Question

Presence of unsaturation in orgenic compounds cun be tested with (a) Fehling's reagent (b) Tollens' reagent (c) Baeyer's reagent (d) Fittig's reaction.

Short answer

(c) Baeyer's reagent

Step-by-step solution

Understanding the Use of Reagents

Presence of unsaturation in organic compounds, such as double or triple bonds, can be tested using specific reagents that react with these types of bonds.

Analyzing Each Option

(a) Fehling's reagent is used to test for the presence of an aldehyde functional group. (b) Tollens' reagent is also used to test for aldehydes, giving a silver mirror with aldehydic compounds. (c) Baeyer's reagent, which is dilute potassium permanganate (KMnO_4), is used to test for unsaturation. It decolorizes when there are double or triple bonds present as they are oxidized to diols. (d) Fittig's reaction involves the coupling of aryl halides with metallic sodium in the presence of a dry ether solvent, which is not a test for unsaturation.

Determining the Correct Reagent for Unsaturation

Based on the analysis of each option, Baeyer's reagent is specifically used for testing the presence of unsaturation in organic compounds due to its ability to react with double or triple bonds.

Key concepts

Baeyer's Reagent

Understanding the role of Baeyer's reagent in detecting unsaturation can simplify the study of organic chemistry. This reagent is an aqueous solution of potassium permanganate (KMnO_4) and sometimes mixed with sodium carbonate (Na_2CO_3) to make it mildly alkaline. Unsaturated compounds such as alkenes and alkynes readily engage in an addition reaction with KMnO_4, causing the purple color of the reagent to fade or become brown. This is due to the conversion of the unsaturated bond into a diol or hydroxyl groups attached to the same carbon.
For students, remembering the color change to a brownish precipitate or discoloration can be a helpful indicator that the test for unsaturation was positive.

• Positive Test: Discoloration to brown precipitate (unsaturation is present).
• Negative Test: No color change (compound is likely saturated).

By associating Baeyer's reagent with the concept of unsaturation in organic compounds, it becomes much easier to grasp its practical application in experiments.

Fehling's Reagent

When diving into the specifics of Fehling's reagent, we encounter a compound that serves a different purpose than testing for unsaturation. Fehling’s reagent comprises two solutions: Fehling's A, which is a solution of copper(II) sulfate (CuSO_4), and Fehling’s B, which is a mixture of potassium sodium tartrate (KNaC_4H_4O_6) and strong alkali like sodium hydroxide (NaOH).
This reagent particularly targets aldehydes; when it is heated with an aldehyde, it causes a redox reaction. This leads to the reduction of the blue copper(II) ion to a red precipitate of copper(I) oxide (Cu_2O) and thereby indicates the presence of aldehyde groups.

• Positive Test: Red precipitate indicates aldehyde.
• Negative Test: No precipitate suggests the absence of an aldehyde group.

Remembering that aldehydes are the target and the color change to red is the goal, can help students discern the function of Fehling's reagent in organic chemistry experiments.

Tollens' Reagent

Moving onto Tollens' reagent, commonly referred to as the 'silver mirror test', this reagent is another tool for distinguishing aldehydes from other functional groups. Tollens' reagent is a solution of silver nitrate (AgNO_3) in ammonia (NH_3). When this reagent comes into contact with an aldehyde group under basic conditions, it oxidizes the aldehyde to a carboxylic acid and in the process, silver ions (Ag^+) are reduced to metallic silver, frequently depositing a reflective coating on the test tube.
The silver mirror effect serves as a hallmark of this test and is also a useful visual aid for students studying functional group analysis:

• Positive Test: Silver mirror indicates an aldehyde.
• Negative Test: No silver deposit implies no aldehyde is present.

Instructive for the identification of aldehydes, Tollens' reagent is a prime example of how specific reagents in organic chemistry are utilized to reveal particular molecular features.

Organic Chemistry Reagents

Broadly speaking, organic chemistry reagents are substances or mixtures used in chemical reactions to detect, measure, examine, or produce other substances in organic chemistry. Each reagent is like a key tailored to unlock information about specific molecular structures or functional groups. For instance, while Baeyer's reagent detects unsaturation, Fehling's and Tollens' reagents are adept at signaling the presence of aldehydes in a compound.
Students should approach these reagents with the understanding that each one has a unique role:

• Baeyer's reagent test for double or triple bonds.
• Fehling's reagent indicates aldehyde groups.
• Tollens' reagent also tests for aldehydes, but does so in a way that results in a silver mirror.

Understanding the function and the result of each reagent can aid in mastering organic chemistry reactions and analyses. It's not just about memorizing the color changes or outcomes but also comprehending the chemistry behind why these changes occur, which will enhance problem-solving skills in the laboratory.