NaBH4, LiAlH4, DIBAL Reduction Mechanism, Carboxylic Acid, Acid Chloride, Ester, & Ketones
TLDRThis chemistry video tutorial delves into the reduction of functional groups like ketones, aldehydes, and carboxylic acids using reducing agents such as sodium borohydride (NaBH4) and lithium aluminum hydride (LiAlH4). It explains the mechanisms and outcomes of these reactions, highlighting the selectivity and reactivity differences between the reducing agents. The video also discusses the use of protecting groups to selectively reduce certain functional groups without affecting others, providing step-by-step procedures for each scenario.
Takeaways
- 🔬 Sodium borohydride (NaBH4) is a mild reducing agent, less reactive than lithium aluminum hydride (LiAlH4).
- ⚗️ NaBH4 can reduce aldehydes and ketones to alcohols but does not reduce carboxylic acids or esters.
- 🚫 LiAlH4 is a much stronger reducing agent, capable of reducing carboxylic acids, esters, and even amides to alcohols or amines.
- 🔄 The reduction mechanism involves the addition of hydrogen atoms: NaBH4 adds one hydrogen to aldehydes/ketones, while LiAlH4 adds two hydrogens to carboxylic acids/esters.
- 🔧 When NaBH4 reduces cyclohexanone, it produces cyclohexanol. Similarly, it converts aldehydes to alcohols.
- 📉 Acid chlorides are reduced to alcohols by NaBH4 through a different mechanism, adding two hydrogens.
- ⚛️ In the reduction mechanism, hydride ions from NaBH4 or LiAlH4 attack the carbonyl carbon, breaking the pi bond and forming an alkoxide ion.
- 🧪 LiAlH4 reduction of carboxylic acids produces alcohols by first converting to a carboxylate ion, then forming a tetrahedral intermediate.
- 🔒 Protecting groups, like ethylene glycol, can protect ketones during reduction, allowing selective reduction of esters.
- 🌡️ Reduction reactions with LiAlH4 or NaBH4 should be carried out in specific conditions to control reactivity and achieve desired products.
Q & A
What are reducing agents and why are they used in organic chemistry?
-Reducing agents are chemicals that can donate electrons to other substances, reducing them to a simpler form. They are used in organic chemistry to convert functional groups into other forms, such as converting ketones to alcohols or reducing esters to alcohols.
What is sodium borohydride (NaBH4) and how does it differ from lithium aluminum hydride (LiAlH4) in terms of reactivity?
-Sodium borohydride (NaBH4) is a mild reducing agent that is less reactive than lithium aluminum hydride (LiAlH4). NaBH4 is used to reduce aldehydes and ketones but does not reduce carboxylic acids or esters. In contrast, LiAlH4 is a more powerful reducing agent capable of reducing a wider range of functional groups, including carboxylic acids, esters, and amides.
Can you explain the reduction of cyclohexanone using sodium borohydride?
-When cyclohexanone is treated with sodium borohydride, the carbonyl group of the ketone is reduced to an alcohol. The hydride ion from NaBH4 attacks the carbonyl carbon, forming an alkoxide ion, which is then protonated to form cyclohexanol.
What happens when an acid chloride reacts with sodium borohydride?
-An acid chloride reacts with sodium borohydride by adding two hydrogen atoms to the carbonyl group, replacing the carbonyl with two hydroxyl groups (OH). The chloride ion is expelled as a leaving group, resulting in the formation of an alcohol.
How does the mechanism of reduction differ between ketones and acid chlorides when using sodium borohydride?
-For ketones, sodium borohydride adds one hydrogen atom to the carbonyl group, forming an alkoxide ion that is then protonated to yield an alcohol. For acid chlorides, two hydrogen atoms are added, first reducing the acid chloride to an aldehyde and then to an alcohol, with the chloride ion being expelled as the leaving group.
What is the role of electronegativity in the reduction of carbonyl compounds using sodium borohydride?
-Electronegativity plays a crucial role in the reduction mechanism. Since hydrogen is more electronegative than boron, it bears a partial negative charge and is attracted to the partially positive carbonyl carbon. This difference in electronegativity facilitates the nucleophilic attack of the hydride ion on the carbonyl carbon.
Can lithium aluminum hydride (LiAlH4) reduce esters and what is the product of this reduction?
-Yes, lithium aluminum hydride can reduce esters. The reduction of an ester by LiAlH4 results in the formation of an alcohol, with the ester group being replaced by two hydrogen atoms and the leaving group (such as an alkoxide ion) being expelled.
How does the reduction of a carboxylic acid by lithium aluminum hydride differ from the reduction of an ester?
-In the reduction of a carboxylic acid by LiAlH4, one of the oxygen atoms is removed, and the carboxylic acid is converted into an alcohol. This involves a two-step process where the hydride ion first acts as a base to remove a hydrogen atom, forming a carboxylate ion, and then as a nucleophile to add two hydrogen atoms to the carbonyl group.
What is the purpose of using protecting groups in organic synthesis, and how are they applied in the reduction of a molecule containing both a ketone and an ester?
-Protecting groups are used to temporarily mask functional groups from reacting under certain conditions. In the case of a molecule with both a ketone and an ester, a protecting group like ethylene glycol can be used to mask the ketone, allowing the selective reduction of the ester with lithium aluminum hydride. After the ester is reduced, the protecting group is removed to restore the ketone.
What is the significance of temperature control in the reduction of esters to aldehydes using diisobutyl aluminum hydride (DIBAL-H)?
-Temperature control is crucial in the reduction of esters to aldehydes with DIBAL-H. The reaction is carried out at low temperatures (-78°C) to prevent the reduction from proceeding further to the alcohol stage. After all the ester has reacted, the solution is warmed to remove the leaving group, resulting in the formation of an aldehyde.
Outlines
🧪 Chemistry of Reducing Agents
This paragraph introduces the topic of reducing agents, focusing on sodium borohydride (NaBH4), lithium aluminum hydride (LiAlH4), and diisobutyl aluminum hydride (DIBAL-H). It explains the reactivity differences among these agents, noting that NaBH4 is milder and less reactive than LiAlH4. The paragraph details how NaBH4 can reduce aldehydes and ketones but not carboxylic acids or esters, whereas LiAlH4 is more powerful and can reduce a wider range of compounds. The summary also covers the reactions of these reducing agents with ketones, aldehydes, and acid chlorides, resulting in the formation of alcohols, and explains the mechanism of reduction involving hydride ion attack on the carbonyl group and subsequent protonation to form the alcohol product.
🌡 Reaction Mechanisms and Selectivity
The second paragraph delves into the mechanisms of reduction reactions with acid chlorides and the selectivity of reducing agents. It describes the two-hydrogen addition required for acid chloride reduction to alcohols, contrasting it with the single hydrogen addition for aldehydes and ketones. The paragraph explains the nucleophilic addition reaction, the formation of tetrahedral intermediates, and the selection of leaving groups based on stability. It also touches on the handling of reducing agents, emphasizing the caution required with LiAlH4 due to its reactivity with water, and introduces the concept of protecting groups to selectively reduce certain functional groups in the presence of others.
🛠️ Reducing Esters and Amides with Lithium Aluminum Hydride
This paragraph discusses the reduction of esters and amides using lithium aluminum hydride (LiAlH4). It outlines the process of converting esters into alcohols and amides into amines, highlighting the addition of two hydrogen atoms to the carbonyl group and the expulsion of the leaving group. The summary describes the mechanism involving the attack of the hydride ion on the carbonyl carbon, the formation of a tetrahedral intermediate, and the subsequent steps leading to the final alcohol or amine product. It also explains the role of water in the final protonation step to yield the alcohol and the stability considerations for leaving groups in these reactions.
🔍 Selective Reduction of Functional Groups
The fourth paragraph explores the selective reduction of functional groups in organic chemistry. It poses a question regarding the behavior of the hydride ion when reacting with carboxylic acids, explaining that the hydride ion acts as a base rather than a nucleophile in this scenario. The summary details the mechanism of carboxylic acid reduction to an alcohol using lithium aluminum hydride, including the formation of a carboxylate ion, the combination of hydrogen atoms to form hydrogen gas, and the Lewis acid-base reaction between the oxygen with a negative charge and the aluminum atom.
🌟 Advanced Reduction Mechanisms with Lithium Aluminum Hydride
This paragraph continues the discussion on the reduction mechanisms involving lithium aluminum hydride, focusing on the reduction of amides to amines and the unique behavior of the hydride ion in these reactions. It describes the process of resonance in amides and how the hydride ion acts as a base to remove a hydrogen atom, leading to the formation of an amine. The summary explains the subsequent steps of the reaction, including the attack of the hydride ion on the carbonyl group, the formation of intermediates, and the final protonation to yield the amine product.
❄️ Low-Temperature Reduction with Dibal-H
The sixth paragraph introduces the use of diisobutyl aluminum hydride (DIBAL-H) for the low-temperature reduction of esters to aldehydes. It explains the unique structure of DIBAL-H, with only one hydrogen atom available for the reduction process, which stops at the aldehyde level. The summary describes the mechanism of this reduction, emphasizing the importance of maintaining a low temperature (-78°C) to prevent further reduction to alcohols and the subsequent warming of the solution to complete the reaction.
🔬 Strategies for Selective Reduction in Complex Molecules
The final paragraph presents strategies for selectively reducing functional groups in complex molecules containing both ketones and esters. It discusses the use of lithium aluminum hydride for the reduction of both functional groups, sodium borohydride for selective reduction of ketones, and the use of protecting groups to selectively reduce esters without affecting ketones. The summary outlines the steps involved in protecting the ketone with ethylene glycol, reducing the ester with lithium aluminum hydride, and then removing the protecting group under acidic conditions to restore the ketone.
Mindmap
Keywords
💡Reducing agents
💡Ketones
💡Aldehydes
💡Carboxylic acids
💡Esters
💡Amides
💡Protecting groups
💡Acid chlorides
💡Nucleophilic addition
💡Tetrahedral intermediate
💡Lewis acid-base reactions
Highlights
Sodium borohydride (NaBH4) is a mild reducing agent suitable for reducing aldehydes and ketones but not carboxylic acids or esters.
Lithium aluminum hydride (LiAlH4) is a more powerful and dangerous reducing agent capable of reducing carboxylic acids, esters, and amides.
NaBH4 converts ketones into secondary alcohols and aldehydes into primary alcohols through the addition of hydrogen atoms.
The reduction of acid chlorides by NaBH4 involves the addition of two hydrogens and the expulsion of the chloride ion.
Electronegativity differences between hydrogen and boron are key to understanding the nucleophilic nature of hydride ions in reduction reactions.
The reduction of ketones to alcohols by NaBH4 involves the protonation of alkoxide ions to form the final alcohol product.
LiAlH4 can reduce esters to alcohols by adding two hydrogen atoms and the ester group is replaced by a hydroxyl group.
Carboxylic acids are reduced to alcohols by LiAlH4 through a two-step process involving the formation and expulsion of a carboxylate ion.
Amides are reduced to amines by LiAlH4 through a complex mechanism involving the transfer of hydrogen atoms and the rearrangement of the nitrogen atom's bonds.
Lithium tri-tert-butoxy aluminum hydride (LiAl(OtBu)3H) is a selective reducing agent that stops at the aldehyde level due to having only one hydrogen atom.
Diisobutyl aluminum hydride (DIBAL-H) is used to selectively reduce esters to aldehydes at low temperatures, preventing further reduction.
Protecting groups, such as ethylene glycol, can be used to selectively reduce functional groups in the presence of others, like reducing an ester without affecting a ketone.
The use of acid catalysis is essential in the formation of protecting groups, such as acetals, from ketones.
Removal of protecting groups can be achieved through acidic conditions, allowing the original functional group to be restored.
The video provides a comprehensive guide on the use of different reducing agents in organic chemistry and their selectivity towards various functional groups.
The importance of understanding electronegativity and partial charges in predicting the outcomes of reduction reactions is emphasized.
The video concludes with practical applications of reduction reactions, including the synthesis of alcohols, aldehydes, and amines from common organic compounds.
Transcripts
5.0 / 5 (0 votes)
Thanks for rating: