Problem 9 - Arihant Objective Approach to Organic Chemistry for IIT JEE and NEET all Medical Entrances Part 4

Question

Which of the following is incorrect? ICBSE (a) \(\mathrm{FeCl}_{3}\) is used in detection of phenol (b) Fehling solution is used in detection of glucose (c) Tollen reagent is used in detection of unsaturation (d) \(\mathrm{NaHSO}_{3}\) is used in detection of carbonyl compound

Step-by-Step Solution

Answer

Option (c) is incorrect because Tollen's reagent is not used for detecting unsaturation.

Step 1: Analyze Option (a)

The compound \( \mathrm{FeCl}_{3} \) is indeed used in the detection of phenol. When phenol is mixed with an iron (III) chloride solution, a violet or purple color appears, indicating the presence of phenol. Therefore, option (a) is correct.

Step 2: Analyze Option (b)

The Fehling solution is a chemical reagent used to distinguish between water-soluble carbohydrate and ketone functional groups. It is specifically used in the detection of reducing sugars such as glucose. The solution changes to a brick-red precipitate in the presence of glucose, confirming reducing sugars. Thus, option (b) is correct.

Step 3: Analyze Option (c)

Tollen's reagent, also known as the silver mirror test, is primarily used to detect aldehydes. It is not used for detecting unsaturation (like double or triple bonds). Thus, option (c) is incorrect because Tollen's reagent isn't used for unsaturation detection.

Step 4: Analyze Option (d)

\( \mathrm{NaHSO}_{3} \) is utilized for the detection and identification of carbonyl compounds. It forms bisulfite addition complexes with aldehydes and certain ketones, indicating the presence of a carbonyl group. Hence, option (d) is correct.

Key Concepts

FeCl3 detection of phenol

Ferric chloride, also known as \( \text{FeCl}_3 \), plays a pivotal role in the identification of phenolic compounds. When phenol is present, the addition of \( \text{FeCl}_3 \) leads to the formation of a complex that exhibits a distinct violet or purple coloration. This color change serves as a qualitative indicator of phenol due to the iron ion's interaction with the oxygen in the phenolic hydroxyl group.

This reaction is widely used because it is both rapid and highly sensitive. Here is how it works:

• The hydroxyl group in phenol forms a complex with \( \text{Fe}^{3+} \) ions.
• A noticeable violet color indicates the presence of phenolic hydroxyl groups.

Whether in a laboratory or academic setting, the \( \text{FeCl}_3 \) test is a classic and efficient method to spot phenols.

Fehling solution and glucose

Fehling's solution is an essential tool in organic chemistry when it comes to detecting reducing sugars, such as glucose. This solution consists of two separate solutions that are mixed just before conducting the test: Fehling's A (contains copper sulfate) and Fehling's B (contains a mixture of sodium potassium tartrate and sodium hydroxide).

When glucose is present in a solution, this is what happens during a Fehling's test:

• Glucose reduces the copper(II) ions to copper(I) ions.
• This reduction leads to the formation of a red brick precipitate of copper(I) oxide \((\text{Cu}_2\text{O})\).

This appearance of a red precipitate indicates the presence of glucose, making Fehling's solution a reliable chemical test for detecting aldehyde functional groups in sugars.

Tollen reagent and aldehydes

Tollen's reagent, commonly associated with its 'silver mirror' test, is highly effective in identifying aldehydes. Unlike ketones, aldehydes can be oxidized easily, and Tollen's reagent exploits this property to reveal their presence. The reagent itself is made up of a solution containing silver nitrate \((\text{AgNO}_3)\) and ammonia.

When testing with Tollen's reagent, the reaction follows this process:

• An aldehyde reduces silver ions \((\text{Ag}^+)\) to metallic silver \((\text{Ag})\).
• The production of silver forms a shiny silver mirror deposit on the inner walls of the reaction vessel.

If you see this unmistakable silver coating, it confirms you've got an aldehyde. This method doesn't work with ketones, as they aren't as easily oxidized, offering specificity to detecting aldehydes.

NaHSO3 and carbonyl compounds

Sodium bisulfite \((\text{NaHSO}_3)\) is an excellent reagent for detecting carbonyl groups in organic compounds. Carbonyl groups include both aldehydes and ketones, although its applications are mainly associated with aldehydes.

The interaction between \(\text{NaHSO}_3\) and carbonyl groups leads to the formation of bisulfite addition products or bisulfite complexes. This chemical behavior can be summarized as follows:

• A carbonyl group reacts with \(\text{NaHSO}_3\) to form a bisulfite compound.
• This reaction forms a white crystalline precipitate, which is a reliable indicator of the presence of certain aldehydes and ketones.

This method is widely employed for compound separation, as the bisulfite addition complexes are often water-soluble, enabling easy purification or isolation in an organic synthesis or analysis process.