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

(a), (c), and (d) are correct.

Step 1: Understanding Oxidizing Agents

An oxidizing agent is a substance that gains electrons in a chemical reaction and in the process, oxidizes another substance. Potassium permanganate (\(\mathrm{KMnO}_{4}\)) is a well-known powerful oxidizing agent.

Step 2: Comparing Oxidizing Strength in Acid Medium

In an acidic medium, \(\mathrm{KMnO}_{4}\) is a stronger oxidizing agent than \(\mathrm{K}_{2}\mathrm{Cr}_{2}\mathrm{O}_{7}\). The oxidizing power is typically measured by their reduction potentials, and \(\mathrm{KMnO}_{4}\) has a higher reduction potential compared to \(\mathrm{K}_{2}\mathrm{Cr}_{2}\mathrm{O}_{7}\).

Step 3: Reaction of Oxidizing Agents with Alcohols

\(\mathrm{K}_{2} \mathrm{Cr}_{2} \mathrm{O}_{7}\) in acidic medium can oxidize secondary alcohols to ketones but is more commonly known to oxidize primary alcohols to carboxylic acids or aldehydes. For alcohol to ketone conversions, it can oxidize secondary alcohols efficiently.

Step 4: Evaluation of Statements

(a) True, because \(\mathrm{KMnO}_{4}\) is a known powerful oxidizing agent. (b) False, since it's the opposite; \(\mathrm{KMnO}_{4}\) is stronger than \(\mathrm{K}_{2}\mathrm{Cr}_{2}\mathrm{O}_{7}\) in acidic medium. (c) True, as \(\mathrm{KMnO}_{4}\) indeed is stronger. (d) True, \(\mathrm{K}_{2} \mathrm{Cr}_{2} \mathrm{O}_{7}\) can oxidize secondary alcohols to ketones.

Key Concepts

Potassium Permanganate

Potassium permanganate, with the chemical formula \(\mathrm{KMnO}_{4}\), is a highly effective oxidizing agent. It's widely used in chemistry due to its strong ability to gain electrons and facilitate oxidation in other substances. This oxidative power makes it valuable in various chemical reactions.

A distinguishing feature of \(\mathrm{KMnO}_{4}\) is its versatility in different conditions. Under acidic, neutral, or even basic environments, \(\mathrm{KMnO}_{4}\) can serve as a reliable oxidizing agent and partakes in a range of organic and inorganic chemical reactions. The striking purple color of \(\mathrm{KMnO}_{4}\) is not just for show. This deep hue signals its active state in reactions, making the process visually informative for chemists. When used in reactions, \(\mathrm{KMnO}_{4}\) is typically reduced to colorless \(\mathrm{Mn}^{2+}\) ions, indicating the end of the oxidation process.

• Useful in water treatment and disinfection due to its strong oxidizing properties.
• Widely used in analytical chemistry for titrations.
• Effective in oxidizing a variety of organic and inorganic substrates.

Potassium Dichromate

Potassium dichromate, \(\mathrm{K}_{2}\mathrm{Cr}_{2}\mathrm{O}_{7}\), is another well-known oxidizing agent frequently employed in chemical laboratories. It is primarily recognized for its bright orange color and usage in oxidative processes, particularly when used in acidic media. In these conditions, it releases \(\mathrm{Cr}^{3+}\) ions, demonstrating its effective oxidizing capability.

Unlike \(\mathrm{KMnO}_{4}\), potassium dichromate has a slightly weaker oxidizing power in acidic media. However, it still boasts significant uses due to its ability to transform a wide range of chemicals through oxidation. Thus, it holds a valuable place in both academic settings and industrial processes.

• Used in breathalyzer devices as one component needed to determine blood alcohol content.
• Capable of oxidizing primary alcohols, secondary alcohols, and aldehydes.
• Critical in laboratory synthesis of chromated compounds and other reagents.

Oxidation of Alcohols

The oxidation of alcohols is a fundamental reaction in organic chemistry, whereby alcohols are converted into more oxidized compounds. Oxidizing agents like potassium permanganate and potassium dichromate are commonly employed to achieve these transformations.

For secondary alcohols, oxidation typically leads to the formation of ketones. Both \(\mathrm{KMnO}_{4}\) and \(\mathrm{K}_{2} \mathrm{Cr}_{2} \mathrm{O}_{7}\) can accomplish this task effectively. These reagents remove hydrogen from the alcohol, forming a carbonyl group and thus converting it into a ketone.

In contrast, primary alcohols can be oxidized to aldehydes or even further to carboxylic acids, depending on the conditions and the choice of oxidizing agent. \(\mathrm{K}_{2} \mathrm{Cr}_{2} \mathrm{O}_{7}\) is particularly effective at pushing the oxidation all the way to carboxylic acids due to its ability to maintain an acidic environment conducive for extensive oxidation.

• Secondary alcohols typically convert to ketones, whereas primary alcohols can become aldehydes or further oxidized to acids.
• The choice of oxidizing agent influences the final product of the reaction.
• Both \(\mathrm{KMnO}_{4}\) and \(\mathrm{K}_{2} \mathrm{Cr}_{2} \mathrm{O}_{7}\) are versatile tools in organic synthesis, aiding the conversion of alcohols.