Question

When propyne is treated with aqueous \(\mathrm{H}_{2} \mathrm{SO}_{4}\) in presence of \(\mathrm{HgSO}_{4}\) the major product is (a) Acetone (b) Propanol (c) Propanal (d) Propyl hydrogen sulphate

Short answer

The major product is acetone (a).

Step-by-step solution

Understanding the Reaction

Propyne is an alkyne, specifically known as methylacetylene with the formula \( ext{CH}_3 ext{C} ext{C} ext{H}\). When this alkyne is treated with aqueous sulphuric acid \( ext{H}_2 ext{SO}_4\) in presence of mercuric sulphate \( ext{HgSO}_4\), it undergoes an acid-catalyzed hydration reaction, leading to the formation of an intermediate enol, which tautomerizes to a more stable keto form.

Formation of the Enol Intermediate

The reaction initially adds water \( ext{H}_2 ext{O}\) across the triple bond of propyne. The \( ext{Hg}^{2+}\) acts as a catalyst, facilitating the addition of the hydroxyl group (OH) and the hydrogen atom from water, leading to the formation of an enol, \( ext{H_2C=CH(OH)CH}_3\).

Tautomerization to Ketone

The enol intermediate is not very stable and undergoes a process called tautomerization, where it rearranges its structure to form a ketone. In this case, the enol \( ext{H}_2 ext{C=CH(OH)CH}_3\) rearranges to form acetone \( ext{CH}_3 ext{COCH}_3\), which is the more stable form due to the keto group being energetically more favorable than the enol group.

Identify the Major Product

After analyzing the reaction steps, the major product of the reaction is acetone, as the enol intermediate undergoes tautomerization to form this stable keto product.

Key concepts

Propyne

Propyne is a simple alkyne, a type of hydrocarbon characterized by at least one carbon-carbon triple bond. It is also known as methylacetylene and has the chemical formula \( \text{CH}_3\text{C}\equiv\text{CH} \). Propyne belongs to the alkynes series, which are unsaturated hydrocarbons. The unsaturation in propynes makes them highly reactive, especially in processes like hydration where they convert into different forms. Propyne and other alkynes are versatile reactants in organic chemistry due to their reactive nature. They are often involved in various reactions where the strong triple bond can break to form new compounds and configurations.

• The triple bond in propyne consists of one strong sigma bond and two pi bonds.
• Propyne is a small, linear molecule, making it an ideal candidate for reactions like hydration.

Tautomerization

Tautomerization is a special type of isomerization. It involves the movement of a proton within a molecule and formation of a double bond in a new position, resulting in an equilibrium between two structural isomers, known as tautomers. In the context of propyne, tautomerization occurs after the formation of the enol intermediate during the acid-catalyzed hydration. The enol form is less stable and tends to rearrange to a more stable keto form. This conversion involves the migration of a hydrogen atom and the shift of a double bond.

• Tautomerization helps stabilize molecules by converting them into their more energetically favored forms.
• In acid-catalyzed reactions, tautomerization can be driven by the presence of acidic conditions which facilitate proton transfers.

Catalysis

Catalysis plays a vital role in chemical reactions by lowering the activation energy needed for the reaction to proceed. Catalysts are substances that speed up reactions without themselves being changed in the process. In the hydration of propyne, mercury(II) sulfate \( \text{HgSO}_4 \) is used as a catalyst. It assists in breaking the strong triple bond of the alkyne and helps in forming the enol intermediate from propyne.- Catalysis increases reaction rate while maintaining thermodynamic principles of the system.- The use of \( \text{Hg}^{2+} \) ions in this reaction is crucial for the stepwise addition of water across the triple bond.

Acid-catalyzed hydration

Acid-catalyzed hydration is a reaction where water is added to a molecule, with an acid serving as a catalyst. In organic chemistry, this method is often used to convert alkynes and alkenes into alcohols or derivatives.In the hydration of propyne, sulfuric acid \( \text{H}_2\text{SO}_4 \) acts as the catalyst that facilitates the reaction process. Water, in the presence of sulfuric acid, adds across the triple bond of propyne to initially form an enol, which subsequently undergoes tautomerization to produce a ketone.- This method is efficient in breaking the less reactive alkyne bond by converting it into a more reactive intermediate.- Using strong acids ensures that the protonation occurs easily, leading to subsequent reactions involving water addition.