Problem 98 - A Complete Resource Book in Chemistry for JEE Main

Question

The product(s) obtained via oxymercuration \(\left(\mathrm{HgSO}_{4}\right.\) \(+\mathrm{H}_{2} \mathrm{SO}_{4}\) ) of but-1-yne would give (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 obtained is (a) \(CCC(C)=O\).

Step 1: Identify the Reaction Type

The reaction involves oxymercuration, specifically the oxymercuration of an alkyne (but-1-yne). Oxymercuration typically adds water across a carbon-carbon multiple bond, resulting in an enol, which can then tautomerize to a more stable ketone.

Step 2: Write the Reaction

The starting material is but-1-yne. The reagents, HgSO4 and H2SO4, will add across the triple bond of the but-1-yne, converting it into an enol intermediate: \[C_4H_6 \xrightarrow{HgSO_4, H_2SO_4, H_2O} C_4H_8O \text{(enol)}\]

Step 3: Understand Tautomerization

The enol formed in the previous step is not stable and undergoes keto-enol tautomerization. This means the enol will convert into a more stable keto form, which is a ketone.

Step 4: Determine the Final Product

The enol formed from but-1-yne will specifically tautomerize to butan-2-one. This is because the addition of water occurs at the more substituted position (Markovnikov addition), leading to the ketone with a methyl group at the second carbon.

Step 5: Match Chemical Structure

Compare the product obtained, butan-2-one, which can be represented as \( CCC(C)=O \), with the given options. This matches option (a): \( CCC(C)=O\).

Key Concepts

Alkyne Chemistry

Alkyne chemistry is a fascinating area that delves into the reactions and properties of alkynes, which are hydrocarbons characterized by the presence of a carbon-carbon triple bond. These triple bonds are highly reactive, making alkynes a central focus in organic reactions.
One important aspect of alkyne chemistry is their ability to participate in addition reactions. Because the triple bond consists of one sigma bond and two pi bonds, it can be broken and transformed into single or double bonds by adding atoms or groups.
Oxymercuration is a specific type of reaction involving alkynes, where reagents like mercury sulfate and sulfuric acid (HgSO4 and H2SO4) help in the addition of water. This reaction typically turns an alkyne into a more stable ketone by going through an enol intermediate, which is a temporary structure where the oxygen and hydrogen add across the triple bond.
Understanding alkyne chemistry is essential for recognizing how different reactions can transform the structure and reactivity of molecules, leading to products that are often more stable.

Keto-Enol Tautomerization

Keto-enol tautomerization is a fundamental concept in organic chemistry that describes the chemical process through which enols and ketones interconvert. An enol is a compound that has an alcoholic group (-OH) directly connected to a carbon-carbon double bond, whereas a ketone features a carbonyl group (C=O). These forms are both structural isomers called tautomers, and they exist in a dynamic equilibrium.
During this tautomerization, the enol form can spontaneously rearrange to become a ketone, which is usually more stable. This involves a proton transfer from the hydroxyl group to another atom, often leading to the migration of a double bond.
In the context of the oxymercuration of an alkyne such as but-1-yne, the initial product is an enol that swiftly undergoes tautomerization to form butan-2-one, a ketone. The conversion from enol to ketone is driven by the thermodynamic stability provided by stronger carbon-oxygen bonds.
Keto-enol tautomerization is a critical concept to grasp because it underscores how molecular structures can dynamically evolve into more stable configurations under certain conditions.

Markovnikov Addition

Markovnikov Addition is a principle in organic chemistry that predicts how a compound will add across a double or triple bond. According to Markovnikov's rule, during the addition of a reagent to an unsymmetrical alkene or alkyne, the more stable carbocation intermediate forms preferentially.

• This means that the hydrogen atom tends to add to the carbon with the most hydrogens already attached, while the other part of the reagent will add to the carbon with the fewest hydrogens.

In the context of alkyne oxymercuration, such as with but-1-yne, Markovnikov addition helps determine the position of water addition. The water adds in such a way that the more substituted carbon (the one attached to more carbon atoms) secures the OH group.
This pattern of addition ensures that the oxymercuration proceeds to give the Markovnikov product. For but-1-yne, this results in the enol intermediate rearranging through keto-enol tautomerization into butan-2-one, a ketone.
Understanding Markovnikov Addition helps predict and control the outcome of reactions involving alkenes and alkynes, greatly benefiting synthetic chemistry and the construction of complex organic molecules.