20.5 Hydride Reduction Reactions | Carboxylic Acid Derivatives | Organic Chemistry
TLDRThis video script delves into the world of hydride reduction reactions, specifically focusing on carboxylic acids and their derivatives. It outlines the use of four key reagents: sodium borohydride, lithium aluminum hydride, diisobutyl aluminum hydride, and lithium tri-tert-butoxy aluminum hydride. Sodium borohydride is highlighted as less reactive, capable of reducing acid chlorides and anhydrides to primary alcohols but not suitable for esters, carboxylic acids, amides, or nitriles. Lithium aluminum hydride, or LAH, is showcased as a more potent reagent, reducing a broader range of carboxylic acid derivatives to primary alcohols, with the exception of amides and nitriles, which yield amines instead. The script also introduces specialty reagents that offer selective reduction, allowing for the formation of aldehydes as products under specific conditions. The video is part of an organic chemistry series aimed at educating viewers throughout the school year, with an invitation to subscribe for updates.
Takeaways
- 🔍 Sodium borohydride and lithium aluminum hydride are key reagents for the reduction of carboxylic acid derivatives, with lithium aluminum hydride being more reactive.
- 🔬 Sodium borohydride is less reactive and can only reduce acid chlorides and anhydrides to primary alcohols, not reacting with esters, carboxylic acids, amides, or nitriles.
- ⚙️ Lithium aluminum hydride (LAH) is capable of reducing a wider range of functional groups, including acid chlorides, anhydrides, carboxylic acids, and esters, all the way to primary alcohols.
- 🌡️ Special conditions like low temperatures are required to selectively stop the reduction at the aldehyde stage with certain reagents, such as lithium tri-tert-butoxy aluminum hydride.
- 📉 Diba (diisobutyl aluminum hydride) is less reactive than LAH and can selectively reduce esters to aldehydes under the right conditions.
- 🔁 Both sodium borohydride and LAH undergo nucleophilic substitution and addition reactions, but the mechanisms differ slightly based on the starting material.
- 🚫 Protic solvents should not be used with LAH due to its high reactivity, which necessitates an acid workup step.
- 🔄 The reduction of amides and nitriles with LAH results in the formation of amines, not primary alcohols, which is a different outcome from other carboxylic acid derivatives.
- ❗ It's important not to confuse the reduction of amides with the Hoffmann rearrangement, which also produces amines but results in the loss of a carbon atom.
- 🔬 Specialty reagents like lithium tri-tert-butoxy aluminum hydride and Diba offer selectivity in reductions, allowing for the formation of aldehydes without further reduction to alcohols under specific conditions.
- 📚 The video script is part of an organic chemistry playlist released weekly, providing a comprehensive guide for students throughout the school year.
Q & A
What are the four major reagents discussed in the transcript for hydride reduction reactions?
-The four major reagents discussed are sodium borohydride, lithium aluminum hydride, diisobutyl aluminum hydride, and lithium tri-tert-butoxy aluminum hydride.
Why is lithium aluminum hydride considered more important than sodium borohydride?
-Lithium aluminum hydride is considered more important because it is more reactive and capable of reducing a wider range of functional groups, including not only ketones and aldehydes but also carboxylic acids, esters, and amides.
What type of solvents should not be used with lithium aluminum hydride?
-Protic solvents should not be used with lithium aluminum hydride due to its high reactivity, which requires an acid workup step.
How does sodium borohydride typically react with acid chlorides and anhydrides?
-Sodium borohydride reacts with acid chlorides and anhydrides through a nucleophilic substitution mechanism, replacing the leaving group with a hydrogen and forming an aldehyde, which further reacts to form a primary alcohol.
What is the primary product of reducing an amide with lithium aluminum hydride?
-The primary product of reducing an amide with lithium aluminum hydride is not a primary alcohol but the corresponding amine, as the reduction results in the loss of the oxygen atom and addition of two hydrogens.
What is the difference between the reduction of an amide with lithium aluminum hydride and the Hoffmann rearrangement?
-The reduction of an amide with lithium aluminum hydride results in an amine without the loss of a carbon atom, whereas the Hoffmann rearrangement also produces an amine but results in the loss of the entire carbonyl group, effectively shortening the carbon chain by one carbon.
What are the conditions required to selectively reduce acid chlorides and anhydrides to aldehydes using lithium tri-tert-butoxy aluminum hydride?
-The reaction must be carried out at very low temperatures, such as -78 degrees Celsius, and with short reaction times to prevent further reduction of the aldehyde to a primary alcohol.
Which reagent is suitable for selectively reducing esters to aldehydes at low temperatures?
-Diibutyl aluminum hydride (also known as DIBAH or DIBAL-H) is suitable for selectively reducing esters to aldehydes at low temperatures.
What is the role of the acid workup step in hydride reduction reactions?
-The acid workup step is necessary to protonate the product of the reduction reaction, especially when using highly reactive reagents like lithium aluminum hydride, ensuring the formation of the desired alcohol or amine product.
How does the reduction mechanism of a carboxylic acid differ when using lithium aluminum hydride?
-While the initial nucleophilic acyl substitution mechanism is similar to other reductions, the carboxylic acid reduction with lithium aluminum hydride ultimately leads to the formation of an aldehyde, which can further react to form a primary alcohol.
What is the significance of the hydride ion in the reduction reactions discussed in the transcript?
-The hydride ion (H-) acts as a nucleophile, attacking the carbonyl carbon of the carboxylic acid derivatives, pushing electrons and facilitating the replacement of the leaving group with a hydrogen, leading to the formation of alcohols or, in the case of amides and nitriles, amines.
Outlines
🧪 Sodium Borohydride and Lithium Aluminum Hydride in Organic Chemistry
The first paragraph introduces the topic of hydride reduction reactions, specifically for carboxylic acids and their derivatives. It outlines the four major reagents to be discussed: sodium borohydride, lithium aluminum hydride, diisobutyl aluminum hydride, and lithium tri-tert-butoxy aluminum hydride. The paragraph emphasizes the importance of lithium aluminum hydride and its higher reactivity compared to sodium borohydride. It also explains that sodium borohydride is limited in its reactivity, only reacting with acid chlorides and anhydrides, not with esters, carboxylic acids, amides, or nitriles. The reduction mechanism of aldehydes to primary alcohols using sodium borohydride is described, highlighting the nucleophilic substitution involved and the option of using either a protic solvent or an acid workup step.
🌡️ Reactivity and Selectivity of Lithium Aluminum Hydride and Specialty Reagents
The second paragraph delves into the reactivity of lithium aluminum hydride (LAH), noting that it is more reactive and requires non-protic solvents. It discusses the reduction of various carboxylic acid derivatives to primary alcohols using LAH, including acid chlorides, anhydrides, carboxylic acids, and esters. The paragraph also covers the reduction of amides and nitriles to amines, which is a different outcome compared to the formation of primary alcohols. It cautions against confusing this reduction with the Hoffman rearrangement, which results in the loss of a carbon atom. The paragraph concludes with an introduction to specialty reducing agents, lithium tri-tert-butoxy aluminum hydride and diisobutyl aluminum hydride, which offer selectivity in reductions, allowing for the formation of aldehydes without further reduction to primary alcohols, given the right conditions.
🔍 Using Specialty Reagents for Controlled Reductions
The third paragraph focuses on the use of diisobutyl aluminum hydride (DIBAL-H) for controlled reductions of esters to aldehydes at low temperatures, preventing further reduction to primary alcohols. It contrasts DIBAL-H with lithium aluminum hydride, which is more reactive and does not allow for such selectivity. The paragraph also mentions that reactions should be carried out at low temperatures and for short durations to ensure the desired aldehyde product is obtained. The video script concludes with a call to action for viewers to like and share the content, and a mention of a study guide and practice problems available on the instructor's website.
Mindmap
Keywords
💡Hydride Reduction
💡Sodium Borohydride
💡Lithium Aluminum Hydride
💡Dialkyl Aluminum Hydrides
💡Lithium Tri-tert-butoxy Aluminum Hydride
💡Carboxylic Acid Derivatives
💡Primary Alcohol
💡Secondary Alcohol
💡Nucleophilic Substitution
💡Protic Solvent
💡Amine
Highlights
Sodium borohydride and lithium aluminum hydride both reduce ketones and aldehydes to alcohols, with lithium aluminum hydride being more reactive.
Sodium borohydride is less reactive and only reacts with acid chlorides and anhydrides, not with esters, carboxylic acids, amides, or nitriles.
Lithium aluminum hydride (LAH) is more reactive and can reduce a wider range of carboxylic acid derivatives, including esters and carboxylic acids.
Reactions with LAH require an acid workup step due to its reactivity with protic solvents.
LAH reduces acid chlorides, anhydrides, carboxylic acids, and esters to primary alcohols.
The reduction of amides and nitriles with LAH yields amines instead of primary alcohols.
Lithium tri-tert-butoxy aluminum hydride and diisobutyl aluminum hydride are specialty reagents that offer selectivity in reductions.
Lithium tri-tert-butoxy aluminum hydride can selectively reduce acid chlorides and anhydrides to aldehydes at low temperatures.
Diisobutyl aluminum hydride is used to selectively reduce esters to aldehydes, also requiring low temperatures to prevent further reduction.
The reduction of aldehydes to primary alcohols is a common outcome with most reducing agents discussed.
The mechanism of reduction with LAH involves nucleophilic substitution and nucleophilic addition steps.
The reduction of carboxylic acids with LAH is slightly different but still results in the formation of aldehydes and subsequent primary alcohols.
The reduction of amides and nitriles should not be confused with the Hoffmann rearrangement, which results in the loss of a carbon.
Hydrazine is another reducing agent that can be used for certain reductions in organic chemistry.
The use of specialty reagents allows for greater control over the reduction process and the products formed.
Temperature control is crucial in selective reductions to prevent unwanted over-reduction.
The choice of reducing agent and reaction conditions can significantly influence the outcome of a reduction reaction.
Transcripts
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