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 \(p\)-substituted phenols (d) Both (b) and (c)

Step-by-Step Solution

Answer

(b) It is given by nitrophenols. This is incorrect.

Step 1: Understand the Libermann Nitroso Test

The Libermann Nitroso Test is used to detect phenols by producing a blue to green color upon reaction. This color change is due to the formation of a compound involving a sodium salt of indophenol, which is an indicator of phenols, especially para-substituted phenols.

Step 2: Analyze Option (a)

This option states that the blue color in the Libermann Nitroso Test is due to the formation of sodium salt of indophenol. This statement is true because the characteristic color change is actually due to the formation of this sodium salt.

Step 3: Analyze Option (b)

Option (b) claims the test is given by nitrophenols. However, nitrophenols typically do not respond to this test as their strongly withdrawing nitro group affects the reactivity needed for the test. Thus, this statement is false.

Step 4: Analyze Option (c)

Option (c) indicates that the test is also given by p-substituted phenols. This is true because the Libermann Nitroso Test is indeed well-known for detecting para-substituted phenols, as they participate in the reaction readily.

Step 5: Analyze Option (d)

Option (d) suggests both (b) and (c) are not true, but since option (c) is true, option (d) itself is false because both statements are not wrong. Therefore, the correct evaluation should focus primarily on the false nature of option (b).

Key Concepts

Phenols Detection

The Libermann Nitroso Test is a chemical test specifically designed to detect phenols, compounds featuring an aromatic ring bonded directly to a hydroxyl group. Phenols distinguish themselves by their ability to engage in unique reactions such as the one in this test, which is not typical of other alcohols.

During the test, phenols react with nitrous acid to form indophenol, a reaction that results in striking color change, usually a blue or green hue. This color changes serve as a visual indicator, making it very simple to confirm the presence of phenols.

Detecting phenols accurately is important in various fields, including organic chemistry and biochemistry, due to their prevalence and significance in both natural and synthetic compounds. Therefore, tests like the Libermann Nitroso are fundamental in the quick and reliable identification of these molecules.

Sodium Salt of Indophenol

The blue color in the Libermann Nitroso Test results from the formation of a compound called the sodium salt of indophenol. This compound is produced when phenols react under the test conditions.

Indophenol, specifically in its sodium salt form, is stable and readily observable due to its distinct coloration. This is why it is crucial to the method's effectiveness in indicating phenols.

Importantly, the formation of sodium salt of indophenol is indicative only for phenolic compounds like para-substituted phenols and not for other groups that contain similar functional groups, which adds to the test's specificity and utility.

Para-substituted Phenols

Para-substituted phenols are a specific type of phenol where a substituent group is in the para position relative to the hydroxyl group on the aromatic ring. This particular positioning often enhances the reactivity required for the Libermann Nitroso Test.

The test is specially sensitive to these compounds, making it a preferred method when trying to reliably detect their presence. Their reactivity in the phenol detection mechanism makes them prime candidates to yield the characteristic indophenol compound.

This enhances the blue to green color change, which is the hallmark of a positive result in the Libermann Nitroso Test, ensuring that para-substituted phenols show conclusive results.

False Test Results in Nitrophenols

Nitrophenols are special variants of phenols that include a nitro group, which significantly alters their chemical properties. Due primarily to this nitro group's electron-withdrawing nature, nitrophenols do not tend to participate positively in the Libermann Nitroso Test.

In fact, the presence of the nitro group often affects the overall reactivity required to produce indophenol, hence misleading or false-negative results may occur. This creates complications when using traditional phenol detection methods for these compounds.

Understanding this limitation is crucial, especially in complex chemical analyses or when testing compounds expected to have mixtures containing both phenolic and nitrophenolic components.