Oxidations - DMP, PCC & Swern - Aldehydes & Ketones (IOC 24)
TLDRThis episode of 'Introductory Organic Chemistry' focuses on oxidation reactions that convert alcohols into aldehydes and ketones. The lecture reviews practice problems involving the use of reducing agents like sodium borohydride and lithium aluminum hydride, and then delves into the Swern, DMP, and PCC oxidation methods. It highlights the mildness and selectivity of DMP oxidation, the chromium waste concerns with PCC, and the detailed mechanism of Swern oxidation, including sigmatropic rearrangement. The video concludes with new practice problems to reinforce the concepts covered.
Takeaways
- 🧪 Sodium borohydride is a reducing agent that typically reduces ketones and aldehydes but not esters, making it a choice for selective reduction.
- 🔍 In the presence of multiple functional groups, lithium aluminum hydride (LiAlH4) can reduce carbonyl compounds, halides, nitro groups, and epoxides, whereas sodium borohydride is more selective.
- 🔬 Partial reduction of esters to aldehydes is challenging due to the tendency of aldehydes to further reduce to alcohols; a common strategy is to fully reduce to alcohol and then use a mild oxidant to convert back to aldehyde.
- ⚠️ Aldehydes are highly reactive electrophiles, making the choice of oxidant crucial to avoid over-oxidation to carboxylic acids.
- 📚 Three main methods for oxidizing alcohols to aldehydes are the Dess-Martin periodinane (DMP), pyridinium chlorochromate (PCC), and Swern oxidation.
- 🌟 DMP oxidation is a mild method for converting alcohols to aldehydes, favored for its selectivity and compatibility with sensitive functional groups.
- 🛠 PCC oxidation is effective but generates chromium waste, which is toxic and requires careful disposal, presenting a downside to its use.
- 🔄 Swern oxidation involves a sigmatropic rearrangement and produces dimethyl sulfide, a malodorous byproduct, but is a reliable method for oxidizing alcohols to aldehydes.
- 🧐 The mechanism of Swern oxidation is a common topic in organic chemistry exams, emphasizing the importance of understanding the step involving triethylamine and the sigmatropic rearrangement.
- 📘 Examples from the literature demonstrate the utility of DMP and Swern oxidations in complex organic synthesis, showcasing their selectivity and efficiency.
- 💡 The lecture concludes with practice problems to reinforce understanding of the oxidation methods discussed and their applications in organic chemistry.
Q & A
What is the main topic of the lecture?
-The main topic of the lecture is the oxidation of primary and secondary alcohols to aldehydes and ketones in organic chemistry.
Why is sodium borohydride preferred over other reducing agents for ketones?
-Sodium borohydride is preferred because it typically reduces ketones and aldehydes but not esters, and ketones are more electrophilic and react faster.
What is the role of LAH (lithium aluminum hydride) in the reduction of carbonyl compounds?
-LAH is a strong reducing agent that can reduce all carbonyl compounds including esters, as well as halides, nitro groups, and epoxides.
Why is sodium borohydride chosen over LAH for a transformation that requires the product to retain a bromide, nitro group, and epoxide?
-Sodium borohydride is chosen to prevent the reduction of the bromide, nitro group, and epoxide, as LAH would reduce these groups in addition to the alcohol.
What is the difficulty in partially reducing an ester to an aldehyde?
-Partial reduction of an ester to an aldehyde is challenging because if the reaction is left too long, the aldehyde tends to be further reduced to an alcohol.
What are the three main methods for converting alcohols to aldehydes discussed in the lecture?
-The three main methods are DMP oxidation, PCC (Pyridinium chlorochromate) oxidation, and Swern oxidation.
What is the advantage of using DMP (Dess-Martin periodinane) for oxidizing alcohols to aldehydes?
-DMP oxidation is mild and selective, often stopping at the aldehyde stage without over-oxidizing to carboxylic acids, and it is well-tolerated by various functional groups.
What is the environmental concern associated with PCC oxidation?
-PCC oxidation generates chromium waste, which is a chromium-6 reagent known to be carcinogenic and toxic, posing a disposal challenge and safety hazard.
What is a sigmatropic rearrangement, and why is it important in the Swern oxidation mechanism?
-A sigmatropic rearrangement is a reaction where a pi bond moves to an adjacent electron-deficient atom. It is important in the Swern oxidation mechanism because it facilitates the transfer of a hydrogen atom from the alcohol to the DMSO, forming the aldehyde.
Why does the Swern oxidation produce an unpleasant smell?
-The Swern oxidation produces dimethyl sulfide as a byproduct, which has an extremely unpleasant smell, often described as resembling rotting broccoli or cabbage.
What is the recommended procedure for performing a Swern oxidation to avoid side reactions?
-The recommended procedure is to first react DMSO with oxalyl chloride and triethylamine at very low temperatures, then add the alcohol gradually while warming the mixture to a slightly higher but still low temperature to prevent decomposition of the sulfonium intermediate.
Outlines
🧪 Organic Chemistry: Oxidation and Reduction of Carbonyl Compounds
This paragraph introduces the topic of oxidation in organic chemistry, focusing on reactions that produce aldehydes and ketones. It reviews practice problems from a previous lecture, discussing the use of sodium borohydride as a reducing agent that selectively reduces ketones and aldehydes, not esters. The instructor emphasizes the importance of choosing the correct reducing agent to avoid unwanted side reactions and the challenges of partially reducing esters to aldehydes. The summary also touches on the use of lithium aluminum hydride (LAH) and its broader reactivity, including the reduction of halides, nitro groups, and epoxides. The paragraph concludes with an introduction to the day's material, which contrasts the reduction of carbonyl compounds with the oxidation of alcohols to aldehydes and ketones, highlighting the use of mild oxidants to achieve the desired products without over-oxidation.
🔬 Selective Oxidation Techniques in Organic Chemistry
The second paragraph delves into the methods of selectively oxidizing alcohols to aldehydes and ketones, with a focus on the use of Dess-Martin periodinane (DMP), pyridinium chlorochromate (PCC), and Swern oxidation. The paragraph explains the mildness and selectivity of DMP oxidation, which is particularly useful for converting alcohols to aldehydes without affecting other functional groups. It also discusses the challenges of using DMP, such as potential unreactivity due to partial hydrolysis, and the possibility of preparing it from scratch. The mechanism of DMP oxidation is outlined, detailing the displacement of an acetate group by the alcohol and the subsequent formation of acetic acid and the desired aldehyde or ketone product. The paragraph includes examples from the literature that demonstrate the effectiveness of DMP in complex organic molecules, showcasing its utility in organic synthesis.
🚯 Environmental and Safety Concerns in Organic Chemistry: PCC and Swern Oxidations
This paragraph addresses the environmental and safety concerns associated with the use of PCC and Swern oxidations due to the generation of chromium waste and unpleasant odors, respectively. It explains the mechanism of PCC oxidation, which involves the alcohol attacking the chromium center and the subsequent formation of aldehydes, while cautioning about the need for dry conditions to prevent the formation of chromic acid. The paragraph also describes the Swern oxidation process, emphasizing the sigmatropic rearrangement step, which is a common point of confusion and a frequent topic in organic chemistry exams. It provides examples of Swern oxidation in the literature, highlighting the clean conversion of alcohols to aldehydes despite the presence of various functional groups. Additionally, it warns of the challenges associated with handling the byproduct dimethyl sulfide, which has a strong, unpleasant odor.
📚 Practice Problems and Assignments in Organic Chemistry
The final paragraph concludes the lecture by assigning practice problems related to the oxidation of primary and secondary alcohols to aldehydes and ketones. It challenges students to predict the products of reactions involving DMP and to understand the reactivity of different types of alcohols with DMP. Additionally, it introduces a variant of the Swern oxidation using DMSO and acetic anhydride as an activator, emphasizing the versatility of oxidation techniques in organic chemistry. The paragraph encourages students to engage with the material by attempting the practice problems and to seek clarification through questions or comments if needed.
Mindmap
Keywords
💡Oxidation
💡Aldehydes
💡Ketones
💡Sodium Borohydride
💡Lithium Aluminum Hydride
💡Dess-Martin Periodinane
💡Pyridinium Chlorochromate
💡Swern Oxidation
💡Sigmatropic Rearrangements
💡Selective Reduction
💡Conjugated Alkene
Highlights
Introduction to the topic of oxidation reactions that produce aldehydes and ketones in organic chemistry.
Explanation of the importance of choosing the right reducing agent to reduce esters or ketones, with a focus on sodium borohydride.
Discussion on the use of lithium aluminum hydride (LAH) for reducing carbonyl compounds, halides, and other functional groups.
The selection of sodium borohydride over LAH to avoid unwanted reduction of sensitive functional groups.
Introduction to the reverse reaction of converting alcohols into aldehydes and ketones.
The difficulty of partial reduction of esters to aldehydes and the common practice of over-reduction.
Three main methods for oxidizing alcohols to aldehydes: DMP oxidation, PCC oxidation, and Swern oxidation.
DMP oxidation as a mild method for accessing aldehydes with a focus on its structure and mechanism.
The reactivity of benzylic and allylic alcohols with DMP and their potential over-oxidation issues.
The issue of unreactive or insoluble DMP and its possible remedies, including recrystallization or synthesis from scratch.
PCC oxidation, its advantages, and the environmental concerns associated with chromium waste.
The mechanism of Swern oxidation, including the sigmatropic rearrangement and the formation of dimethyl sulfide.
The unpleasant smell of dimethyl sulfide produced in Swern oxidation and its impact on lab safety.
Examples from the literature demonstrating the effectiveness of DMP, PCC, and Swern oxidations on various substrates.
The importance of understanding the mechanism of Swern oxidation for organic chemistry exams.
Practice problems assigned to reinforce the concepts of oxidation of primary and secondary alcohols to aldehydes and ketones.
Final thoughts on the utility of the discussed oxidation methods and their practical applications in organic chemistry.
Transcripts
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