Question

Which is an incorrect statement regarding oxymercuration-demercuration? (a) Here rearrangement takes place (b) In the second step demercuration occurs that is, \(\mathrm{NaBH}_{4}\) reduces- \(\mathrm{HgOAc}_{2}\) group to hydrogen(c) The net reaction is addition of water according to Markovnikov's rule (d) In the first step oxymercuration occurs, that is, water and \(\mathrm{Hg}(\mathrm{OAc})_{2}\) add to double bond

Short answer

The incorrect statement is (a), as oxymercuration-demercuration occurs without rearrangement.

Step-by-step solution

Understand Oxymercuration-Demercuration

Oxymercuration-demercuration is a two-step chemical reaction that converts an alkene into an alcohol without rearrangement. In the first step, oxymercuration, the alkene reacts with mercuric acetate (Hg(OAc)_2) followed by water, resulting in an organomercury intermediate. The second step, demercuration, involves reducing the mercury element using sodium borohydride (NaBH_4) to get the final alcohol product.

Evaluate Statement (a)

Statement (a) claims that rearrangement occurs during the reaction. However, oxymercuration-demercuration is known for proceeding without carbocation rearrangement, which is one of its main advantages over other hydration reactions.

Evaluate Statement (b)

Statement (b) correctly describes the demercuration step where the organomercury compound is reduced by NaBH_4, replacing the HgOAc group with hydrogen, completing the conversion to an alcohol.

Evaluate Statement (c)

Statement (c) is accurate as oxymercuration-demercuration results in Markovnikov addition of water across the double bond, where the OH group is added to the more substituted carbon atom.

Evaluate Statement (d)

Statement (d) correctly describes the first step, oxymercuration, where Hg(OAc)_2 and water add across the alkene's double bond to form the organomercury intermediate.

Key concepts

Markovnikov's Rule

Markovnikov's Rule is a guiding principle in organic chemistry that predicts the outcome of certain addition reactions, particularly for alkenes. According to the rule, when you add a protic acid (an acid that can donate protons) to an unsymmetrical alkene, the hydrogen atom of the acid bonds to the carbon with more hydrogen atoms already attached, and the other part of the acid, such as a halide or a hydroxyl group, bonds to the carbon with fewer hydrogen atoms.
This rule helps chemists predict which isomer will be formed when different reactions occur.

• In the context of oxymercuration-demercuration, Markovnikov's Rule is followed because the hydroxyl group ( OH) ends up on the more substituted carbon atom of the originally present double bond.
• The advantage of this reaction is that it allows transformation without the rearrangement of carbon atoms, which is often a problem in other hydration reactions that progress via carbocations.

This predictable outcome of Markovnikov's Rule makes these types of reactions very useful in synthetic organic chemistry, allowing scientists to produce specific isomers that might be necessary for a particular application.

Carbocation Rearrangement

Carbocation rearrangement is a significant consideration in many chemical reactions that involve carbocations as intermediates. A carbocation is an ion with a positively charged carbon atom. These are generally unstable and in many reactions will attempt to rearrange to a more stable state, typically becoming more substituted during the course of the reaction.
However, in the case of oxymercuration-demercuration, this rearrangement does not occur. This is because the reaction does not form a free carbocation. Instead, it forms a bridge-like intermediate, where the mercury element stabilizes the structure.

• This feature is beneficial because it ensures the reaction progresses predictably, adhering to Markovnikov's Rule without resulting in unexpected rearrangements of the carbon skeleton.
• Because of the absence of carbocation rearrangement, oxymercuration-demercuration is often preferred in a synthetic setting over reactions that proceed through a carbocation, such as direct hydration using sulfuric acid, which can produce different isomers.

By avoiding carbocation rearrangements, chemists can consistently achieve the desired chemical structure without the risk of creating multiple by-products.

Hydration of Alkenes

Hydration of alkenes is a common reaction in organic chemistry where water is added to a carbon-carbon double bond (alkene) to form an alcohol. This reaction is crucial in converting alkenes into more functional alcohols, which are valuable intermediates in synthesis.
One of the traditional methods of hydrating alkenes involves the direct use of water in the presence of an acid catalyst. However, this can lead to carbocation rearrangements and thus a mixture of products. This is where oxymercuration-demercuration becomes advantageous:

• Oxymercuration: Involves using mercuric acetate and water to add across the double bond, without forming a free carbocation, thereby avoiding rearrangements.
• Demercuration: Uses sodium borohydride to replace the mercury intermediate with hydrogen, completing the transformation into an alcohol while maintaining the integrity of the carbon framework.

This method ensures the Markovnikov addition of water to the alkene, where the hydroxyl group ends up on the more substituted carbon atom, providing a cleaner and more straightforward approach to synthesizing alcohols from alkenes.