Aldehydes and Ketones | Summary and Q&A

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May 5, 2018
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The Organic Chemistry Tutor
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Aldehydes and Ketones

TL;DR

The video discusses the reduction reactions of ketones and aldehydes using sodium borohydride and lithium aluminum hydride, as well as the addition reactions using Grignard reagents and weak bases. It also covers the distinction between direct addition and conjugate addition reactions.

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Questions & Answers

Q: What is the difference between the reduction reactions of ketones and aldehydes using sodium

borohydride and lithium aluminum hydride?

Both sodium borohydride and lithium aluminum hydride can reduce ketones and aldehydes into alcohols. However, sodium borohydride is milder and typically reduces ketones to secondary alcohols, while lithium aluminum hydride is stronger and can reduce both ketones and aldehydes to primary alcohols.

Q: How does the mechanism of reduction using sodium borohydride or lithium aluminum hydride work?

In the first step, the hydride ion attacks the carbonyl carbon, forming an alkoxide ion. In the second step, the alkoxide ion reacts with a proton source, such as H3O+, to convert it into an alcohol. This mechanism involves the addition of hydride ions to the carbonyl carbon.

Q: Can sodium borohydride or lithium aluminum hydride reduce esters or carboxylic acids?

Sodium borohydride is not strong enough to reduce esters or carboxylic acids. However, lithium aluminum hydride can reduce both esters and carboxylic acids to alcohols.

Q: What is the difference between a direct addition reaction and a conjugate addition reaction?

In a direct addition reaction, a strong base attacks the carbonyl carbon, leading to the addition of a nucleophile to the molecule. In a conjugate addition reaction, a weak base attacks the beta carbon adjacent to the carbonyl carbon, resulting in the addition of a nucleophile to the molecule. The choice between direct and conjugate addition depends on the strength of the base and the accessibility of the carbon atom.

Summary & Key Takeaways

  • Reduction reactions of ketones and aldehydes using sodium borohydride and lithium aluminum hydride result in the formation of alcohols, with the type of alcohol depending on the reagent used.

  • Addition reactions of ketones and aldehydes with Grignard reagents and weak bases can lead to the formation of alcohols or addition products depending on the strength of the base.

  • The distinction between direct addition (strong base) and conjugate addition (weak base) reactions is important in determining the major product.

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