Problem 12 - 43 Years JEE ADVANCED (1978-2020) + JEE MAIN Chapterwise & Topicwise Solved Papers Chemistry

Question

The product(s) obtained via oxymercuration of 1 - butyne would be [1999-2 Marks] (a) CCC(C)=O (b) \(\mathrm{CH}_{3}-\mathrm{CH}_{2}-\mathrm{CH}_{2}-\mathrm{CHO}\) (c) \(\mathrm{CH}_{3}-\mathrm{CH}_{2}-\mathrm{CHO}+\mathrm{HCHO}\) (d) \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{COOH}+\mathrm{HCOOH}\)

Step-by-Step Solution

Answer

The product is butan-2-one, corresponding to option (a).

Step 1: Understand the Oxymercuration Reaction

Oxymercuration is a reaction involving the addition of Hg(OAc)₂ (mercury acetate), followed by reduction, to transform an alkyne or alkene into a ketone or alcohol. For a terminal alkyne like 1-butyne, the reaction generally adds an OH group across the triple bond with Markovnikov selectivity, leading to the formation of an enol.

Step 2: Apply Oxymercuration to 1-Butyne

1-butyne is CH₃-C≡C-CH₂. During oxymercuration, an Hg(OAc) group and an OH group are initially added across the triple bond, forming an unstable enol.

Step 3: Enol-Keto Tautomerism

The unstable enol formed from oxymercuration undergoes tautomerization to yield a ketone. For 1-butyne, the enol will rearrange to form butan-2-one (CH₃-CO-CH₂-CH₃). This is the ketone product that results from the rearrangement.

Step 4: Analyze the Given Options

The question asks which compound results from the oxymercuration of 1-butyne. Given the enol tautomerization to butan-2-one, this is consistent with (a) as the final product: CCC(C)=O represents butan-2-one in SMILES notation.

Key Concepts

Markovnikov selectivity

In the world of organic chemistry, Markovnikov selectivity is a crucial concept. This principle helps us understand how the addition of atoms or groups takes place across a multiple bond in a molecule. When dealing with oxymercuration, this concept becomes especially important.
Markovnikov's rule states that, in the addition of a molecule like hydrogen halide to an unsymmetrical alkene or alkyne, the hydrogen atom attaches to the carbon with the greater number of hydrogen atoms. This rule elucidates that the more electrophilic part of the reagent (in this case, the OH group from the oxymercuration agent) attaches to the more substituted carbon.
In the oxymercuration of 1-butyne, Markovnikov selectivity guides the addition of the OH group to the carbon atom that is more substituted within the triple bond.

• This leads to the formation of an enol intermediate that is less stable.
• The subsequent tautomerization of this enol produces a stable ketone.

Understanding this selectivity helps predict the structure of the oxymercuration product.

enol-keto tautomerism

Enol-keto tautomerism is a fascinating and significant aspect of organic reactions, especially when dealing with oxymercuration. This process describes the chemical reaction in which an enol, a compound that contains both a hydroxyl group and a carbon-carbon double bond, transforms into a keto form, which is more stable.
The enols are typically less stable because of the favorability of the keto form, which is usually lower in energy due to the stronger carbonyl bond compared to the enol's hydroxyl and alkene configuration.

• In the case of 1-butyne's oxymercuration, an enol intermediate is initially formed.
• During enol-keto tautomerism, this enol undergoes a rearrangement.

The enol efficiently transitions into a ketone, resulting in butan-2-one. This process of shifting the structure is not just a simple switch but a crucial realignment leading to more thermodynamically stable products.

alkyne to ketone transformation

Transforming an alkyne into a ketone is a valuable reaction in synthetic organic chemistry. Oxymercuration efficiently allows for this transformation by leveraging the concepts of Markovnikov selectivity and enol-keto tautomerism.
Initially, when the oxymercuration reagent adds across the carbon-carbon triple bond of the alkyne, it forms an enol. However, as we have seen, enols are unstable under these conditions.

• This instability spurs the compound to undergo tautomerism, producing a ketone as the final product.
• For 1-butyne, we reach the stable ketone form known as butan-2-one.

Transformations like these are fundamental in the synthesis of complex molecules, providing a way to change functional groups and thus alter the chemical properties of the molecule. By understanding the mechanism through which an alkyne transforms into a ketone, chemists can strategically devise synthesis pathways for new compounds.