8.4 Addition of an Alcohol | Acid-Catalyzed Addition and Alkoxymercuration-Demercuration | OChem
TLDRThe video script discusses two methods of adding an alcohol across an alkene in organic chemistry: acid-catalyzed addition and alkoxymercuration-demercuration. The first method is similar to acid-catalyzed hydration, but with an alcohol instead of water, leading to Markovnikov addition without stereoselectivity and potential carbocation rearrangements. The second method involves a nucleophilic attack by the alkene on a mercuric ion, followed by a backside attack from the alcohol, resulting in an anti-addition with Markovnikov regiochemistry and anti-stereoselectivity. The script is part of a new organic chemistry playlist released weekly during the 2020-21 school year, aimed at educating viewers on various chemical reactions and mechanisms.
Takeaways
- 🍷 **Markovnikov's Rule in Alcohol Addition**: The addition of an alcohol across an alkene follows Markovnikov's rule, where the hydrogen (H) adds to the less substituted carbon, and the alkoxy or hydroxyl group (OR) adds to the more substituted carbon.
- 🔄 **Carbocation Intermediates**: Acid-catalyzed addition of alcohol involves a carbocation intermediate, which is prone to rearrangements, leading to Markovnikov's rule adherence and no stereoselectivity.
- ⚙️ **Mechanism Similarity**: The acid-catalyzed addition of alcohol is analogous to acid-catalyzed hydration, differing only in the use of an alcohol instead of water.
- 🥃 **Alkoxymercuration-Demercuration**: This reaction involves the addition of an alcohol (OR group) across an alkene, with a mechanism similar to oxymercuration-demercuration but using an alcohol instead of water.
- ⏲️ **Stereoselectivity in Alkoxymercuration**: Unlike acid-catalyzed addition, alkoxymercuration-demercuration exhibits anti-stereoselectivity, with the added groups ending up on opposite sides when forming two chiral centers.
- 🚫 **No Carbocation Rearrangements**: In alkoxymercuration-demercuration, there is no carbocation intermediate, hence no rearrangements occur during the reaction.
- 📚 **Study Materials**: The speaker is releasing a series of organic chemistry lessons weekly throughout the 2020-21 school year, with study guides, quizzes, chapter tests, and practice exams available on chatsprep.com.
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- 🧪 **Protonation in Alcohols**: Just as in water, protonation occurs in alcohols when reacting with H2SO4, leading to the formation of H3O+ or, in the case of alcohol, a protonated alcohol.
- 🔬 **Nucleophilic Attack**: In both acid-catalyzed addition and alkoxymercuration-demercuration, the alkene acts as a nucleophile, attacking the electrophile (protonated alcohol or mercuric ion, respectively).
- 🔄 **Solvent Role**: In the acid-catalyzed addition, the alcohol acts both as a solvent and a reactant, deprotonating to form the final product.
- 🔄 **Mercuranium Ion Formation**: In alkoxymercuration, the alkene attacks the mercury ion to form a three-membered ring called a mercuranium ion, which then undergoes a backside attack by the alcohol to open the ring.
Q & A
What is the addition of an alcohol across an alkene?
-The addition of an alcohol across an alkene is a chemical reaction where an alcohol adds across a double bond in an alkene, resulting in the formation of a new compound with an H and an OR group, where R depends on the alcohol used.
What are the two methods mentioned for the addition of an alcohol to an alkene?
-The two methods mentioned are acid-catalyzed addition of alcohol and alkoxymercuration-demercuration. The first is analogous to acid-catalyzed hydration but uses an alcohol instead of water, while the second is similar to oxymercuration-demercuration but also uses an alcohol in the first step.
Why is the acid-catalyzed addition of an alcohol subject to carbocation rearrangements?
-The acid-catalyzed addition of an alcohol goes through a carbocation intermediate, which can undergo rearrangements due to the instability of the carbocation and the tendency to form a more stable, more substituted carbocation.
What is the Markovnikov's rule and how does it apply to the addition of an alcohol to an alkene?
-Markovnikov's rule states that in the addition of a protic acid or an alcohol to an alkene, the hydrogen atom will be added to the carbon with the greater number of hydrogen atoms (the less substituted carbon), and the hydroxyl group or the alkyl group will be added to the other carbon. This rule is followed in both the acid-catalyzed addition and the alkoxymercuration-demercuration reactions.
Why is there no stereoselectivity in the acid-catalyzed addition of an alcohol?
-There is no stereoselectivity in the acid-catalyzed addition of an alcohol because the reaction goes through a planar carbocation intermediate, which does not have a defined stereochemistry, leading to a non-stereoselective addition.
What is the difference between the acid-catalyzed addition of an alcohol and alkoxymercuration-demercuration in terms of intermediates?
-The acid-catalyzed addition of an alcohol involves a carbocation intermediate, while the alkoxymercuration-demercuration does not. In alkoxymercuration-demercuration, the reaction proceeds through a three-membered ring mercuranium ion intermediate, which does not involve a carbocation.
What is the anti-stereoselectivity in the context of alkoxymercuration-demercuration?
-Anti-stereoselectivity refers to the spatial arrangement where two new chiral centers formed in a reaction are on opposite sides of the molecule. In alkoxymercuration-demercuration, the backside attack on the mercuranium ion leads to the anti addition, where the added groups are on opposite sides.
How does the mechanism of acid-catalyzed hydration relate to the mechanism of acid-catalyzed addition of an alcohol?
-The mechanism of acid-catalyzed hydration is nearly identical to that of acid-catalyzed addition of an alcohol. The main difference is the electrophile involved: in hydration, it's a protonated water molecule, while in the addition of an alcohol, it's a protonated alcohol molecule.
What is the role of the alcohol in the acid-catalyzed addition mechanism?
-In the acid-catalyzed addition mechanism, the alcohol serves as the solvent and also as the electrophile that reacts with the alkene. After the formation of the carbocation intermediate, the alcohol acts as a nucleophile to attack the carbocation, leading to the final product.
What is the final product of the alkoxymercuration-demercuration reaction?
-The final product of the alkoxymercuration-demercuration reaction is an ether, which is formed after the demercuration step where the mercury is replaced by a hydrogen atom.
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Outlines
🧪 Acid-Catalyzed Alcohol Addition to Alkenes
The first paragraph discusses the addition of an alcohol across an alkene, which results in the addition of a hydrogen (H) and an alkyl or aryl group (R) from the alcohol. Two methods are highlighted: acid-catalyzed addition and alkoxymercuration-demercuration. The acid-catalyzed addition is compared to acid-catalyzed hydration, but with alcohol as the solvent instead of water. This method leads to Markovnikov's rule, where the hydrogen atom is added to the less substituted carbon, and the alkyl or aryl group is added to the more substituted carbon. It is also mentioned that this process goes through a carbocation intermediate, which is prone to rearrangements, and lacks stereoselectivity.
🔬 Alkoxymercuration-Demercuration Mechanism
The second paragraph focuses on the alkoxymercuration-demercuration process, which is similar to oxymercuration-demercuration but uses an alcohol instead of water. This method also follows Markovnikov's rule and exhibits anti-stereoselectivity, meaning that if two chiral centers are formed, they will be on opposite sides. The mechanism for the first step is described in detail, where the alkene acts as a nucleophile and attacks the mercury in mercuric acetate, forming a three-membered ring called a mercuranium ion. Methanol, acting as the solvent, performs a backside attack, leading to the opening of the three-membered ring and the formation of a positive formal charge intermediate. The solvent then deprotonates, resulting in the final product of the alkoxymercuration step. The second step, demercuration with NaBH4, replaces the mercury with a hydrogen atom, completing the reaction. The paragraph concludes with an invitation for viewers to like, share, and explore additional resources such as study guides, quizzes, and practice exams available through the instructor's premium courses.
Mindmap
Keywords
💡Alcohol Addition
💡Acid-Catalyzed Hydration
💡Carbocation Intermediate
💡Markovnikov's Rule
💡Stereoselectivity
💡Alkoxymercuration-Demercuration
💡Mercuranium Ion
💡Backside Attack
💡Chiral Centers
💡Deprotonation
💡Ether
Highlights
Addition of an alcohol across an alkene results in the addition of an H and an OR group, depending on the alcohol used.
There are two main methods for this reaction: acid-catalyzed addition and alkoxymercuration-demercuration.
Acid-catalyzed addition is analogous to acid-catalyzed hydration, but uses an alcohol as the solvent instead of water.
Alkoxymercuration-demercuration is similar to oxymercuration-demercuration, but uses an alcohol in the first step.
Acid-catalyzed addition goes through a carbocation intermediate, making it subject to carbocation rearrangements.
The acid-catalyzed addition follows Markovnikov's rule and lacks stereoselectivity.
Alkoxymercuration-demercuration also follows Markovnikov's rule and exhibits anti-stereoselectivity.
Unlike acid-catalyzed addition, alkoxymercuration-demercuration does not go through a carbocation intermediate and is not subject to rearrangements.
The mechanism of acid-catalyzed hydration is nearly identical to that of acid-catalyzed addition.
In alkoxymercuration-demercuration, a three-membered ring called a mercuranium ion is formed.
Backside attack on the mercuranium ion leads to the anti-addition of the alcohol.
The solvent, in this case, an alcohol, deprotonates the intermediate in both acid-catalyzed and alkoxymercuration-demercuration reactions.
The alkoxymercuration-demercuration mechanism is exactly analogous to oxymercuration, but forms an ether as a product instead of an alcohol.
The lesson is part of a new organic chemistry playlist being released weekly throughout the 2020-21 school year.
The channel offers a study guide, quizzes, chapter tests, and practice final exams as part of its premium courses.
The lesson provides a detailed comparison between acid-catalyzed hydration and acid-catalyzed addition of alcohol.
The transcript humorously suggests the term 'acid-catalyzed inebriation' as an alternative name for the reaction involving alcohol.
The alkene acts as a nucleophile in both acid-catalyzed addition and alkoxymercuration-demercuration reactions.
Mercuric acetate dissociates to a small extent in both oxymercuration-demercuration and alkoxymercuration-demercuration.
Transcripts
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