Problem 133 - A Complete Resource Book in Chemistry for JEE Main

Question

1-propanol and 2 -propanol can be best distinguished by (a) oxidation with \(\mathrm{KMnO}_{4}\) followed by reaction with Fehling solution (b) oxidation with acidic dichromate followed by reaction with Fehling solution (c) oxidation by heating with copper followed by reaction with Fehling solution (d) oxidation with concentrated \(\mathrm{H}_{2} \mathrm{SO}_{4}\) followed by reaction

Step-by-Step Solution

Answer

Option (c) oxidation by heating with copper distinguishes them.

Step 1: Understand the Alcohol Types

1-Propanol is a primary alcohol, while 2-propanol is a secondary alcohol. The distinction between these types is crucial for determining how they react with oxidizing agents. Primary alcohols can be oxidized to aldehydes and further to carboxylic acids, whereas secondary alcohols can be oxidized to ketones.

Step 2: Consider the Oxidizing Agent Reactions

Fehling’s solution reacts with aldehydes to form a red precipitate of cuprous oxide. It does not react with ketones. This means, for the Fehling’s test to differentiate between 1-propanol and 2-propanol, the 1-propanol must first be oxidized to an aldehyde.

Step 3: Analyze Each Option

- Option (a): KMnO₄ is a strong oxidizing agent that will convert 1-propanol to a carboxylic acid, not just an aldehyde. - Option (b): Acidic dichromate is also a strong oxidizing agent, similar to KMnO₄, and will take primary alcohols to carboxylic acids. - Option (c): Heating with copper can gently oxidize primary alcohols to aldehydes, suitable for Fehling's test. - Option (d): H₂SO₄ is not typically used as an oxidizing agent alone in such a reaction.

Step 4: Choose the Appropriate Method

Heating with copper for option (c) allows the primary alcohol (1-propanol) to be oxidized to an aldehyde, which will react with Fehling’s solution. The secondary alcohol (2-propanol) will be converted to a ketone, which does not react with Fehling's solution.

Key Concepts

Primary Alcohols

Primary alcohols are a type of alcohol where the hydroxyl group (-OH) is attached to a carbon atom that is only connected to one other carbon atom. This makes the carbon attached to the -OH group end the carbon chain. An example is 1-propanol, where the hydroxyl group is on the first carbon.

Primary alcohols can be oxidized in a two-step process to form aldehydes and carboxylic acids. First, the primary alcohol is oxidized to an aldehyde. With further oxidation, the aldehyde can be turned into a carboxylic acid.

During oxidation, an alcohol loses electrons, often by gaining an oxygen or losing hydrogen. This concept is crucial when considering reactions with different oxidizing agents since the degree of oxidation affects the final product of the reaction.

Secondary Alcohols

Secondary alcohols have their hydroxyl group (-OH) attached to a carbon atom that is connected to two other carbon atoms. This structure creates more branching compared to primary alcohols. An example is 2-propanol, often known as isopropanol.

When oxidized, secondary alcohols form ketones, which do not undergo further oxidation to carboxylic acids. This is because the carbon atom carrying the -OH group does not have an additional hydrogen atom required for further oxidation beyond a ketone.

Unlike primary alcohols that can become more acidic through further oxidation, secondary alcohols reach their terminus at the ketone stage. This is useful in distinguishing them from primary alcohols, especially in tests that target aldehydes or carboxylic acids, such as the Fehling’s test.

Oxidizing Agents

Oxidizing agents play a fundamental role in converting alcohols to their oxidized forms. Different oxidizing agents can achieve varying levels of oxidation depending on their strength.

• Potassium permanganate (\( \mathrm{KMnO}_{4} \)) is a powerful oxidizing agent, turning primary alcohols into carboxylic acids directly.
• Acidic dichromate is another strong oxidizer, also creating carboxylic acids from primary alcohols.
• Copper, when used at elevated temperatures with primary alcohols, gently oxidizes them to aldehydes, perfect for attempts to identify aldehydes using the Fehling’s test.

It's important to choose the right oxidizing agent for the desired oxidation product whether an aldehyde, ketone, or carboxylic acid is needed. This is especially important when distinguishing between primary and secondary alcohols.

Fehling's Test

Fehling's test is a chemical reaction specifically for detecting aldehyde functional groups. This classical method uses Fehling’s solution, consisting of copper(II) sulfate and potassium sodium tartrate in an alkaline solution.

The test works as follows: if an aldehyde is present, the solution will react, resulting in the reduction of blue copper(II) ions to a red cuprous oxide precipitate. This change is visible, making it a useful qualitative test for aldehydes.

However, this test is not applicable for ketones, such as those derived from secondary alcohol oxidation, since ketones do not reduce the copper(II) ions. This characteristic is what makes Fehling's test useful in distinguishing between primary and secondary alcohols, where primary alcohols (through their aldehyde intermediates) will give a positive result, while secondary alcohols will not.