Problem 98 - A Complete Resource Book in Chemistry for JEE Main

Question

An organic compound (A) reacts with sodium metal and forms (B). On heating with concentrated \(\mathrm{H}_{2} \mathrm{SO}_{4}\), (A) gives diethyl ether. (A) and (B) are respectively (a) \(\mathrm{C}_{2} \mathrm{H}_{5} \mathrm{OH}\) and \(\mathrm{C}_{2} \mathrm{H}_{5} \mathrm{ONa}\) (b) \(\mathrm{C}_{3} \mathrm{H}_{7} \mathrm{OH}\) and \(\mathrm{CH}_{3} \mathrm{ONa}\) (c) \(\mathrm{CH}_{3} \mathrm{OH}\) and \(\mathrm{CH}_{3} \mathrm{ONa}\) (d) \(\mathrm{C}_{4} \mathrm{H}_{9} \mathrm{OH}\) and \(\mathrm{C}_{4} \mathrm{H}_{9} \mathrm{ONa}\)

Step-by-Step Solution

Answer

(A) is \(\text{C}_2\text{H}_5\text{OH}\) and (B) is \(\text{C}_2\text{H}_5\text{ONa}\), option (a).

Step 1: Identify Reaction with Sodium

The compound (A) reacts with sodium metal to form compound (B). Typically, an alcohol reacts with sodium to form a sodium alkoxide and hydrogen gas. Therefore, (A) must be an alcohol, and (B) is the corresponding sodium alkoxide.

Step 2: Reaction with Concentrated H2SO4

Compound (A) is heated with concentrated \(\text{H}_2\text{SO}_4\) to form diethyl ether. This reaction is typical for ethanol, \(\text{C}_2\text{H}_5\text{OH}\), which undergoes dehydration in the presence of \(\text{H}_2\text{SO}_4\) to form ether. Thus, (A) should be \(\text{C}_2\text{H}_5\text{OH}\).

Step 3: Match the Compounds (A) and (B)

Based on the observations in Step 2, (A) is \(\text{C}_2\text{H}_5\text{OH}\). When \(\text{C}_2\text{H}_5\text{OH}\) reacts with sodium, the product formed is \(\text{C}_2\text{H}_5\text{ONa}\), a sodium alkoxide. Therefore, (B) should be \(\text{C}_2\text{H}_5\text{ONa}\). This matches option (a).

Step 4: Verify the Chosen Option

Option (a) suggests \(\text{C}_2\text{H}_5\text{OH}\) and \(\text{C}_2\text{H}_5\text{ONa}\) as (A) and (B) respectively. This fits the reaction conditions provided: \(\text{C}_2\text{H}_5\text{OH}\) forming diethyl ether under the given conditions matches the requirement, confirming that option (a) is correct.

Key Concepts

Alcohol

Alcohols are organic compounds characterized by one or more hydroxyl groups (-OH) attached to a carbon atom. A common example of an alcohol is ethanol \( ext{C}_2 ext{H}_5 ext{OH}\).

Here are some features of alcohols:

• They are polar molecules due to the presence of the hydroxyl group, which forms hydrogen bonds with water, making low-molecular-weight alcohols soluble in water.
• Alcohols can react with metals, such as sodium, to produce hydrogen gas and a compound called a sodium alkoxide. This property is used to determine the alcohol type in a reaction.
• They display both acidic and basic properties, but are generally considered weak acids compared to other acid groups.

Recognizing alcohols and their ability to form alkoxides is crucial in many organic synthesis reactions.

Ether Formation

The process of ether formation is a key transformation in organic chemistry, allowing for the creation of ethers, which are characterized by an oxygen atom connected to two alkyl or aryl groups. One classical method to form ethers is the dehydration of alcohols.

Notably, diethyl ether can be synthesized by heating ethanol \( ext{C}_2 ext{H}_5 ext{OH}\) with a strong acid like concentrated sulfuric acid \( ext{H}_2 ext{SO}_4\).

• The reaction removes a molecule of water (hence, 'dehydration') to join two alcohol molecules into an ether (\( ext{C}_2 ext{H}_5 ext{O} ext{C}_2 ext{H}_5\)).
• This specific reaction is known as "Williamson ether synthesis" when it's carried out with alcohols and a dehydrating agent.
• Ethers are generally less reactive than alcohols, but they play important roles as solvents in chemical reactions.

Ether formation through dehydration illustrates how versatile alcohols can be in synthesis pathways.

Sodium Alkoxide Reaction

Sodium alkoxide formation is a classic reaction where an alcohol reacts with sodium metal. This is a suitable method to study how alcohols behave under basic conditions.

Here's how it works:

• When ethanol, for example, reacts with sodium, it forms sodium ethoxide (\( ext{C}_2 ext{H}_5 ext{ONa}\)) plus hydrogen gas (\( ext{H}_2\)).
• This reaction confirms the presence of an alcohol because only compounds with hydrogen-containing ionizable groups, like \(-OH\), will liberate \( ext{H}_2\) when reacting with active metals.
• The alkoxide ion \(( ext{RO}^{-})\) formed is a strong base and nucleophile, playing a significant role in further synthesis, such as Williamson ether synthesis.

Understanding alkoxide ion formation is central to predicting behavior in various organic reactions.

Dehydration Reaction

Dehydration reactions are transformative processes in organic chemistry where water is removed from a molecule. For alcohols, this typically involves heating with a strong acid such as sulfuric acid.

Let's dive into some specifics:

• In the case of ethanol, dehydration results in the formation of diethyl ether or, if conditions favor different pathways, could lead to the production of different alkenes.
• Dehydration is an elimination reaction that often results in the rearrangement of molecules to form more stable products.
• This type of reaction is integral to understanding how to manipulate and convert simple alcohols into valuable industrial solvents and precursors for other chemical reactions.

Grasping dehydration processes ensures that you can predict and control product outcomes in synthetic chemistry.

Chemical Reactions

Chemical reactions are fundamental processes by which substances interact to form new products. Each reaction is characterized by changes in molecular structures and energy release or absorption.

Understanding essential terms and processes:

• Reactions can be simple substitutions, like sodium displacing hydrogen in alcohol to form sodium alkoxide, to complex arrangements like dehydration.
• They involve reactants (starting materials), products (end materials), and conditions under which the reactions occur.
• These reactions are vital in organic chemistry to synthesize new molecules, study reaction mechanisms, and develop new materials and drugs.

Learning about types of chemical reactions helps in predicting how substances will interact under various conditions, thereby enabling the control over reaction pathways.