}_{4}\). \(\mathrm{KMnO}_{4}\) in acidic conditions is a strong oxidizing agent.

Step 2: Consider the Structure of Naphthalene

Naphthalene is a polycyclic aromatic hydrocarbon composed of two fused benzene rings. It's represented as \(C_{10}H_{8}\), with the rings sharing two carbon atoms.

Step 3: Understand the Oxidation Process

Under oxidative conditions with \(\mathrm{KMnO}_4\), the oxidation of aromatic hydrocarbons like naphthalene often leads to breaking of the aromatic rings and formation of dicarboxylic acids.

Step 4: Determine the Oxidation Product

Given that naphthalene has two aromatic rings, oxidation results in the opening and oxidation of these rings to form phthalic acid. This is because both rings of naphthalene open up and tend to form a chain with carboxylic acid (\(-\mathrm{COOH}\)) groups at the end.

Key Concepts

KMnO4 as oxidizing agent

Potassium permanganate (\(\mathrm{KMnO}_{4}\)) is a potent oxidizing agent, especially in acidic conditions. It is commonly used in organic chemistry to add oxygen or remove hydrogen from molecules, thereby oxidizing them. This process often leads to the breakdown of complex organic compounds into simpler, more stable structures.
When \(\mathrm{KMnO}_{4}\) is used in acidic conditions, it effectively supplies oxygen that can attack and oxidize various functional groups present in organic compounds. This is especially useful for oxidizing aromatic structures, like naphthalene, which we are exploring in this exercise.
In the presence of acids, \(\mathrm{KMnO}_{4}\) transforms complex organic structures by opening up aromatic rings and introducing oxygen in the form of hydroxyl or carboxyl groups. This makes it a key player in converting hydrocarbons to acids, such as transforming naphthalene to phthalic acid.

structure of naphthalene

Naphthalene is a fascinating molecule characterized by its unique structure as a polycyclic aromatic hydrocarbon. It consists of two benzene rings fused together, sharing a pair of carbon atoms. This fusion gives it the chemical formula \(\mathrm{C}_{10}\mathrm{H}_{8}\).
In this arrangement, naphthalene exhibits the typical stability associated with aromatic compounds, with delocalized electrons spreading over the rings. This delocalization contributes to naphthalene's overall stability and means it can undergo reactions like oxidation, under the right conditions.
Understanding the structure of naphthalene is crucial, especially when considering its reaction pathways. The dual-ring system makes naphthalene more reactive to oxidizing agents that can break these rings apart and introduce new functional groups, transforming the molecule substantially.

oxidation to phthalic acid

When the oxidation process occurs with naphthalene using \(\mathrm{KMnO}_{4}\) as the oxidizing agent, it typically results in the formation of phthalic acid. During this reaction, the strong oxidative conditions lead to the breakdown of naphthalene’s aromatic rings.
Here’s how the transformation happens:

• The oxidation disrupts the aromatic stability of the naphthalene by attacking the fused benzene rings.
• The rings cleave open, undergoing further oxidation that adds oxygen atoms.
• This results in the formation of a dicarboxylic acid, specifically phthalic acid, where the \(-\mathrm{COOH}\) groups replace some of the carbon-hydrogen bonds.

This oxidation not only changes the structure but also the compound’s properties, converting it from a solid, volatile hydrocarbon to a compound that is acidic and crystalline in nature. Such transformations highlight the power of oxidation reactions in altering organic molecules.