Question 1

Why does lithium aluminum hydride cause over-reduction of carboxylic acids to aldehydes?

Accepted Answer

Lithium aluminum hydride (LAH) is an extremely strong reducing agent that reduces carboxylic acids faster than aldehydes can be isolated. The aldehyde product forms instantly but is immediately reduced to a primary alcohol because aldehydes are more easily reduced than carboxylic acids. This over-reduction makes LAH unsuitable for controlled aldehyde synthesis from carboxylic acids.

Question 2

What milder reducing agents can selectively convert carboxylic acids to aldehydes?

Accepted Answer

Aluminum hydride derivatives like lithium tri-tert-butoxyaluminum hydride and diisobutylaluminum hydride (DIBAL-H) are milder alternatives to LAH. These alkylaluminum hydrides have increased steric bulk, making them less reactive and allowing selective reduction of carboxylic acid derivatives to aldehydes without over-reduction to primary alcohols.

Question 3

How do organolithium reagents produce ketones directly from carboxylic acids?

Accepted Answer

Carboxylic acids react rapidly with two equivalents of organolithium reagents to form a dianion intermediate. Protonation of this dianion yields a hydrate, which loses water to give the corresponding ketone. Unlike aldehydes, ketones form directly without over-reduction, making this method efficient for ketone synthesis from carboxylic acids.

Question 4

What is the mechanism for preparing aldehydes from nitriles using DIBAL-H?

Accepted Answer

DIBAL-H reduces nitriles to form an aluminum complex intermediate. Upon hydrolysis, this complex yields the corresponding aldehyde. This partial reduction is selective because DIBAL-H's steric bulk prevents further reduction of the aldehyde product, making it ideal for aldehyde synthesis from nitriles.

Question 5

How do Grignard and organolithium reagents convert nitriles to ketones?

Accepted Answer

Grignard and organolithium reagents reduce nitriles to form imine intermediates. Subsequent acid hydrolysis of these imines yields the corresponding ketones. This two-step process allows selective ketone formation from nitriles through imine intermediates, providing an alternative route to ketone synthesis from nitrogen-containing precursors.

Question 6

What is the difference between aldehyde and ketone formation from carboxylic acids?

Accepted Answer

Aldehydes require milder reducing agents like DIBAL-H to prevent over-reduction to primary alcohols, while ketones form directly from carboxylic acids using organolithium reagents without over-reduction. This fundamental difference reflects the varying reactivity of aldehydes versus ketones toward strong reducing agents.

Question 7

Why are carboxylic acid derivatives preferred over free carboxylic acids for aldehyde synthesis?

Accepted Answer

Carboxylic acid derivatives like acyl chlorides, esters, and nitriles are more easily reduced to aldehydes using milder aluminum hydride agents. These derivatives avoid the over-reduction problem encountered with free carboxylic acids and LAH, allowing selective conversion to aldehydes through controlled reduction pathways.

Preparação de Aldeídos e Cetonas a partir de Nitrilas e Ácidos Carboxílicos

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