Problem 150 - A Complete Resource Book in Chemistry for JEE Main

Question

Which of the following statements are correct? (a) \(\mathrm{KMnO}_{4}\) is a powerful oxidising agent. (b) \(\mathrm{KMnO}_{4}\) is a weaker oxidising agent than \(\mathrm{K}_{2} \mathrm{Cr}_{2} \mathrm{O}_{7}\) in acid medium.(c) \(\mathrm{KMnO}_{4}\) is a stronger oxidising agent than \(\mathrm{K}_{2} \mathrm{Cr}_{2} \mathrm{O}_{7}\) in acid medium. (d) \(\mathrm{K}_{2} \mathrm{Cr}_{2} \mathrm{O}_{7}\) oxidises a secondary alcohol to a ketone.

Step-by-Step Solution

Answer

Statements (a), (c), and (d) are correct; statement (b) is incorrect.

Step 1: Analyze Statement (a)

The statement (a) claims that \(\mathrm{KMnO}_{4}\) is a powerful oxidizing agent. In chemical reactions, \(\mathrm{KMnO}_{4}\) is commonly used for its strong oxidizing properties. It can oxidize a wide variety of organic and inorganic substances in acidic, neutral, or alkaline media.

Step 2: Analyze Statement (b)

Statement (b) indicates that \(\mathrm{KMnO}_{4}\) is a weaker oxidizing agent than \(\mathrm{K}_{2}\mathrm{Cr}_{2}\mathrm{O}_{7}\) in an acid medium. Contrary to this statement, \(\mathrm{KMnO}_{4}\) is generally considered a stronger oxidizing agent than \(\mathrm{K}_{2}\mathrm{Cr}_{2}\mathrm{O}_{7}\), especially in acidic conditions.

Step 3: Analyze Statement (c)

Statement (c) claims that \(\mathrm{KMnO}_{4}\) is a stronger oxidizing agent than \(\mathrm{K}_{2}\mathrm{Cr}_{2}\mathrm{O}_{7}\) in an acid medium. This statement is correct, as \(\mathrm{KMnO}_{4}\) has a higher oxidation potential in acid medium compared to \(\mathrm{K}_{2}\mathrm{Cr}_{2}\mathrm{O}_{7}\).

Step 4: Analyze Statement (d)

Statement (d) suggests that \(\mathrm{K}_{2}\mathrm{Cr}_{2}\mathrm{O}_{7}\) oxidizes a secondary alcohol to a ketone. This statement is true, as \(\mathrm{K}_{2}\mathrm{Cr}_{2}\mathrm{O}_{7}\) in acidic conditions is typically used to oxidize secondary alcohols to ketones.

Key Concepts

Potassium permanganate (KMnO4)

Potassium permanganate, or \(\mathrm{KMnO}_{4}\), is a ubiquitous oxidizing agent known for its vibrant purple color. It is famous for its ability to accept electrons and transform into manganese dioxide (\(\mathrm{MnO}_2\)), making it highly effective in various chemical reactions.

Some key points about \(\mathrm{KMnO}_{4}\) as an oxidizing agent include:

• It is versatile and works in different environments: acidic, neutral, or alkaline.
• It is often used to break down organic substances in environmental and analytical chemistry.
• It is often employed for the purification of water, as it helps in degrading impurities due to its high oxidative capability.

In acidic solutions, \(\mathrm{KMnO}_{4}\) becomes even more robust, as the standard electrode potential (\( E^a \)) is higher, boosting its reactivity and ability to oxidize stronger compounds.

Dichromate (K2Cr2O7)

Dichromate, denoted as \(\mathrm{K}_{2}\mathrm{Cr}_{2}\mathrm{O}_{7}\), is another powerful oxidizing agent often used in organic chemistry. This bright orange compound is highly effective in acidic solutions.

Here are some essential characteristics of \(\mathrm{K}_{2}\mathrm{Cr}_{2}\mathrm{O}_{7}\):

• It can convert primary alcohols into aldehydes and secondary alcohols into ketones.
• It generates chromium ions (\(\mathrm{Cr}^{3+}\)) as a byproduct once it donates oxygen atoms in reactions.
• It is also utilized in breathalyzer tubes as it reacts with ethanol vapors, demonstrating its reactive capabilities.

While in acidic conditions, it isn’t as potent as \(\mathrm{KMnO}_{4}\), it remains a critical reagent in many laboratory and industrial processes.

Acidic oxidation reactions

Acidic oxidation reactions are chemical processes that involve the transfer of electrons from a substance (getting oxidized) to an oxidizing agent in an acidic medium. This type of reaction is crucial in transforming various compounds into more oxidized forms.

In an acidic medium, oxidizing agents like \(\mathrm{KMnO}_{4}\) and \(\mathrm{K}_{2}\mathrm{Cr}_{2}\mathrm{O}_{7}\) excel due to their increased reactivity. Here’s how this works:

• The acidic environment facilitates the addition of protons which can assist in breaking bonds during oxidation.
• \(\mathrm{KMnO}_{4}\) in acid turns into \(\mathrm{Mn}^{2+}\) ions through a series of electron transfer steps.
• \(\mathrm{K}_{2}\mathrm{Cr}_{2}\mathrm{O}_{7}\) yields \(\mathrm{Cr}^{3+}\) ions, actively participating in oxidation reactions.
• The oxidation potential of agents like \(\mathrm{KMnO}_{4}\) is higher in acidic conditions, allowing stronger oxidations.

Such reactions are integral to converting alcohols to carboxylic acids or ketones, aligning with common transformations in organic synthesis.