Oxymercuration Demercuration Reaction Mechanism
TLDRThis video tutorial delves into the oxymercuration-demercuration reaction of alkanes, specifically using butane as an example. It explains the regioselective oxymercuration step, where the OH group attaches to the more substituted carbon, and the subsequent demercuration step that converts alkenes into alcohols with Markovnikov addition. The mechanism, involving electrophilic attack and resonance stabilization, is detailed. The video also covers the reaction's anti-addition nature, resulting in a racemic mixture of products, and encourages viewers to practice predicting major products for given alkenes.
Takeaways
- 🧪 The video discusses the oxymercuration-demercuration reaction of alkanes, specifically using butane as an example.
- 🌡 The first step, oxymercuration, involves the addition of mercury acetate and water to the alkene, resulting in the OH group attaching to the more substituted (secondary) carbon.
- 🔍 The reaction is regioselective, meaning it preferentially occurs at a specific location on the molecule, in this case, the more substituted carbon.
- ⚔️ In the second step, demercuration, sodium borohydride is used to remove the mercury and replace it with a hydrogen atom, converting the alkene into an alcohol.
- 🔬 The mechanism involves the ionization of mercury acetate to form an electrophile, which then reacts with the nucleophilic alkene to form an intermediate.
- 🔄 The intermediate goes through a resonance stabilization, favoring the structure with the positive charge on the secondary carbon.
- 💧 Water attacks the secondary carbon in an anti-addition manner, leading to the formation of the oxymercuration product.
- 🧪 A base is used to remove a hydrogen atom from the intermediate to complete the oxymercuration step.
- 🔬 The final step involves the addition of sodium borohydride to achieve the final alcohol product.
- 🌀 The reaction yields a racemic mixture, indicating the presence of both R and S enantiomers of the alcohol.
- 📚 The video includes practice problems to illustrate the application of the oxymercuration-demercuration reaction to different alkenes and emphasizes the lack of rearrangement due to the partial positive charge on the carbon.
Q & A
What is the oxymercuration-demercuration reaction?
-The oxymercuration-demercuration reaction is a two-step chemical process that converts alkenes into alcohols with Markovnikov addition. It involves the initial addition of mercury acetate and water to the alkene (oxymercuration), followed by the reduction of the organomercury intermediate with sodium borohydride (demercuration).
Why is the oxymercuration step regioselective?
-The oxymercuration step is regioselective because the OH group adds to the more substituted carbon (secondary carbon) of the double bond, while the mercury atom adds to the less substituted carbon (primary carbon). This selectivity is due to the stabilization of the positive charge on the secondary carbon through resonance with the mercury atom.
What role does sodium borohydride play in the demercuration step?
-Sodium borohydride acts as a reducing agent in the demercuration step. It removes the mercury atom from the organomercury intermediate and replaces it with a hydrogen atom, resulting in the formation of an alcohol.
How does the oxymercuration-demercuration mechanism involve resonance stabilization?
-The mechanism involves the formation of a cyclic intermediate where the positive charge is distributed between the mercury atom and the carbon atom through resonance. The secondary carbon with a partial positive charge is more stable, making it the major resonance contributor and thus the preferred site for the water attack.
What is the significance of the anti-addition in the oxymercuration step?
-The anti-addition in the oxymercuration step refers to the approach of the water molecule from the opposite side of the alkene's double bond. This results in the formation of a new bond with the secondary carbon and the breaking of the bond with the mercury atom, leading to the final alcohol product.
Why does the oxymercuration-demercuration reaction not lead to rearrangements?
-The reaction does not lead to rearrangements because there is no formation of a true carbocation intermediate with a full positive charge. The carbon in the intermediate has a partial positive charge, which is stabilized by resonance with the mercury atom, preventing the need for a rearrangement to a more stable carbocation.
What is the stereochemistry outcome of the oxymercuration-demercuration reaction?
-The reaction results in a racemic mixture, meaning both R and S enantiomers of the alcohol are formed. This is due to the fact that the reaction involves two distinct steps, each capable of producing either enantiomer.
How can you predict the major product of an oxymercuration-demercuration reaction with a given alkene?
-To predict the major product, identify the most substituted carbon atom of the double bond and place the OH group on it. Then, consider the demercuration step to determine the final alcohol product, keeping in mind that a racemic mixture will be formed.
What happens if the alkene has a quaternary carbon adjacent to the double bond?
-If the alkene has a quaternary carbon adjacent to the double bond, the oxymercuration-demercuration reaction will still proceed without rearrangement. The OH group will be placed on the more substituted carbon of the double bond, and the reaction will not be affected by the presence of the quaternary carbon.
Can you provide an example of how to work through an oxymercuration-demercuration reaction problem?
-Certainly. For an alkene with a double bond between two secondary carbons, place the OH group on either carbon, as they are equivalent. Then, perform the demercuration step with sodium borohydride to replace the mercury atom with a hydrogen atom, resulting in the formation of the alcohol product.
Outlines
🧪 Oxymercuration Demercuration Reaction Mechanism
This paragraph introduces the oxymercuration-demercuration reaction of alkanes, specifically using butane as an example. The process involves reacting butane with mercury acetate and water, leading to the oxymercuration step where the OH group attaches to the more substituted (secondary) carbon of the alkene, and the mercury to the less substituted (primary) carbon. This reaction is regioselective. The demercuration step follows, utilizing sodium borohydride to remove the mercury and replace it with a hydrogen atom, converting alkenes into alcohols. The paragraph also explains the mechanism, highlighting the electrophilic nature of mercury acetate and the nucleophilic attack of the alkene, resulting in a cyclic intermediate stabilized by resonance. The regioselectivity is attributed to the stabilization of the positive charge on the secondary carbon by resonance, leading to an anti-addition of water in the oxymercuration step.
📚 Stereochemistry and Practice Problems of the Reaction
The second paragraph delves into the stereochemistry of the oxymercuration-demercuration reaction, noting that it results in a racemic mixture of R and S enantiomers. The paragraph emphasizes the absence of rearrangements in this reaction due to the lack of a true carbocation intermediate. It provides guidance on predicting the major product of the reaction with practice problems, stressing that the OH group will be placed on the more substituted carbon of the double bond, leading to a racemic mixture. The paragraph encourages viewers to pause and work through examples, considering the regioselectivity and anti-addition nature of the reaction.
🔍 Further Examples and Reaction Outcomes
The final paragraph continues with additional examples to illustrate the reaction's outcomes. It discusses scenarios where the double bond's carbon atoms have the same substitution, allowing the alcohol group to be placed on either carbon, resulting in different stereoisomers. The paragraph explains that for 3-pentanol, there is only one stereoisomer due to the non-chiral carbon bearing the OH group, while 2-pentanol can have two stereoisomers based on the OH group's position. The video concludes with an invitation to watch the next video on related topics and mentions a resource for organic chemistry exam preparation.
Mindmap
Keywords
💡Oxymercuration Demercuration
💡Alkanes
💡Mercuric Acetate
💡Regioselective
💡Sodium Borohydride
💡Markovnikov's Rule
💡Electrophile
💡Nucleophile
💡Resonance
💡Stereochemistry
💡Racemic Mixture
Highlights
Introduction to the oxymercuration-demercuration reaction of alkanes.
Reaction of butane with mercury acetate and water to initiate oxymercuration.
Oxymercuration is regioselective, with the OH group attaching to the more substituted carbon.
Mercury adds to the less substituted carbon atom in the alkene.
Demercuration step involves using sodium borohydride to remove mercury and add a hydrogen atom.
Conversion of alkenes into alcohols with Markovnikov addition through the reaction.
Explanation of the mechanism involving mercury acetate ionization and electrophilic attack by the alkene.
Formation of a cyclic intermediate with resonance stabilization.
Resonance structures and the identification of the most stable contributor.
Water's role in attacking the secondary carbon atom due to its partial positive charge.
Anti-addition nature of the oxymercuration step.
Use of a base to remove a hydrogen atom in the final oxymercuration step.
Stereochemistry discussion resulting in a racemic mixture of butanol.
Practice problem walkthrough with no rearrangements in the oxymercuration reaction.
Prediction of the major product for given alkenes undergoing oxymercuration and demercuration.
Discussion on the stereoisomers obtained from the reaction.
Final thoughts on the reaction's anti-addition and the products formed.
Invitation to the next video on alcoxymercuration-demercuration reactions.
Transcripts
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