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 ethanol (\(\mathrm{C}_{2} \mathrm{H}_{5} \mathrm{OH}\)) and (B) is sodium ethoxide (\(\mathrm{C}_{2} \mathrm{H}_{5} \mathrm{ONa}\)).

Step 1: Identify Reaction with Sodium

We need to determine a compound (A) that reacts with sodium metal to produce a compound (B). In these scenarios, compound (A) is typically an alcohol, as alcohols react with sodium to form alkoxide ions. The general reaction is: \[ \text{R-OH} + \text{Na} \rightarrow \text{R-ONa} + \frac{1}{2}\text{H}_2 \]This means (A) is an alcohol and (B) is the corresponding sodium alkoxide.

Step 2: Analyze Formation of Diethyl Ether

The problem states that heating (A) with concentrated \(\mathrm{H}_2 \mathrm{SO}_4\) gives diethyl ether \((\text{C}_2 \text{H}_5)\text{O}(\text{C}_2 \text{H}_5)\). This reaction suggests that the alcohol (A) should be ethanol \((\text{C}_2 \text{H}_5\text{OH})\) because heating ethanol with sulfuric acid can produce diethyl ether by dehydration. \[2 \text{C}_2 \text{H}_5\text{OH} \xrightarrow{\text{conc. } \text{H}_2 \text{SO}_4} \text{C}_2 \text{H}_5\text{-O-C}_2 \text{H}_5 + \text{H}_2\text{O}\]

Step 3: Determine the Correct Pair (A and B)

Given the reactions, (A) should be \(\text{C}_2 \text{H}_5\text{OH}\) (ethanol), and \(\text{B}\), resulting from the reaction with sodium, should be \(\text{C}_2 \text{H}_5\text{ONa}\). This matches option (a).

Step 4: Verify against Provided Options

From the analysis:- (a) \(\mathrm{C}_{2} \mathrm{H}_{5} \mathrm{OH}\) (ethanol) and \(\mathrm{C}_{2} \mathrm{H}_{5} \mathrm{ONa}\) is consistent with both the sodium reaction and ether formation.- Check other options: - (b) Propanol (\(\mathrm{C}_{3} \mathrm{H}_{7} \mathrm{OH}\)) doesn't form diethyl ether. - (c) Methanol (\(\mathrm{CH}_{3} \mathrm{OH}\)) would form methyl ether, not diethyl ether. - (d) Butanol (\(\mathrm{C}_{4} \mathrm{H}_{9} \mathrm{OH}\)) would not result in diethyl ether.Thus, the answer is option (a).

Key Concepts

Alcohols

In organic chemistry, alcohols are one of the most essential and versatile groups of compounds. They are characterized by the presence of a hydroxyl group (-OH) attached to a carbon atom. This group significantly influences the chemical properties of alcohols.

• Naming: The IUPAC naming convention for alcohols involves adjusting the alkane name by replacing the '-e' suffix with '-ol'. For instance, ethane becomes ethanol, symbolizing a two-carbon chain with a hydroxyl group.
• Physical properties: Alcohols generally have higher boiling points compared to hydrocarbons of the same molecular weight due to their ability to form hydrogen bonds.
• Reactivity: Alcohols can act as both acids and bases, but their main reactions are nucleophilic substitution and elimination. They easily undergo reactions with active metals like sodium.

When alcohols react with sodium, they form alkoxide ions, which is a critical property for understanding many chemical processes.

Ether Formation

Ethers are a class of organic compounds characterized by having an oxygen atom connected to two alkyl or aryl groups. When discussing ether formation, particularly diethyl ether, it involves the dehydration of ethanol under acidic conditions. Here are key aspects to remember:

• Esterification: The synthesis of ethers can be achieved from alcohols, a common method involves acid-catalyzed dehydration.
• Reaction Mechanism: For diethyl ether formation, two molecules of ethanol undergo a dehydration reaction. This is promoted by concentrated sulfuric acid, acting as both a catalyst and a dehydrating agent.
• Product Characterization: Diethyl ether is a well-known solvent with the molecular formula \((\text{C}_2 \text{H}_5)\text{O}(\text{C}_2 \text{H}_5)\), indicating two ethyl groups linked by an oxygen atom.

This reaction is a classic example of how alcohols can be converted into ethers, demonstrating the transformation and utility of functional groups.

Reaction with Sodium

Alcohols have a special reactivity with alkali metals like sodium, resulting in the formation of alkoxide ions and hydrogen gas. This reaction is useful for preparing other compounds in organic synthesis.

• Chemical equation: The general reaction can be represented as \[\text{R-OH} + \text{Na} \rightarrow \text{R-ONa} + \frac{1}{2}\text{H}_2\]. The 'R' denotes the alkyl group in the alcohol.
• Role of the Reaction: This reaction demonstrates the amphoteric nature of alcohols, as it acts as an acid in the presence of a stronger base, sodium.
• Resulting Products: The formed sodium alkoxide is a useful base for further chemical reactions, while hydrogen gas evolves as a byproduct.

This interaction is fundamental in organic chemistry to both study alcohol reactivity and prepare for subsequent reactions.

Dehydration Reactions

Dehydration reactions are a crucial type of reaction in organic chemistry where water is removed from a molecule. In the context of alcohols, dehydration typically leads to the formation of alkenes or ethers.

• Mechanism for Ethers: When ethanol is heated with concentrated sulfuric acid, it undergoes dehydration to form diethyl ether, a pivotal industrial and laboratory solvent.
• Reaction Conditions: The concentrated acid serves as a catalyst as well as a dehydrating agent that absorbs the water produced during the reaction.
• Product Versatility: Depending on the specific conditions, dehydration of alcohols can either form alkenes (via E1 or E2 mechanisms) or ethers, emphasizing the versatile nature of this reaction.

Understanding dehydration reactions provides insight into the versatility and usefulness of alcohols, especially in synthesizing new compounds like ethers.