Lithium Aluminum Hydride LiAlH4 Reduction Reaction + Mechanism

Leah4sci
26 Feb 201609:36
EducationalLearning
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TLDRThis video explores carbonyl reduction reactions using lithium aluminum hydride (LiAlH4), a potent reducing agent for converting aldehydes, ketones, carboxylic acids, and their derivatives into alcohols and amines. The script delves into the mechanisms of these reactions, highlighting the differences between ester and aldehyde/ketone reductions, and the importance of protonation in product formation. It also emphasizes the chirality of alcohols produced from asymmetric ketones and the unique reduction process of carboxylic acids, providing a comprehensive understanding of organic chemistry's reduction techniques.

Takeaways
  • πŸ” Lithium aluminum hydride (LiAlH4), also known as LAH, is a stronger reducing agent compared to sodium borohydride, which was discussed in a previous video.
  • 🌟 LAH has a unique structure with a positive spectator ion and a negatively charged hydride anion, which prefers to have only three bonds.
  • βš”οΈ LAH is capable of reducing a wide range of carbonyl compounds, typically converting them into alcohols, with the reduction mechanism involving the breaking of the Ο€ bond and the addition of hydrogen to both carbon and oxygen.
  • 🍸 Both ketones and aldehydes can be reduced by LAH to secondary and primary alcohols, respectively, with the potential for chiral products if the starting material is asymmetric.
  • πŸ§ͺ LAH also reacts with carboxylic acids and their derivatives, such as esters, through a different mechanism involving the breaking of the Ο€ bond and the removal of the electronegative leaving group.
  • πŸ”„ The reduction of esters by LAH involves an initial nucleophilic attack by the hydride, followed by the collapse of the Ο€ bond and the formation of an aldehyde intermediate.
  • πŸ§ͺ The reduction of carboxylic acids by LAH is unique, as it involves the attack on the acidic hydrogen rather than the carbonyl carbon, leading to the formation of a carboxylate and hydrogen gas.
  • πŸŒ€ The final step in the reduction process often involves the introduction of an acid, such as H3O+, to protonate the oxygen and complete the formation of the primary alcohol.
  • πŸ“š The script emphasizes the importance of understanding the source of hydrogen atoms in the products, distinguishing between those from LAH and those from the protonating solvent.
  • πŸ”¬ The mechanism for reducing aldehydes and ketones with LAH is similar to that with sodium borohydride, but the ester reduction mechanism is distinct and involves the formation and subsequent removal of a leaving group.
  • πŸ“š The video promises a follow-up on the similarities and differences between reduction using sodium borohydride and lithium aluminum hydride, and additional resources such as practice quizzes and cheat sheets are available on the presenter's website.
Q & A

  • What is lithium aluminum hydride (LiAlH4) commonly referred to as and why is it stronger than sodium borohydride?

    -Lithium aluminum hydride is commonly referred to as LAH. It is stronger than sodium borohydride because it acts as a better nucleophile and is capable of reducing a wider range of carbonyl compounds, including carboxylic acids and their derivatives, not just aldehydes and ketones.

  • How does the general structure of lithium aluminum hydride differ from that of sodium borohydride?

    -The general structure of lithium aluminum hydride consists of a positive spectator ion and an anion (AlH4-) which prefers to have only three bonds, similar to sodium borohydride. However, in LAH, the anion has a fourth bond to hydrogen, adding an extra pair of electrons and a formal charge of minus 1, making it more reactive.

  • What happens when lithium aluminum hydride reacts with carbonyl compounds?

    -When lithium aluminum hydride reacts with carbonyl compounds, such as ketones and aldehydes, it typically reduces them to alcohols. The reaction involves breaking the pi bond and adding hydrogen to both the carbon and oxygen atoms.

  • What type of alcohol is produced when a ketone is reduced with lithium aluminum hydride?

    -A ketone, when reduced with lithium aluminum hydride, is converted into a secondary alcohol.

  • How does the reduction of an aldehyde by lithium aluminum hydride differ from that of a ketone?

    -An aldehyde, when reduced by lithium aluminum hydride, is converted into a primary alcohol, whereas a ketone is reduced to a secondary alcohol.

  • What is special about the alcohol product when starting with an asymmetric ketone like 2-butanone?

    -When starting with an asymmetric ketone like 2-butanone, the alcohol product, 2-butanol, will be on a chiral carbon, resulting in a racemic mixture due to the flat, sp2 hybridized carbonyl starting structure.

  • Can lithium aluminum hydride reduce carboxylic acids and their derivatives? If so, what is the end product?

    -Yes, lithium aluminum hydride can reduce carboxylic acids and their derivatives. The end product is typically a primary alcohol, with the hydrogen on the oxygen atom coming from the protonation step.

  • What is the role of the leaving group in the reduction of esters by lithium aluminum hydride?

    -In the reduction of esters by lithium aluminum hydride, the leaving group (OCH3) is crucial. It gets kicked out and broken off after the addition of hydrogen from lithium aluminum hydride, allowing the formation of an aldehyde intermediate.

  • How does the mechanism of reducing aldehydes and ketones with lithium aluminum hydride differ from that with sodium borohydride?

    -The mechanism for reducing aldehydes and ketones with lithium aluminum hydride is the same as with sodium borohydride, involving nucleophilic attack by the hydride ion on the partially positive carbonyl carbon.

  • What is unique about the reduction mechanism of carboxylic acids with lithium aluminum hydride?

    -The unique aspect of the reduction mechanism of carboxylic acids with lithium aluminum hydride is that the hydride attacks the acidic hydrogen instead of the carbonyl carbon, breaking it off and forming a carboxylate and Al(AlH4)-, which then undergoes further reduction to form a primary alcohol.

  • What happens when deuterium (D3O+) is used as the acid in the final protonation step instead of H3O+?

    -When deuterium (D3O+) is used instead of H3O+ in the final protonation step, the hydrogen atoms on carbon would come from lithium aluminum hydride, and the hydrogen atom on oxygen would come from the deuterium, resulting in a primary alcohol with deuterium at the hydroxyl group (OD instead of OH).

  • What is the final product of the reduction of an amide by lithium aluminum hydride?

    -The final product of the reduction of an amide by lithium aluminum hydride is a primary amine, as the reaction results in the complete loss of the oxygen atom from the amide group.

Outlines
00:00
πŸ” Carbonyl Reduction with Lithium Aluminum Hydride

This paragraph introduces the use of lithium aluminum hydride (LiAlH4), a powerful reducing agent, for carbonyl reduction reactions. It explains that LiAlH4 is stronger than sodium borohydride and can reduce aldehydes and ketones to alcohols. The reaction mechanism involves breaking the pi bond and adding hydrogen to both carbon and oxygen. The paragraph also discusses the reduction of carboxylic acids and their derivatives, highlighting the process of removing the electronegative group and the formation of primary alcohols. It mentions the potential for chiral products and the formation of racemic alcohols from asymmetric ketones. The reduction of esters and amides is also covered, with a focus on the unique mechanisms involved, especially the ester reduction that results in an aldehyde intermediate before yielding a primary alcohol. The paragraph concludes by emphasizing the strong reducing nature of LiAlH4 and its reactivity with aldehydes in solution.

05:01
🌟 Mechanistic Insights into Ester and Carboxylic Acid Reductions

The second paragraph delves deeper into the mechanisms of ester and carboxylic acid reduction by lithium aluminum hydride. It describes the initial attack of the hydride on the ester's carbonyl carbon, leading to the formation of an intermediate aldehyde. The subsequent steps involve the collapse of the pi bond and the departure of the leaving group (OCH3), forming a primary alcohol. The paragraph also explains the unique reduction mechanism of carboxylic acids, where the hydride attacks the acidic hydrogen instead of the carbonyl carbon, resulting in the formation of a carboxylate and hydrogen gas. The reaction continues with the hydride attacking the carbonyl carbon, leading to the formation of another aldehyde intermediate. The final step involves the introduction of acid to protonate the oxygen, converting the aldehyde into a primary alcohol. The paragraph concludes by emphasizing the importance of understanding these mechanisms, especially the source of hydrogen atoms in the final products, and hints at the upcoming comparison between sodium borohydride and lithium aluminum hydride in the next video.

Mindmap
Keywords
πŸ’‘Lithium Aluminum Hydride (LiAlH4)
Lithium aluminum hydride, commonly abbreviated as LAH, is a strong reducing agent used in organic chemistry. It is stronger than sodium borohydride and is capable of reducing a wide range of carbonyl compounds. In the video, LAH is described as having a general structure with a positive spectator ion and an anion that prefers to have only three bonds, which is characteristic of its reactivity. The video script explains how LAH can reduce aldehydes and ketones to alcohols, and it also reacts with carboxylic acids and their derivatives.
πŸ’‘Carbonyl Reduction
Carbonyl reduction refers to the chemical process of converting a carbonyl group (C=O) into an alcohol (OH) group. The video script discusses how LAH can reduce carbonyl compounds such as aldehydes and ketones to their corresponding alcohols. This is a key reaction in organic synthesis, and the script provides a detailed mechanism for how LAH facilitates this reduction.
πŸ’‘Aldehydes
Aldehydes are a class of organic compounds containing a terminal carbonyl group (R-CHO). In the context of the video, aldehydes are reduced by LAH to primary alcohols. The script explains that the reduction of aldehydes involves the addition of hydrogen to the carbonyl carbon and the migration of the pi bond to the oxygen atom, resulting in the formation of a chiral alcohol.
πŸ’‘Ketones
Ketones are organic compounds with a carbonyl group bonded to two other carbon atoms (R-CO-R'). The script mentions that ketones are reduced by LAH to secondary alcohols. This reduction process involves the same mechanism as aldehydes, but the resulting alcohol has a chiral center due to the presence of two different groups attached to the carbonyl carbon.
πŸ’‘Chiral Center
A chiral center is a carbon atom that is bonded to four different groups, resulting in non-superimposable mirror images called enantiomers. The video script explains that the reduction of both aldehydes and ketones by LAH can lead to the formation of chiral centers, which is significant in the study of stereochemistry and the synthesis of optically active compounds.
πŸ’‘Carboxylic Acids
Carboxylic acids are organic compounds containing a carboxyl group (-COOH). The script describes how LAH can reduce carboxylic acids to primary alcohols. This reduction involves the loss of the hydroxyl group and the addition of hydrogen to the carbonyl carbon, resulting in a primary alcohol with two hydrogens added from LAH.
πŸ’‘Carboxyl Derivatives
Carboxyl derivatives are compounds derived from carboxylic acids by the replacement of the hydroxyl group with another functional group. The video script explains that LAH can also reduce these derivatives, such as esters, to primary alcohols. The reduction mechanism involves the cleavage of the pi bond and the removal of the leaving group, which is different from the reduction of aldehydes and ketones.
πŸ’‘Ester Reduction
Ester reduction refers to the chemical process where an ester is converted to an alcohol through the action of a reducing agent. The script provides a detailed mechanism for the reduction of esters by LAH, which involves the initial attack of the hydride on the carbonyl carbon and the subsequent removal of the leaving group, resulting in the formation of an aldehyde intermediate and eventually a primary alcohol.
πŸ’‘Leaving Group
A leaving group is a part of a molecule that departs during a chemical reaction, often carrying away a pair of electrons. In the context of ester reduction by LAH, the script explains that the methoxy group (OCH3) acts as a leaving group, which is removed to allow the formation of an aldehyde intermediate and ultimately a primary alcohol.
πŸ’‘Primary Alcohol
A primary alcohol is an alcohol with the hydroxyl group (-OH) attached to a carbon atom that is bonded to only one other carbon atom. The script describes the formation of primary alcohols as the end product of the reduction of aldehydes, ketones, esters, and carboxylic acids by LAH. This is significant as primary alcohols are important intermediates in organic synthesis.
πŸ’‘Protonating Solvent
A protonating solvent is a substance that donates protons (H+) to another molecule, facilitating certain chemical reactions. The video script mentions the use of a protonating solvent, such as H3O+, in the final step of the reduction reactions to protonate the oxygen atom, leading to the formation of water and the final alcohol product.
Highlights

Lithium aluminum hydride (LiAlH4) is a stronger reducing agent than sodium borohydride, capable of reducing a wider range of carbonyl compounds.

LiAlH4 has a unique structure with a positive spectator ion and an anion that prefers to have only three bonds.

The reduction of carbonyl compounds with LiAlH4 typically results in the formation of alcohols.

Ketones are reduced to secondary alcohols, while aldehydes are reduced to primary alcohols with potential chirality.

LiAlH4 can also reduce carboxylic acids and their derivatives, necessitating the removal of the electronegative group.

Ester reduction with LiAlH4 involves a unique mechanism, starting with a nucleophilic attack by the hydride.

In ester reduction, a leaving group is formed, which is eventually kicked out to form an aldehyde intermediate.

The final step in ester reduction involves protonation to form a primary alcohol.

Carboxylic acid reduction with LiAlH4 is distinct, involving an initial attack on the acidic hydrogen.

Aluminum hydride's reactivity allows it to further react with the intermediate to form a primary alcohol.

Amide reduction with LiAlH4 results in the formation of a primary amine, unlike the alcohol formation in other reductions.

The mechanism for reducing aldehydes and ketones with LiAlH4 is similar to that with sodium borohydride.

LiAlH4 is a strong reducing agent, and its reaction with aldehydes can lead to further reduction steps.

The use of deuterium as the acid in the reduction can result in the formation of deuterated alcohol products.

The video series includes a comparison between reduction using sodium borohydride and lithium aluminum hydride.

Additional resources such as practice quizzes and cheat sheets are available on the presenter's website.

Transcripts

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