Problem 14 - 43 Years JEE ADVANCED (1978-2020) + JEE MAIN Chapterwise & Topicwise Solved Papers Chemistry

Question

When propyne is treated with aqueous \(\mathrm{H}_{2} \mathrm{SO}_{4}\) in presence of \(\mathrm{HgSO}_{4}\) the major product is [1983-1 Mark| (a) propanal (b) propyl hydrogensulphate (c) acetone (d) propanol

Step-by-Step Solution

Answer

The major product is acetone, option (c).

Step 1: Understanding the Reaction

Propyne, which is an alkyne, reacts with aqueous \( \mathrm{H}_2\mathrm{SO}_4 \) in the presence of \( \mathrm{HgSO}_4 \) via a hydration process. This reaction converts alkynes into ketones or aldehydes through a mechanism involving the formation of an enol intermediate.

Step 2: Identifying the Mechanism

The reaction typically follows Markovnikov's rule. The alkyne first interacts with \( \mathrm{Hg}^{2+} \) to form a mercurinium ion. Water then adds across the triple bond, leading to an enol after demercuration. This enol tautomerizes to its more stable keto form.

Step 3: Recognizing the Enol to Keto Tautomerization

For propyne, the enol form initially produced is \( CH_3C(OH)=CH_2 \). This enol undergoes keto-enol tautomerization. During this process, the hydroxyl group moves from the terminal carbon to form a carbonyl group, resulting in the ketone.

Step 4: Determining the Major Product

Given the structure of propyne and the mechanism described, this tautomerization yields acetone, \( (CH_3)_2CO \), as the major product. The carbonyl group forms on the more substituted carbon due to Markovnikov's rule, favoring the stabilization of the carbocation intermediate.

Key Concepts

Alkynes

Alkynes are a fascinating class of organic compounds characterized by a carbon-carbon triple bond. Due to this triple bond, alkynes are unsaturated hydrocarbons, giving them unique reactivity compared to their alkanes and alkenes counterparts.
One of the crucial features of alkynes is their linear geometry around the triple bond, which is a result of sp hybridization.
This linear structure results in interesting reactions, particularly addition reactions, where atoms or groups are added across the triple bond, opening opportunities for various synthetic transformations.
These reactions often result in alkenes or other useful products.

• Low reactivity under non-catalytic conditions
• Sp hybridization leading to 180° bond angles
• Potential for transformation into ketones and aldehydes

Hydration Reaction

The hydration of alkynes is a significant reaction in organic chemistry, involving the addition of water \( (H_2O) \) across the triple bond. This results in an intermediate state known as an enol, which is a compound with an \( –OH \) group bonded directly to a carbon that also forms a double bond with another carbon.
In the presence of acids, such as sulfuric acid \( (H_2SO_4) \), and mercury salts like \( HgSO_4 \), this reaction is accelerated and follows a specific mechanism.
The reaction's key purpose is to convert unsaturated compounds into more stable forms, typically achieving this by transforming alkynes into ketones or aldehydes. The immediate formation of the enol intermediate is crucial for the subsequent steps of the reaction.

Keto-Enol Tautomerization

Keto-enol tautomerization is a fascinating chemical equilibrium between two structural isomers – enols and their associated keto forms.
In this tautomerization, an initial enol form, having an alcohol and a carbon-carbon double bond, rearranges to form a ketone or an aldehyde. This shift involves the movement of a hydrogen atom and a double bond, generally leading to the more stable keto form.

• This process is acid or base-catalyzed, depending on the specific reaction conditions.
• The keto form is often favored due to its lower energy state and greater thermodynamic stability.
• For propyne, this tautomerization results in acetone, as the keto form stabilizes the entire molecule.

The keto form usually dominates the equilibrium because carbonyl compounds (ketones and aldehydes) tend to be more thermodynamically stable than their enol counterparts.

Markovnikov's Rule

Markovnikov's Rule is a basic principle guiding the addition of reagents to unsymmetrical alkenes and alkynes. It predicts the regioselectivity of certain chemical reactions, particularly those that involve hydrogen atoms being added across double or triple bonds.
According to Markovnikov's rule, in a hydration reaction, the hydrogen atom from water tends to bond with the less substituted carbon atom. Meanwhile, the hydroxyl group attaches itself to the more substituted carbon.
This behavior is linked to the stability of intermediates formed during the reaction, often carbocations, which are more stable when located on more substituted carbon atoms. In the case of propyne, following Markovnikov's rule ensures that the reaction proceeds efficiently towards the formation of acetone, a more stable and preferred product.

• Helps determine the position of substituents in reaction products
• Works by stabilizing positive intermediates
• For acetone formation, the reacted site is at the more substituted carbon, in line with Markovnikov's prediction