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) \( \mathrm{C}_2 \mathrm{H}_5 \mathrm{OH} \) and \( \mathrm{C}_2 \mathrm{H}_5 \mathrm{ONa} \).

Step 1: Identify the Reaction with Sodium

The problem states that compound A reacts with sodium metal to form compound B. This indicates that compound A is an alcohol. When an alcohol reacts with sodium, it forms an alkoxide and hydrogen gas. For example, ethanol (\( \mathrm{C}_2 \mathrm{H}_5 \mathrm{OH} \)) reacts with sodium to form sodium ethoxide (\( \mathrm{C}_2 \mathrm{H}_5 \mathrm{ONa} \)).

Step 2: Determine Reaction with Concentrated Sulfuric Acid

The problem states that compound A, when heated with concentrated \( \mathrm{H}_2 \mathrm{SO}_4 \), gives diethyl ether. The formation of diethyl ether involves the dehydration of ethanol (\( \mathrm{C}_2 \mathrm{H}_5 \mathrm{OH} \)) in the presence of acid to form ether. This suggests that compound A must be ethanol, which readily forms diethyl ether when treated with acid.

Step 3: Match the Compounds to Options

Given that compound A is ethanol (\( \mathrm{C}_2 \mathrm{H}_5 \mathrm{OH} \)) that reacts with sodium to form sodium ethoxide (\( \mathrm{C}_2 \mathrm{H}_5 \mathrm{ONa} \)), the corresponding pair in the options is option (a): \( \mathrm{C}_2 \mathrm{H}_5 \mathrm{OH} \) and \( \mathrm{C}_2 \mathrm{H}_5 \mathrm{ONa} \). Therefore, the correct answer is option (a).

Key Concepts

Ethanol

Ethanol, commonly known as ethyl alcohol, is a colorless, flammable liquid. It's a key ingredient in alcoholic beverages but also serves crucial roles in various industrial and chemical processes. Chemically, it is represented by the formula \( \mathrm{C}_2 \mathrm{H}_5 \mathrm{OH} \). Understanding ethanol involves recognizing its dual nature as both a drinkable alcohol and an essential chemical reagent.

In reactions, ethanol often participates as a reactant, providing a hydroxyl group \( (\mathrm{-OH}) \). This characteristic makes it a prime candidate for participating in substitution reactions like forming alkoxides. Ethanol's ability to donate a proton (\( \mathrm{H}^+ \)) from its hydroxyl group underlies many of its chemical transformations.

Key attributes of ethanol include:

• Polar structure, allowing it to mix with water.
• Combustibility, making it useful as a fuel.
• Capacity to form ethers like diethyl ether when subjected to specific conditions like heating with acids.

The versatility of ethanol makes it foundational in both organic chemistry labs and various industries.

Sodium Alkoxide

Sodium alkoxide is a compound formed when an alcohol reacts with metallic sodium. In this context, the alkoxide of interest is sodium ethoxide, produced when ethanol reacts with sodium metal.

The reaction can be summarized as follows:

\[ \mathrm{2} \; \mathrm{C}_2 \mathrm{H}_5 \mathrm{OH} + \mathrm{2Na} \rightarrow \mathrm{2} \; \mathrm{C}_2 \mathrm{H}_5 \mathrm{ONa} + \mathrm{H}_2 \uparrow \]

Here, ethoxide ions replace the hydroxyl hydrogen, yielding hydrogen gas as a byproduct. Sodium alkoxides like sodium ethoxide are crucial intermediates in organic synthesis.

Key properties of sodium alkoxides include:

• Strong basicity, as they readily donate\/accept protons.
• Usage as bases or nucleophiles in chemical reactions.
• Soluble in organic solvents, facilitating their role in non-aqueous conditions.

Sodium alkoxides are not only essential in laboratory syntheses but also play roles in industrial applications that involve converting alcohols into other chemical forms.

Dehydration Reaction

Dehydration reactions are important for forming complex molecules by removing water. In the context of ethanol, when it is heated with a strong acid like sulfuric acid, dehydration results in the formation of ethers.

The general dehydration equation is:
\[ \mathrm{C}_2 \mathrm{H}_5 \mathrm{OH} \xrightarrow{\text{\( H_2SO_4 \), \text{heat}}} \mathrm{CH}_3 \mathrm{CH}_2 \mathrm{OCH}_2 \mathrm{CH}_3} + \mathrm{H}_2\mathrm{O} \]

Here, two ethanol molecules lose water, bonding together to form diethyl ether. Dehydration reactions are widely used in organic chemistry for synthesizing new compounds.

Important features of dehydration reactions include:

• Requires acid catalysts and heat to proceed.
• Often result in symmetric or mixed ethers depending on reactants.
• Accompanied by the loss of one molar equivalent of water.

Understanding dehydration reactions helps in designing pathways for synthesizing a broad range of simple and complex chemicals.

Diethyl Ether Formation

Diethyl ether formation is a classic example of how alcohols can transform into ethers through dehydration. This particular process involves ethanol molecules under specific conditions.

The synthesis of diethyl ether typically requires heating ethanol with concentrated sulfuric acid. This induces a dehydration reaction, in which two ethanol molecules combine, foregoing a water molecule:

• The hydroxyl group of one ethanol loses an \( \mathrm{H}^+ \), while another ethanol provides an \( \mathrm{H}^- \) to form water.
• The resulting carbocation intermediate facilitates the linkage of two ethyl groups, creating diethyl ether.

Diethyl ether's formation emphasizes how simple structure manipulations can produce a functional and useful chemical compound in both the lab and industry.

Its characteristics include:

• Low boiling point, providing utility as a solvent.
• Colorless and highly volatile nature.
• Role as an anesthetic in earlier medical practices.

In summary, diethyl ether formation from ethanol highlights the interplay between fundamental chemical reactions and practical applications in chemistry.