Problem 41 - 43 Years JEE ADVANCED (1978-2020) + JEE MAIN Chapterwise & Topicwise Solved Papers Chemistry

Question

An alcohol \(A\), when heated with conc. \(\mathrm{H}_{2} \mathrm{SO}_{4}\) gives an alkene \(B\). When \(B\) is bubbled through bromine water and the product obtained is dehydrohalogenated with excess of sodamide, a new compound \(C\) is obtained. The compound \(C\) gives \(D\) when treated with warm dilute \(\mathrm{H}_{2} \mathrm{SO}_{4}\) in presence of \(\mathrm{HgSO}_{4} . D\) can also be obtained either by oxidizing \(A\) with \(\mathrm{KMnO}_{4}\) or from acetic acid through its calcium salt. Identify \(A, B, C\) and \(D\).

Step-by-Step Solution

Answer

A is ethanol, B is ethene, C is ethyne, and D is acetaldehyde.

Step 1: Identify the Alkene Formation Process

The alcohol \( A \) when heated with concentrated \( \mathrm{H}_{2}\mathrm{SO}_{4} \) undergoes dehydration to form an alkene \( B \). This is a typical mechanism where an alcohol loses water (\( \text{H}_2\text{O} \)) to form an alkene. For simple alcohols like ethanol, this reaction produces ethene as the alkene. Therefore, if \( A \) is ethanol, \( B \) would be ethene.

Step 2: Analyze Reaction with Bromine Water

Alkene \( B \), when bubbled through bromine water, forms a dibromoalkane through an anti-addition reaction. Ethene would react with bromine (\( \mathrm{Br}_2 \)) to form 1,2-dibromoethane.

Step 3: Dehydrohalogenation with Sodamide

The product from Step 2 is treated with excess sodamide (\( \text{NaNH}_2 \)), which typically results in dehydrohalogenation. This process removes hydrogen halides from vicinal dihalides to form an alkyne. Applying this to 1,2-dibromoethane, the product \( C \) would be ethyne (acetylene).

Step 4: Formation of Compound D

Compound \( C \) (ethyne) is treated with warm dilute \( \mathrm{H}_{2} \mathrm{SO}_{4} \) in the presence of \( \mathrm{HgSO}_{4} \), known as the hydration of alkynes, which typically gives a ketone or aldehyde. Ethyne, through this reaction, gives acetaldehyde (ethanal), which is compound \( D \).

Step 5: Identify Compound through Alternative Synthesis

Compound \( D \) is also obtainable by oxidizing the alcohol \( A \) with \( \mathrm{KMnO}_4 \), which would convert ethanol (\( A \)) directly to acetaldehyde (ethanal, \( D \)). Furthermore, calcium acetate through pyrolysis reforms acetaldehyde, confirming \( D \) as ethanal.

Key Concepts

Dehydration Reaction

A dehydration reaction is a type of chemical reaction where water molecules are eliminated from a substance. In organic chemistry, this is often seen when alcohols lose water to form alkenes. For instance, when ethanol, a simple alcohol, is heated with concentrated sulfuric acid (\(\text{H}_2\text{SO}_4\)), it undergoes dehydration. This leads to the formation of ethene, a common alkene, as ethanol sheds a water molecule.

• Dehydration reactions are often catalyzed by acids.
• They involve the formation of a double bond, converting an alcohol to an alkene.
• Heat is generally required for these reactions to take place.

When studying dehydration reactions, one common question is identifying the starting alcohol and the resulting alkene. This is particularly important in synthesizing alkenes for further chemical transformations.

Dehydrohalogenation

Dehydrohalogenation is a reaction where hydrogen halides are removed from a compound, often resulting in the formation of a double or triple bond. This reaction is a critical step in converting saturated compounds like vicinal dihalides into unsaturated ones such as alkynes.
In our context, 1,2-dibromoethane, formed from ethene reacting with bromine, is treated with sodamide (\(\text{NaNH}_2\)). Sodamide deprotonates the compound, removing hydrogens and bromides to produce ethyne (acetylene).

• Dehydrohalogenation effectively increases unsaturation, often leading to the creation of alkynes from dibromo compounds.
• It is widely used in organic chemistry to form carbon-carbon triple bonds.
• Sodamide is a strong base used to remove hydrogen halides in these reactions.

This step is crucial in synthesizing alkynes, highly reactive compounds utilized in various organic synthesis pathways.

Alkyne Hydration

The hydration of alkynes is a process that involves adding water to an alkyne, usually catalyzed by mercury (II) sulfate (\(\text{HgSO}_4\)) in the presence of dilute sulfuric acid (\(\text{H}_2\text{SO}_4\)). This reaction typically converts alkynes to ketones or aldehydes through an enol intermediate.
In the given example, ethyne (acetylene) undergoes hydration to form acetaldehyde (ethanal).

• The reaction proceeds via the formation of a rarely isolated entity known as enol.
• Enols are unstable and quickly isolate to form the more stable ketone or aldehyde.
• This reaction is important for creating ketones or aldehydes from simpler precursors.

Alkyne hydration is a valuable tool in organic chemistry to synthesize carbonyl-containing compounds from alkynes, showcasing the versatility of alkynes in synthesis.

Oxidation of Alcohols

The oxidation of alcohols is a crucial organic chemistry process that involves converting alcohols into aldehydes, ketones, or carboxylic acids. This transformation often uses oxidizing agents such as potassium permanganate (\(\text{KMnO}_4\)). In this specific example, ethanol is oxidized to form acetaldehyde (ethanal), which corresponds to compound D in the sequence.

• Primary alcohols can be oxidized to aldehydes or further to carboxylic acids.
• Secondary alcohols are primarily oxidized to ketones.
• Tertiary alcohols do not undergo oxidation easily under normal conditions.

Understanding how oxidation affects different alcohols is essential for predicting the outcomes of these reactions. This process not only provides pathways to essential functional groups but also interconnects different functional classes of compounds in organic synthesis.