19.7b Wittig Reaction | Organic Chemistry
TLDRThe provided script is an educational lesson focusing on the Wittig reaction, a fundamental concept in organic chemistry. The lesson explains the process of converting a carbon-oxygen bond into a carbon-carbon double bond using a Wittig reagent, typically a phospholide. The instructor emphasizes the importance of choosing the correct alkyl halide for the reaction, highlighting the role of an SN2 reaction in the formation of the phospholide. The mechanism is detailed, starting with a nucleophilic attack by the phospholide, leading to the formation of an intermediate alkoxide. This is followed by a four-membered ring formation and a cyclic electron movement, resulting in the desired alkene product and a byproduct. The script also touches on the preparation of the phospholide from an alkyl halide using triphenylphosphine and a strong base like butyllithium. The lesson concludes by briefly mentioning other types of Wittig reagents and encourages students to engage with the content by liking, sharing, and seeking further resources on the topic.
Takeaways
- 🧪 The Wittig reaction is a method to convert a carbon-oxygen bond into a carbon-carbon double bond using a phosphorane, which is a phosphorus ylide.
- 🔍 The most common Wittig reagent is phospho-illid, which is typically used in undergraduate courses.
- 📚 The lesson is part of an organic chemistry playlist released weekly, and subscribing to the channel can keep you updated on new content.
- ⚙️ The mechanism of the Wittig reaction involves a nucleophilic attack by the phospho-illid on a ketone or aldehyde, forming an intermediate alkoxide.
- 🔬 The intermediate is a four-membered ring that collapses to yield the alkene product and a byproduct, which can be recycled.
- 🧠 Retrosynthetic analysis is key to predicting the reactants for a Wittig reaction, starting from the desired alkene and working backward to the less substituted carbon.
- 🏗 The phospho-illid is synthesized from an alkyl halide through an SN2 reaction with triphenylphosphine, followed by deprotonation using a strong base like butyl lithium.
- 🔄 Phosphorus can exceed the octet rule, which is why phospho-illid has a polar bond with a partial negative charge on carbon and a partial positive charge on phosphorus.
- 📉 The less substituted side of the alkene is derived from an alkyl halide to facilitate an SN2 reaction, which prefers a less hindered backside attack.
- ⚖️ The mechanism's final step involves a cyclic movement of electrons, forming both a carbon-carbon double bond and a phosphorous-oxygen double bond.
- 📚 There are other types of Wittig reagents involving sulfur, but they are less commonly encountered, especially in undergraduate studies.
Q & A
What is the Wittig reaction?
-The Wittig reaction is a chemical reaction that involves the conversion of a carbon-oxygen bond into a carbon-carbon double bond using a phosphorane, commonly known as a Wittig reagent.
How is the Wittig reagent represented in its nucleophilic form?
-The Wittig reagent is represented with a negative formal charge on carbon, making it a carbanion and a nucleophile. It can also be represented with a resonance structure where the negative charge is delocalized through a double bond with the phosphorus.
What is the role of triphenylphosphine in the Wittig reaction?
-Triphenylphosphine is used in an SN2 reaction with an alkyl halide to form a phosphonium ion, which is a key intermediate in the formation of the phosphorane used in the Wittig reaction.
Why is an alkyl halide chosen for the Wittig reaction?
-An alkyl halide is chosen because it will be involved in an SN2 reaction to form the phosphonium ion. It is preferred to be the least substituted side of the alkene to facilitate the backside attack required for the SN2 mechanism.
What is the purpose of using a strong base like butyllithium in the Wittig reaction?
-Butyllithium is used as a strong base to deprotonate one of the hydrogens on the carbon of the phosphonium ion, leading to the formation of the phosphorane, which is the active nucleophile in the Wittig reaction.
What is the final product of the Wittig reaction?
-The final product of the Wittig reaction is an alkene, which is formed through a series of steps including nucleophilic attack, formation of a four-membered ring, and a cyclic rearrangement that results in a carbon-carbon double bond.
What is the byproduct formed during the Wittig reaction?
-The byproduct formed during the Wittig reaction is a phosphine oxide, which can be recycled in the reaction process.
Why is the Wittig reaction considered unique among carbon nucleophiles?
-The Wittig reaction is unique because it involves the conversion of a C=O bond into a C=C bond using a phosphorane as the nucleophile, which is a distinct mechanism compared to other carbon nucleophile reactions.
What is the significance of the four-membered ring in the Wittig reaction mechanism?
-The four-membered ring is an intermediate in the Wittig reaction mechanism. It is formed after the nucleophilic attack and collapses to give the final alkene product through a cyclic rearrangement of electrons.
How does the choice of the less substituted side of the alkene affect the Wittig reaction?
-The less substituted side of the alkene is chosen to originate from an alkyl halide because it facilitates the SN2 reaction, which requires a less hindered backside attack for the nucleophile to displace the leaving group.
What is the role of the phenyl groups (Ph) in the Wittig reagent?
-The phenyl groups in the Wittig reagent are part of the triphenylphosphine molecule that is bonded to the phosphorus. They provide additional stabilization to the carbanion through inductive effects and help to make the phosphorane a better nucleophile.
Why are Wittig reagents typically not covered in undergraduate courses?
-Wittig reagents, specifically phospho-illid and its variations, are often not covered in undergraduate courses due to their complexity and the fact that the majority of students may not encounter them in their studies. The focus is usually on the most common type of Wittig reagent.
Outlines
🧪 Wittig Reaction Overview
The first paragraph introduces the Wittig reaction, emphasizing its uniqueness among carbon nucleophiles. It explains that the reaction involves converting a carbon-oxygen bond into a carbon-carbon double bond using a common type of Wittig reagent. The video is part of an organic chemistry series, and viewers are encouraged to subscribe for updates. The key takeaway is that if a synthesis transforms a carbon-oxygen bond into a carbon-carbon double bond, it likely involves a Wittig reaction. The process is also discussed from a retro synthesis perspective, starting from the desired alkene and working backward to predict the reactants, which would be an alkyl halide and a ketone or aldehyde. The role of the alkyl halide in an SN2 reaction is highlighted, with the less substituted side of the alkene originating from it.
🔬 Wittig Reaction Mechanism
The second paragraph delves into the mechanism of the Wittig reaction. It begins with the nucleophilic attack of the phosphorane (phospholide) on the carbonyl carbon of the ketone or aldehyde, leading to an intermediate alkoxide. The positively charged phosphorus and negatively charged oxygen then participate in a reaction to form a four-membered ring. This ring subsequently collapses, resulting in the formation of a carbon-carbon double bond and a phosphorus-oxygen double bond, yielding the alkene product and a byproduct. The paragraph also backtracks to explain the formation of the phospholide from an alkyl halide, involving an SN2 reaction with triphenylphosphine and deprotonation using a strong base like butyllithium. The phospholide is then combined with the ketone or aldehyde to form the alkene, completing the Wittig reaction.
📚 Wittig Reagents and Additional Resources
The third paragraph briefly mentions other types of Wittig reagents that involve sulfur but notes that the majority of students will likely only encounter phospholides. It encourages viewers to like and share the lesson for wider exposure and directs those interested in further study to a premium course on Chatsprep.com for additional resources and practice problems on aldehydes and ketones.
Mindmap
Keywords
💡Wittig Reaction
💡Phosphorane
💡Nucleophile
💡Alkyl Halide
💡Triphenylphosphine
💡Butyl Lithium
💡SN2 Reaction
💡Alkene
💡Ketone
💡Aldehyde
💡Resonance Structure
Highlights
The Wittig reaction is a unique method for converting a carbon-oxygen bond into a carbon-carbon double bond.
The most common Wittig reagent used is the phosphorane, which is derived from an alkyl halide.
The reaction involves a nucleophilic attack by the phosphorane on a ketone or aldehyde.
The phosphorane is stabilized by the positive formal charge on phosphorus, making the carbon a nucleophile.
The mechanism includes the formation of a four-membered ring intermediate.
The final step involves a cyclic movement of electrons, resulting in the formation of a carbon-carbon double bond and a phosphorus-oxygen double bond.
The selection of the alkyl halide for the phosphorane is critical for the SN2 reaction.
The less substituted side of the alkene originates from the alkyl halide for better SN2 reactivity.
Triphenylphosphine is used to convert the alkyl halide into the phosphorane through an SN2 reaction.
Butyl lithium is a strong base used to deprotonate the intermediate to form the phosphorane.
The Wittig reaction is commonly used in organic chemistry for the synthesis of alkenes.
The phosphorane can be represented with different resonance structures, but it remains a potent nucleophile.
The Wittig reaction has a retro synthesis perspective, allowing chemists to predict reactants from the product.
The reaction is part of a series of organic chemistry lessons released weekly throughout the school year.
The video provides a comprehensive guide on the Wittig reaction, including its mechanism and practical considerations.
There are variations of Wittig reagents, including those involving sulfur, but phosphorane is the most commonly encountered in undergraduate courses.
The video encourages viewers to subscribe for weekly updates on new lessons and to use the provided study guide for practice.
Transcripts
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