Problem 166 - A Complete Resource Book in Chemistry for JEE Main

Question

Select wrong statements: (a) Phenols turn blue litmus to red. (b) Reactivity of methanol with sodium metal is more than that of isopropyl alcohol (c) Methanol gives iodoform test. (d) Phenol reacts with \(\mathrm{Na}_{2} \mathrm{CO}_{3}\) and liberates \(\mathrm{CO}_{2}\) gas

Step-by-Step Solution

Answer

Statements (c) and (d) are wrong.

Step 1: Analyze Phenols and Litmus Test

Phenols are weak acids and generally turn blue litmus paper to red, indicating their acidic nature. Thus, statement (a) is correct as phenols do cause blue litmus to turn red.

Step 2: Evaluate Reactivity of Alcohols

The reactivity of alcohols with sodium depends on the structure. Methanol (CH3OH) is a primary alcohol and reacts more vigorously with sodium compared to isopropyl alcohol, which is a secondary alcohol. Thus, statement (b) is correct.

Step 3: Check for Iodoform Test

Methanol (CH3OH) does not give a positive iodoform test as it does not have a methyl ketone group or a structure leading to the necessary intermediate. Thus, statement (c) is incorrect.

Step 4: Examine Reaction of Phenol with Sodium Carbonate

Phenol, being a weak acid, does not react with sodium carbonate (Na2CO3) to release CO2 gas, as it is not acidic enough to displace carbonic acid (H2CO3). Therefore, statement (d) is incorrect.

Key Concepts

Phenol Acidity

Phenols are a group of chemical compounds that consist of a hydroxyl group (\(\text{-OH}\)) attached directly to an aromatic ring. This configuration is what gives phenols their unique properties, including their acidity. Phenols are considered to be weak acids. This classification is due to their ability to ionize slightly in water, producing phenoxide ions and hydrogen ions.

When phenol is dissolved in water, the release of hydrogen ions creates a mildly acidic solution that can turn blue litmus paper to red. This simple litmus test indicates the acidic nature of phenol.

Despite being a weak acid, phenol's acidity is more pronounced than that of aliphatic alcohols, due to the stabilization of the phenoxide ion by resonance distribution of the negative charge throughout the aromatic ring. Understanding phenol acidity is crucial because it guides how phenol interacts with other chemicals, especially bases.

Iodoform Test

The iodoform test is a chemical reaction used to identify certain alcohols or ketones possessing a methyl group in their molecular structure. Specifically, it detects for the presence of the \(\text{-COCH}_3\) group.

This test is particularly useful in organic chemistry because it provides insight into a compound's structure based on the reaction outcome.

Methanol (\(\text{CH}_3\text{OH}\)) does not give a positive iodoform test. This is because methanol lacks the necessary methyl ketone group or the potential to form such a group, which is essential for the iodoform reaction to occur. Only alcohols with structures like isopropanol or ethanol, which can be oxidized to a methyl ketone, will afford a positive result, producing a yellow precipitate of iodoform (\(\text{CHI}_3\)).

• The presence of a methyl ketone group is tested by adding iodine and a base, like NaOH.
• If a compound, such as acetone, is present, the solution yields a yellow precipitate confirming a positive test.

Thus, methanol’s structure disallows such a transformation, leading to a negative iodoform test result.

Sodium Carbonate Reaction

The interaction of phenol with sodium carbonate (\(\text{Na}_2\text{CO}_3\)) reveals insights into phenol's acidity. Although phenol is classified as a weak acid, it is not strong enough to react with sodium carbonate effectively.

Sodium carbonate is a salt derived from carbonic acid, and it typically reacts with acids to release carbon dioxide gas. However, because phenol does not have sufficient acidic strength, it cannot displace carbonic acid (\(\text{H}_2\text{CO}_3\)) to liberate \(\text{CO}_2\) gas.

This characteristic provides an indication of relative acid strength.

• Phenol will not cause effervescence when mixed with \(\text{Na}_2\text{CO}_3\).
• Instead, stronger acids like hydrochloric acid will react immediately, releasing bubbles of carbon dioxide.

This concept is foundational in distinguishing phenolics from more acidic substances, contributing to a better understanding of chemical reactivity patterns.