Question

When 1-butyne undergoes oxymercuration with the help of \(\mathrm{HgSO}_{4}+\mathrm{H}_{2} \mathrm{SO}_{4}\), the product(s) formed is/are (a) \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{COOH}+\mathrm{HCOOH}\) (b) \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{COCH}_{3}\) (c) \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{COOH}\) (d) \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{CHO}\)

Short answer

The product formed when 1-butyne undergoes oxymercuration is \(\mathrm{CH_{3}CH_{2}COCH_{3}}\), which corresponds to option (b).

Step-by-step solution

Understand the Oxymercuration Reaction Mechanism

Oxymercuration is a reaction in which an alkene or alkyne undergoes hydration, with the addition of a hydroxyl group and a substituent from the mercuric reagent across the multiple bond. For alkynes like 1-butyne, the reaction proceeds with anti-markovnikov addition, resulting in the hydration of the alkyne to a carbonyl-containing product (aldehyde or ketone, depending on the specific conditions).

Determine the Regiochemistry of the Product

Since 1-butyne is terminal, the reaction with hydration agents such as \( \mathrm{HgSO_{4}} \) and \( \mathrm{H_{2}SO_{4}} \) will add an \(OH\) group to the more substituted carbon. This is the application of Markovnikov's rule for oxymercuration. The resulting intermediate is a mercurial enol, which will then be reorganized into a ketone.

Identify the Final Product

After the enol rearranges to a ketone, the final product would be a keto-alcohol, which further dehydrates to give the corresponding methyl ketone. Therefore, the product for 1-butyne undergoing oxymercuration will be \(\mathrm{CH_{3}CH_{2}COCH_{3}}\), which corresponds to option (b).

Key concepts

1-Butyne Hydration

Hydration of 1-butyne involves the addition of a water molecule across the triple bond of the molecule, resulting in the formation of a carbonyl compound, either an aldehyde or ketone. This specific hydration is interesting because it uses a technique called oxymercuration, which avoids the direct addition of water and instead uses a mercury catalyst (like HgSO4) to facilitate the process.

In the case of 1-butyne, a terminal alkyne, the reaction particularly yields methyl ketone as the final product. This is due to the nature of the chemical reaction and the placement of the triple bond at the end of the carbon chain. The reaction begins with the addition of mercury catalyst, followed by the addition of a hydroxyl group, leading to the creation of a ketone upon the rearrangement of the intermediate compound.

Markovnikov's Rule

Markovnikov's rule is an essential principle in organic chemistry that predicts the outcome of certain chemical reactions, specifically the addition of acids to alkenes or alkynes. The rule states that with the addition of a protic acid to an alkene or alkyne, the hydrogen atom (from the acid) will attach to the carbon with the greater number of hydrogen atoms, while the more electronegative part (such as the OH group) will attach to the carbon with fewer hydrogen atoms – that is, the more substituted carbon.

This rule helps explain why in the hydration of 1-butyne, the OH group adds to the second carbon atom rather than the first. By understanding Markovnikov's rule, chemists can predict the regiochemistry of the addition reactions, allowing for greater control over the resulting molecular structure.

Organic Reaction Mechanisms

Organic reaction mechanisms describe the step-by-step sequence of elementary reactions by which an overall chemical change occurs. They are central to studying organic chemistry because they allow chemists to understand how and why reactions occur. Mechanisms involve the breaking and forming of bonds, movement of electrons, and the intermediates formed during reactions.

In oxymercuration, the mechanism includes the formation of a mercurium ion intermediate and subsequent enol, which eventually rearranges to a ketone. Understanding the mechanism helps elucidate why certain products are formed, which intermediates are the most stable, and helps predict the outcome of similar chemical reactions. This knowledge is critical for designing new syntheses and manipulating organic reactions to produce desired products.

Aldehydes and Ketones

Aldehydes and ketones are two closely related classes of organic compounds characterized by the presence of a carbonyl group (C=O). Aldehydes have at least one hydrogen atom connected to the carbonyl carbon, while ketones have two alkyl or aryl groups. These compounds are known for their diverse chemical reactivity and their importance in both biological processes and industrial applications.

The product of 1-butyne hydration under oxymercuration conditions is a ketone, which can be distinguished from aldehydes by its IR spectroscopy distinct absorption band and its chemical reactivity. For instance, ketones generally undergo nucleophilic additions and can be further transformed into other compounds, such as alcohols or carboxylic acids, under certain conditions. This transformative ability makes them particularly important in chemical synthesis.