Problem 123 - A Complete Resource Book in Chemistry for JEE Main

Question

The compound 'A' when treated with methyl alcohol and few drops of \(\mathrm{H}_{2} \mathrm{SO}_{4}\) gave smell of winter green. The compound 'A' is (a) oxalic acid (b) tartaric acid (c) salicylic acid (d) succinic acid

Step-by-Step Solution

Answer

Compound 'A' is salicylic acid (c).

Step 1: Understanding the Reaction

The reaction described involves the treatment of compound 'A' with methyl alcohol and sulfuric acid, producing a wintergreen smell. This is indicative of the formation of methyl salicylate, also known as oil of wintergreen.

Step 2: Recognizing Chemical Clues

Methyl salicylate is an ester commonly formed from salicylic acid when it reacts with methanol. The reaction proceeds as an esterification, catalyzed by \(\mathrm{H}_{2} \mathrm{SO}_{4}\).

Step 3: Identifying Compound 'A'

Since the reaction produces methyl salicylate, compound 'A' has to be salicylic acid because it is the precursor to methyl salicylate in the presence of methanol and an acid catalyst.

Key Concepts

Esterification

Esterification is a fascinating chemical reaction where an alcohol and an acid combine to form an ester. This process releases water as a byproduct. It's a central reaction in organic chemistry, allowing the transformation of one type of molecule into another.

In our context, esterification involves reacting an acid like salicylic acid with an alcohol, such as methanol. This reaction is quite common in organic synthesis due to its simplicity and efficiency.

The esterification process can be represented by the general chemical equation:

• Carboxylic Acid + Alcohol → Ester + Water

To make this reaction occur, an acid catalyst is usually added to accelerate the reaction. The acid donates protons, facilitating the union of the alcohol and the acid, forming the ester. In many lab settings, this reaction is a fundamental experiment that highlights how molecular structures can be altered through chemical reactions.

Methyl Salicylate

Methyl salicylate, often recognized by its strong minty fragrance, is commonly known as oil of wintergreen. It is an ester that emerges from the esterification of salicylic acid with methanol.

This compound is not just a laboratory curiosity — it has practical applications in everyday life.

• Used in flavorings, perfumery, and as a liniment for sore muscles.
• It has anti-inflammatory properties and is often found in topical analgesics.

The formation of methyl salicylate from salicylic acid is a classic example of how chemical manipulation can create substances with entirely different properties and uses.

Detecting the formation of methyl salicylate through its distinctive wintergreen scent is a neat way to demonstrate the reaction. This smell acts as a sensory indicator of the successful completion of the esterification process. The esterification of salicylic acid to form methyl salicylate shows the versatility of high school chemistry experiments in teaching real-world applications.

Sulfuric Acid Catalysis

Sulfuric acid (\(\mathrm{H}_{2} \mathrm{SO}_{4}\)) plays a pivotal role in facilitating the esterification reaction. It's used as a catalyst that speeds up the chemical reaction without being consumed by it. This strong acid acts by donating protons (\( H^+ \)) which aids in the breaking and forming of bonds during the esterification.

Here's how it works:

• The proton from sulfuric acid protonates the carbonyl oxygen of the carboxylic acid, increasing the positive charge on the carbonyl carbon.
• This positive charge makes it easier for the alcohol's oxygen to attack, forming a tetrahedral intermediate.
• The intermediate collapses, leading to the formation of the ester and the release of water.

The presence of sulfuric acid not only hastens the reaction speed but also often increases the yield of the ester. This efficiency makes sulfuric acid invaluable in industrial and laboratory settings for conducting esterification reactions

In the case of forming methyl salicylate, sulfuric acid ensures that salicylic acid reacts efficiently with methanol to produce that characteristic wintergreen smell.