Propose a mechanism for the acid-catalyzed hydrolysis of cyclohexanone ethylene acetal.

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Identify the functional group in cyclohexanone ethylene acetal. The acetal group consists of two ether linkages (-OR) attached to the same carbon atom. Acetals are stable under basic conditions but can be hydrolyzed in the presence of an acid.

Protonate one of the ether oxygen atoms using the acid catalyst (e.g., H⁺). This step increases the electrophilicity of the carbon atom attached to the oxygen, making it more susceptible to nucleophilic attack.

Facilitate the departure of the first leaving group (an alcohol molecule, CH₂OH) by breaking the C-O bond. This step forms a resonance-stabilized oxonium ion intermediate.

Add water (H₂O) as a nucleophile to the oxonium ion intermediate. The water molecule attacks the electrophilic carbon, forming a new C-O bond and generating a protonated diol intermediate.

Deprotonate the diol intermediate to yield the final product, cyclohexanone, and regenerate the acid catalyst. This completes the acid-catalyzed hydrolysis mechanism.

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Key Concepts

Here are the essential concepts you must grasp in order to answer the question correctly.

Acid-Catalyzed Hydrolysis

Acid-catalyzed hydrolysis is a reaction where an acetal is converted back to its corresponding carbonyl compound and alcohol in the presence of an acid. The acid protonates the acetal oxygen, making it more susceptible to nucleophilic attack by water. This process typically involves the formation of a carbocation intermediate, which is stabilized by the surrounding solvent and leads to the cleavage of the acetal bond.

Acetals and Their Formation

Acetals are organic compounds formed from the reaction of an aldehyde or ketone with an alcohol, resulting in a structure where two alkoxy groups are attached to the same carbon atom. In the case of cyclohexanone ethylene acetal, cyclohexanone reacts with ethylene glycol to form the acetal. Understanding the structure and stability of acetals is crucial for predicting their reactivity in hydrolysis reactions.

Mechanism of Nucleophilic Substitution

The mechanism of nucleophilic substitution involves the attack of a nucleophile on an electrophilic carbon, leading to the displacement of a leaving group. In the context of acid-catalyzed hydrolysis, water acts as the nucleophile that attacks the carbocation formed after the protonation of the acetal. This mechanism is essential for understanding how the acetal bond is broken and how the original carbonyl compound is regenerated.

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Textbook Question

In the mechanism for acetal hydrolysis shown, the ring oxygen atom was protonated first, the ring was cleaved, and then the methoxy group was lost. The mechanism could also be written to show the methoxy oxygen protonating and cleaving first, followed by ring cleavage. Draw this alternative mechanism.

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Textbook Question

Propose a mechanism for the acid-catalyzed reaction of cyclohexanone with ethylene glycol to give cyclohexanone ethylene acetal.

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Textbook Question

Show what alcohols and carbonyl compounds give the following derivatives.

(a)

(b)

(c)

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