Problem 115 - A Complete Resource Book in Chemistry for JEE Main

Question

The end product in the following sequence of reactions is \(\mathrm{HC} \equiv \mathrm{CH} \frac{1 \% \mathrm{HgSO}_{4}}{20 \% \mathrm{H}_{2} \mathrm{SO}_{4}}-\mathrm{A} \stackrel{\mathrm{CH}_{3} \mathrm{Mg} \mathrm{X}}{\mathrm{H}_{2} \mathrm{O}} \longrightarrow \mathrm{B}[\mathrm{O}]\) (a) ethanol (b) acetone (c) acetic acid (d) isopropyl alcohol

Step-by-Step Solution

Answer

(d) isopropyl alcohol

Step 1: Analyzing the given sequence of reactions

We are given a sequence of reactions starting with acetylene, \[ \text{HC} \equiv \text{CH} \xrightarrow{1\% \text{HgSO}_4/20\% \text{H}_2\text{SO}_4} \text{A} \] These conditions involve mercury(II) sulfate in sulfuric acid, which typically converts alkynes to ketones via a hydration reaction.

Step 2: Hydration of acetylene to find compound A

The addition of water (hydration) in the presence of mercury(II) sulfate and sulfuric acid will cause the acetylene (\(\text{HC} \equiv \text{CH}\)) to form acetaldehyde. However, it generally creates an enol first, which tautomerizes to form acetaldehyde:\[(\text{CH}_3\text{CHO}) \] Here, compound A is then acetaldehyde (ethanal).

Step 3: Reaction of compound A with Grignard reagent to find compound B

Next, the reaction is:\[ \text{CH}_3\text{CHO} \xrightarrow{\text{CH}_3\text{MgX}/\text{H}_2\text{O}} \text{B} \] A Grignard reagent like the methyl magnesium halide \(\text{CH}_3\text{MgX}\) adds to the carbonyl carbon of acetaldehyde forming a secondary alcohol upon hydrolysis. After the Grignard reaction, compound B is isopropyl alcohol \((\text{CH}_3)_2\text{CHOH}\).

Step 4: Final identification of the product

The sequence of reactions forms isopropyl alcohol as the end product. The given reactions first hydrate acetylene to ethanol, which after reacting with a Grignard reagent, results in the final product.

Key Concepts

Alkyne Hydration

Alkyne hydration is a chemical process where alkynes such as acetylene (\( \text{HC} \equiv \text{CH} \)) undergo hydration, meaning they add water across the triple bond. This reaction typically requires certain conditions to proceed, involving catalysts like mercury(II) sulfate (\( \text{HgSO}_4 \)) in the presence of strong acids such as sulfuric acid (\( \text{H}_2\text{SO}_4 \)).

When acetylene is hydrated, it initially forms an enol, a type of molecule that contains both a carbon-carbon double bond and a hydroxyl group (alcohol). However, enols are generally unstable and quickly rearrange to form more stable compounds through a process called tautomerization.

• Initial product: Enol
• Stable product: Acetaldehyde (\( \text{CH}_3\text{CHO} \))

This transformation is important in organic chemistry because it converts simple linear alkynes into aldehydes or ketones, which serve as key intermediates in various chemical syntheses.

Grignard Reaction

The Grignard reaction is an essential tool in organic chemistry, primarily used to form carbon-carbon bonds. This reaction involves Grignard reagents, which are organomagnesium compounds typically represented as \( \text{R-MgX} \), where \( \text{R} \) is an alkyl or aryl group and \( \text{X} \) is a halide.

These reagents are highly reactive, especially towards carbonyl compounds like aldehydes and ketones. When acetaldehyde (\( \text{CH}_3\text{CHO} \)) reacts with a Grignard reagent (\( \text{CH}_3\text{MgX} \)), the Grignard reagent adds to the carbonyl group forming an intermediate, which upon hydrolysis, results in a secondary alcohol:

• Intermediate: Alkoxide
• Final product: Isopropyl alcohol (\( (\text{CH}_3)_2\text{CHOH} \))

The Grignard reaction is favored for its ability to effectively build complex organic molecules by forming new carbon-carbon bonds, essential in synthesizing alcohols like isopropyl alcohol.

Tautomerization

Tautomerization is a chemical phenomenon where compounds called tautomers exist in dynamic equilibrium due to the shift of a hydrogen atom and the rearrangement of a double bond. This process is vital in the transformation of enols to more stable compounds like aldehydes or ketones.

In the context of alkyne hydration, after hydrating acetylene, the product is an enol. This enol form is not as stable and rapidly converts through tautomerization to acetaldehyde:

• Enol: Less stable, transient
• Aldehyde: More stable, long-lived

Tautomerization is crucial as it dictates the final structure of the hydrated product. In many organic pathways, this concept helps stabilize molecules, ultimately affecting the direction and efficiency of reactions. Understanding this process includes realizing its impact on molecular structure and reaction mechanisms in chemistry.