Problem 1 - A Complete Resource Book in Chemistry for JEE Main

Question

Aldehydes and ketones are distinguished by using (a) Tollen's reagent (b) Lucas reagent (c) Borshe reagent (d) all of these

Step-by-Step Solution

Answer

Use Tollen's reagent to distinguish aldehydes from ketones.

Step 1: Recall Reagent Purposes

Begin by recalling the purpose of each reagent listed in the options. Tollen's reagent is used to distinguish aldehydes from ketones. Lucas reagent is used for distinguishing alcohols with different classes (primary, secondary, tertiary). Borshe reagent is generally a less common name, possibly mislabelled here.

Step 2: Focus on Aldehydes and Ketones

Since the question asks about distinguishing aldehydes and ketones, focus specifically on reagents used in this context. Identify Tollen's reagent as the reagent known for this specific purpose, as it produces a silver mirror with aldehydes but not with ketones.

Step 3: Confirm Answer

Verify that Tollen's reagent is used to create a silver mirror with aldehydes, but not with ketones. Lucas reagent does not apply to aldehydes and ketones, confirming it's not suitable for this distinction.

Key Concepts

Tollen's reagent

Tollen's reagent, a solution containing silver nitrate ( AgNO_3 ) and ammonia, is a fascinating tool in organic chemistry. It is used to distinguish between aldehydes and ketones. The reagent works on the premise that aldehydes can be easily oxidized, whereas ketones cannot. Here's how it functions:

• When an aldehyde is present, Tollen's reagent is reduced to metallic silver.
• This causes a characteristic formation of a shiny, silver mirror on the inner wall of the test tube.

In contrast, ketones do not react with Tollen's reagent, which helps chemists quickly determine the presence of an aldehyde through simple visual inspection. Tollen's reagent is especially valued for its simplicity and specific selectivity for aldehydes over ketones.

Lucas reagent

Although not directly used for distinguishing aldehydes and ketones, Lucas reagent is a notable pitstop in the journey of organic chemistry reagents. A mixture of concentrated hydrochloric acid ( HCl ) and zinc chloride ( ZnCl_2 ), it is primarily utilized to distinguish primary, secondary, and tertiary alcohols.

• Lucas reagent reacts with alcohols to form alkyl chlorides, creating a cloudy solution if a reaction occurs.
• Tertiary alcohols react almost immediately giving a cloudy appearance within seconds to minutes.
• Secondary alcohols form cloudiness more slowly.
• Primary alcohols either react slowly or not at all, keeping the solution clear for a longer time.

Lucas reagent is a useful tool, especially when quickly categorizing alcohols, but it doesn't play a role in distinguishing aldehydes from ketones.

Silver mirror test

The silver mirror test is another name for the reaction involving Tollen's reagent to detect aldehydes. This test is named for the striking silver mirror that forms when an aldehyde is present.

The way it works is straightforward and visually remarkable:

• When the reagent is mixed with an aldehyde solution, it oxidizes the aldehyde to a carboxylic acid.
• Simultaneously, the silver ions from the reagent are reduced to metallic silver.
• This silver deposits on the container wall, forming a mirror-like coating.

This test is a highly reliable method for confirming the presence of aldehydes due to its distinctive visual cue. Furthermore, it holds educational value for illustrating redox reactions, where one substance is oxidized and another is reduced. However, it's important to note that because ketones are more difficult to oxidize, they do not give a similar result and hence are differentiated from aldehydes through this method.