Problem 59 - A Complete Resource Book in Chemistry for JEE Main

Question

One can distinguish between \(\mathrm{HCOOH}\) and \(\mathrm{CH}_{3}\) COOH with (a) \(\mathrm{H}_{2} \mathrm{SO}_{4}\) (b) \(\mathrm{FeCl}_{3}\) (c) \(\mathrm{NaHCO}_{3}\) (d) Tollen's reagent

Step-by-Step Solution

Answer

(d) Tollen's reagent.

Step 1: Understanding the Compounds

Formic acid (\(\text{HCOOH}\)) and acetic acid (\(\text{CH}_3\text{COOH}\)) differ structurally and chemically. Formic acid has a simpler structure, whereas acetic acid has a methyl group attached to the carboxyl group. This difference can impact how each compound reacts with specific reagents.

Step 2: Analyze Each Reagent's Reaction

To distinguish between the acids, we evaluate how each reagent reacts with the acids: (a) \(\text{H}_2\text{SO}_4\) is a strong acid; both formic and acetic acid will not show characteristic differences when reacted with \(\text{H}_2\text{SO}_4\).(b) \(\text{FeCl}_3\) is used for detecting phenols, not suitable for distinguishing these acids.(c) Both acids react with \(\text{NaHCO}_3\), producing carbon dioxide gas; no distinct reaction.(d) Tollen's reagent is primarily used to test aldehydes. However, formic acid acts as an aldehyde and will reduce Tollen's reagent to form a silver mirror, while acetic acid will not.

Step 3: Identify the Distinguishing Reagent

From the analysis, only Tollen's reagent (option d) can be used to distinguish between \(\text{HCOOH}\) and \(\text{CH}_3\text{COOH}\). Formic acid can reduce Tollen's reagent due to its aldehyde-like properties, resulting in a silver mirror, whereas acetic acid will not induce this change.

Key Concepts

Tollen's Reagent

Tollen's reagent is a chemical solution specifically designed to test for the presence of aldehydes in a compound. This reagent is made by combining ammoniacal silver nitrate, creating a solution that contains silver ions. When an aldehyde is present, it reduces the silver ions to metallic silver. This unique reaction forms a reflective silver mirror on the inside of the test tube, which is a tell-tale indication of an aldehyde presence.

Interestingly, formic acid (chemically known as \( ext{HCOOH}\)) behaves similarly to an aldehyde. This is due to the direct linkage of its carbon atom to a hydrogen atom, a characteristic it shares with aldehydes. Therefore, when formic acid is tested with Tollen's reagent, it will indeed form a silver mirror. In contrast, acetic acid does not participate in this reaction, making Tollen's reagent an effective tool to distinguish between these two acids.

Formic Acid

Formic acid is the simplest carboxylic acid with the chemical formula \( ext{HCOOH}\). It has a unique distinction among carboxylic acids because of its aldehyde-like behavior. This distinctive behavior is due to its chemical structure, which allows it to reduce mild oxidizing agents such as Tollen's reagent.

Formic acid is commonly found in nature and plays a role in various biological processes. It's produced by ants as part of their chemical defense strategy. In industrial applications, formic acid is used for leather finishing and as a preservative in livestock feed. Its ability to act like an aldehyde makes it useful in scientific settings for specific chemical reactions and tests.

• Natural source: Ant venom
• Common usage: Leather finishing, preservative
• Characteristic reaction: Silver mirror with Tollen's reagent

Acetic Acid

Acetic acid, with the molecular formula \( ext{CH}_3 ext{COOH}\), is a more complex carboxylic acid compared to formic acid. It contains a methyl group attached to its carboxyl group, distinguishing it structurally and behaviorally from formic acid.

Unlike formic acid, acetic acid does not have the aldehyde-like properties required to reduce Tollen's reagent, which is why it doesn't form a silver mirror. Acetic acid is popularly known as the main component of vinegar, providing a sour taste and pungent smell. It is used extensively in the food industry as a condiment and preservative.

• Common form: Vinegar
• Key feature: No reaction with Tollen's reagent
• Industrial uses: Food additive, chemical synthesis

This distinction makes acetic acid easily identifiable from formic acid when using Tollen’s reagent.