Ch. 16 - Reactions of Aldehydes and Ketones • More Reactions of Carboxylic Acid Derivatives

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Bruice 8th Edition

Ch. 16 - Reactions of Aldehydes and Ketones • More Reactions of Carboxylic Acid Derivatives

Problema 88

Capitolo 16, Problema 88

In the presence of an acid catalyst, acetaldehyde forms a trimer known as paraldehyde. Because it induces sleep when it is administered to animals in large doses, paraldehyde is used as a sedative or hypnotic. Propose a mechanism for the formation of paraldehyde.

Guida verificata passo dopo passo

Step 1: Protonation of acetaldehyde - In the presence of an acid catalyst such as HCl, the carbonyl oxygen of acetaldehyde (CH₃CHO) is protonated, increasing the electrophilicity of the carbonyl carbon. This step forms a resonance-stabilized carbocation intermediate.

Step 2: Nucleophilic attack - A second molecule of acetaldehyde acts as a nucleophile and attacks the carbocation intermediate formed in Step 1. This results in the formation of a dimer intermediate with a hydroxyl group and a new carbon-carbon bond.

Step 3: Protonation of the hydroxyl group - The hydroxyl group in the dimer intermediate is protonated by the acid catalyst, converting it into a better leaving group (water). This step prepares the molecule for further reaction.

Step 4: Formation of the trimer - A third molecule of acetaldehyde attacks the carbocation formed in Step 3, leading to the formation of a cyclic trimer structure. This step involves the creation of three carbon-oxygen bonds in the cyclic structure.

Step 5: Deprotonation and stabilization - The final step involves deprotonation of the cyclic trimer to yield paraldehyde, a stable cyclic trimer of acetaldehyde. This completes the mechanism.

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Acid-Catalyzed Reactions

Acid-catalyzed reactions involve the use of an acid to increase the rate of a chemical reaction. In organic chemistry, acids can protonate reactants, making them more electrophilic and facilitating nucleophilic attacks. Understanding this concept is crucial for analyzing how acetaldehyde reacts under acidic conditions to form paraldehyde.

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Acid Catalyzed

Aldol Condensation

Aldol condensation is a reaction where aldehydes or ketones react in the presence of a base or acid to form β-hydroxy aldehydes or ketones, which can further dehydrate to yield α,β-unsaturated carbonyl compounds. This mechanism is essential for understanding how acetaldehyde can undergo self-condensation to form paraldehyde through a series of aldol reactions.

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Crossed Aldol

Trimerization

Trimerization is a chemical process where three molecules of a reactant combine to form a single product. In the case of acetaldehyde, trimerization leads to the formation of paraldehyde. Recognizing the significance of this process helps in visualizing how multiple acetaldehyde units can interact to create a larger, cyclic compound.

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Percorso guidato

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Shown below is the ¹H NMR spectrum of the alkyl bromide used to make the phosphonium ylide that reacts with a ketone in a Wittig reaction to form a compound with molecular formula C₁₁H₁₄. What product is obtained from the Wittig reaction?

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What carbonyl compound and what phosphonium ylide are needed to synthesize the following compounds?

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When a cyclic ketone reacts with diazomethane, the next larger cyclic ketone is formed. This is called a ring-expansion reaction. Draw a mechanism for the following ring-expansion reaction.

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Identify compounds A and B:

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Propose a mechanism for each of the following reactions:

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