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

Option (b) is not true.

Step 1: Understand the question

We need to identify which of the given statements about the Libermann nitroso test is not true. The options describe various aspects of this test.

Step 2: Analyze Option (a)

Option (a) states that the blue color observed in the test is due to the formation of sodium salt of indophenol. In the Libermann nitroso test, this is true, as the test results in a blue color due to this compound's formation.

Step 3: Analyze Option (b)

Option (b) claims that the test is given by nitrophenols. This is false, as nitrophenols typically do not give a positive result in the Libermann nitroso test.

Step 4: Analyze Option (c)

Option (c) states that the test is also given by para-substituted phenols. This is true, as para-substituted phenols can participate in the Libermann nitroso test, resulting in color changes.

Step 5: Analyze Option (d)

Option (d) combines (b) and (c), effectively stating that both options do not apply to the test. Since Option (b) is incorrect and Option (c) is correct, Option (d) is not completely correct.

Key Concepts

Indophenol

Indophenol is an important compound associated with the Libermann nitroso test. If you're curious about why the test shows a blue color, here's why: This blue color is due to the formation of indophenol. Specifically, during the test, the reaction takes place leading to the creation of a blue compound, which is actually the sodium salt of indophenol.
Indophenols are compounds where the presence of a nitrogen group plays a crucial role in the coloring process.

• In chemical terms, indophenol refers to a group of dyes characterized by this distinct blue or green color.
• These dyes are widely used as indicators because of their strong color change property.

For students, understanding this process is not just about memorizing reactions but linking the visual outcome with chemical transformations in the lab. Such connections make learning chemistry both functional and fascinating.

Nitrophenols

Nitrophenols, while relevant to many chemical discussions, particularly react differently in the context of the Libermann nitroso test. Unlike the para-substituted phenols that may react in this test, nitrophenols are less likely to give a positive result.
In chemistry, nitrophenols are distinguished by a nitro group ( ) bonded to a phenol molecule. This structure influences their properties significantly leading to:

• They exhibit strong acidity compared to basic phenols.
• They have a higher tendency to engage in substitution reactions rather than addition reactions.

For students trying to connect the dots between structure and reactivity, it’s key to remember that not all phenol-derived compounds behave similarly in all reaction settings. Nitrophenols, in this test, exemplify this difference strikingly.

Para-Substituted Phenols

Para-substituted phenols are phenol molecules with a substituting group positioned at the para location relative to the hydroxyl group.
These compounds play a notable role in the chemical reactions they can partake in, including the Libermann nitroso test. Their effective engagement in such reactions is crucial for identifying compounds in lab tests due to several features:

• They can engage in coupling reactions, thanks to their reactive spots.
• The para position is spatially favorable for many reactions, making it a common pattern in chemical transformations.

When studying these phenols, remember that the para positioning significantly impacts their chemical behavior. This positioning allows for better stabilization and distribution of charge during reactions, influencing how these compounds interact under testing conditions.