Problem 103 - A Complete Resource Book in Chemistry for JEE Main

Question

Which is not true about Libermann nitroso test? (a) Here the blue colour is due to formation of sodium salt of indophenol (b) It is given by nitrophenols (c) It is also given by \(\mathrm{p}\)-substituted phenols (d) Both (b) and (c)

Step-by-Step Solution

Answer

Statement (d) is not true.

Step 1: Understand the Libermann Nitroso Test

The Libermann Nitroso Test is a chemical test used to detect phenols, especially para-substituted phenols, by producing a blue or green color upon oxidation. This test involves the formation of indophenol sodium salt, which is responsible for the blue coloration.

Step 2: Evaluate statement (a)

Statement (a) states that the blue color is due to the formation of the sodium salt of indophenol. This is true because the characteristic blue color in the Libermann Nitroso Test is indeed due to this reaction.

Step 3: Evaluate statement (b)

Statement (b) claims that the test is given by nitrophenols. Nitrophenols, particularly ortho-nitrophenols, can also give a positive test. However, not all nitrophenols might lead to the characteristic positive result expected from para-substituted phenols.

Step 4: Evaluate statement (c)

Statement (c) suggests that the test is given by para-substituted phenols. This is true as the Libermann Nitroso test is known for detecting para-substituted phenols specifically, due to the substitution pattern enhancing the formation of indophenol sodium salt.

Step 5: Evaluate statement (d)

Statement (d) states that both (b) and (c) are incorrect. Since (c) is actually true while (b) is less definitively true, statement (d) must also be incorrect because it incorrectly asserts that (c) is untrue.

Key Concepts

Understanding Phenols

Phenols are a class of chemical compounds that consist of a hydroxyl group (-OH) bonded directly to an aromatic hydrocarbon. Unlike alcohols, the aromatic ring of phenol contributes to its unique chemical properties.
Phenols are known for being more acidic than typical alcohols. This is due to the ability of the aromatic ring to stabilize the negative charge on the oxygen after the hydrogen ion (H\(^+\)) is released.
The acidity of phenols makes them react readily with bases, forming phenoxide salts.

• This characteristic is exploited in various chemical tests, including the Libermann Nitroso Test.
• The presence of the hydroxyl group also makes phenols reactive towards electrophiles.
• Phenols can participate in oxidation reactions, aromatic substitution reactions, and form esters when reacted with acids.

Phenols show unique color changes in chemical tests due to their ability to form stable complexes and conjugated systems.

Exploring Para-Substituted Phenols

Para-substituted phenols are a specific type of phenols where a substituent other than hydrogen is positioned opposite the hydroxyl group on the aromatic ring. The para position is known for having significant effects on the chemical behavior of these compounds
due to its influence on resonance and electron distribution.
The para substitution can affect the acidity, stability, and reactivity of the phenol compound.

• For example, electron-withdrawing groups in the para position generally make the phenol more acidic by stabilizing the phenoxide ion.
• Conversely, electron-donating groups can decrease acidity, making the phenol less reactive towards bases.

The Libermann Nitroso Test is specifically responsive to para-substituted phenols.
The unique positioning of the substituent enhances the formation of indophenol sodium salt, which is crucial for the characteristic color change observed in the test.

The Role of Indophenol Sodium Salt

Indophenol sodium salt is a compound formed during the Libermann Nitroso Test, which gives the test its distinctive blue coloration. This salt is a result of the oxidation reaction that involves phenols, particularly para-substituted phenols.
The formation of indophenol sodium salt is facilitated by the interaction of the phenoxide ion with nitrous acid, resulting in the production of this colored compound.

• The distinctive blue color is due to the presence of a conjugated system in the indophenol sodium salt, allowing for significant absorption of light in the visible spectrum.
• This indicates a positive result in the test and confirms the presence of phenols, especially when para-substituted.

The understanding of this color change is critical for students, as it helps identify phenolic compounds efficiently in laboratory settings.