Problem 62 - A Complete Resource Book in Chemistry for JEE Main

Question

A and B react with Na gives \(\mathrm{H}_{2}\) gas and by reaction of both \(\mathrm{A}\) and \(\mathrm{B}\) ethyl acetate is formed then \(\mathrm{A}\) and \(\mathrm{B}\) are (a) \(\mathrm{CH}_{3} \mathrm{COOH}, \mathrm{C}_{2} \mathrm{H}_{5} \mathrm{OH}\) (b) \(\mathrm{CH}_{3} \mathrm{COOH}, \mathrm{CH}_{3} \mathrm{OH}\) (c) \(\mathrm{C}_{3} \mathrm{H}_{7} \mathrm{COOH}, \mathrm{C}_{3} \mathrm{H}_{7} \mathrm{OH}\) (d) \(\mathrm{HCOOH}, \mathrm{CH}_{3} \mathrm{COOH}\)

Step-by-Step Solution

Answer

Option (a) \(\mathrm{CH}_{3}\mathrm{COOH}, \mathrm{C}_{2}\mathrm{H}_{5}\mathrm{OH}\) is the correct answer.

Step 1: Understanding the Reaction with Na

When A and B react with sodium (Na) to form hydrogen gas \( \mathrm{H}_{2} \), it indicates that both A and B must contain acidic hydrogen that can react to release \( \mathrm{H}_{2} \). Carboxylic acids and alcohols both have this ability because they contain hydrogen ions that can react with sodium to form hydrogen gas.

Step 2: Identifying the Reaction that Forms Ethyl Acetate

Ethyl acetate is formed by the reaction between acetic acid \(\mathrm{CH}_{3}\mathrm{COOH}\) and ethanol \(\mathrm{C}_{2}\mathrm{H}_{5}\mathrm{OH}\). This is an esterification reaction where acetic acid acts as the acid and ethanol acts as the alcohol to form the ester ethyl acetate and water.

Step 3: Matching with Given Options

Based on the information, A must be acetic acid \(\mathrm{CH}_{3}\mathrm{COOH}\) and B must be ethanol \(\mathrm{C}_{2}\mathrm{H}_{5}\mathrm{OH}\) because they can both release hydrogen with sodium and react together to form ethyl acetate. Comparing with options, only option (a) \(\mathrm{CH}_{3}\mathrm{COOH}, \mathrm{C}_{2}\mathrm{H}_{5}\mathrm{OH}\) matches the criteria.

Key Concepts

Carboxylic Acids

Carboxylic acids are a class of organic compounds that contain a carboxyl group (-COOH). They are well-known for their acidic properties and ability to dissociate in water.
This dissociation ability is primarily due to the presence of an acidic hydrogen atom connected to an oxygen within the carboxyl group.
The most common example of a carboxylic acid is acetic acid ( \( \text{CH}_3 \text{COOH} \)).

Carboxylic acids are important in many chemical reactions and are very versatile in the world of chemistry. They can react with bases to form salts and can also participate in esterification reactions.

• They donate a proton (H⁺) easily due to the pull of electronegative oxygen atoms.
• The resulting anion (\( \text{COO}^- \)) is stabilized by resonance, making them acidic.
• Carboxylic acids also play a key role in biochemical pathways and are involved in making aromas and flavors.

Understanding carboxylic acids is vital for grasping their reactivity in esterification and reactions with metals like sodium.

Hydrogen Gas Formation

Hydrogen gas formation is a key indicator of certain chemical reactions involving the release of gas. When compounds with acidic hydrogen react with metals like sodium (Na), hydrogen gas (cH_{2}e) is often produced.
This happens because the reactive metal displaces hydrogen ions, causing them to pair up as hydrogen molecules and exit the compound as gas.

In the context of the reaction between compounds like carboxylic acids or alcohols with sodium,

• The hydrogen ions from these compounds combine to form hydrogen gas.
• This gas is lightweight, colorless, and flammable.
• The release of cH_{2}e serves as evidence of the substitution reaction occurring when a metal like sodium interferes.

Recognizing the formation of hydrogen gas assists students in understanding how certain compounds interact with one another and transform through chemical processes.

Acidic Hydrogen

Acidic hydrogen refers to hydrogen atoms in a molecule that can be easily ionized and released as a proton (\( \text{H}^+ \)) in reactions.
These hydrogens are typically attached to electronegative atoms, such as oxygen, which increase their tendency to lose hydrogen as a hydrogen ion.

In carboxylic acids, the hydrogen attached to the -OH group in the carboxyl is acidic.

• Acidic hydrogen is central to the acidity of compounds like acetic acid (cCH_{3}ecCOOHe).
• When an acidic hydrogen releases a proton, the remaining molecule becomes more stable owing to resonance or electron distribution.
• These protons can react to form other compounds, such as salts or gases, when paired with suitable reactants.

Grasping the concept of acidic hydrogen is critical for understanding how these atoms affect the reactivity and properties of the molecules they are part of.

Sodium Reaction

The sodium reaction is a classic example of how metals interact with various compounds containing acidic hydrogens, such as alcohols or acids.
When sodium comes into contact with these compounds, it can lead to vigorous reactions that release energy and form hydrogen gas.

This reaction shows the ability of sodium to strip the \( \text{H}^+ \) from the compound and combine two \( \text{H}^+ \) ions to form cH_{2}e gas.

• This is known as a reaction with a reactive metal, where sodium acts to displace hydrogen.
• It is energetic and visually noticeable due to the production of bubbles (from gas) and sometimes even heat.
• This process is utilized to demonstrate metal reactivity and energy changes in chemical reactions.

Understanding how sodium reacts with compounds helps explain the broader category of chemical reactivity and how new products, like hydrogen gas, are formed.