Ch. 18 - Ketones and Aldehydes

Wade9th EditionOrganic ChemistryISBN: 9780135213728Not the one you use?Change textbook

Wade 9th Edition

Ch. 18 - Ketones and Aldehydes

Problem 28d

Chapter 18, Problem 28d

Propose a mechanism for the acid-catalyzed hydrolysis of the acetal given in Problem 18-26(f).

Verified step by step guidance

Step 1: Protonation of one of the oxygen atoms in the acetal structure occurs under acidic conditions. This increases the electrophilicity of the carbon atom attached to the protonated oxygen.

Step 2: The bond between the protonated oxygen and the carbon breaks, leading to the formation of a carbocation intermediate and a neutral alcohol molecule.

Step 3: Water, acting as a nucleophile, attacks the carbocation intermediate, forming a protonated hemiacetal intermediate.

Step 4: Deprotonation of the protonated hemiacetal occurs, resulting in the formation of a neutral hemiacetal.

Step 5: The process repeats for the second oxygen atom in the acetal structure, ultimately leading to the formation of two molecules of alcohol and one molecule of aldehyde or ketone, depending on the original acetal structure.

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Key Concepts

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

Acetal Structure and Formation

Acetals are organic compounds formed from the reaction of an aldehyde or ketone with an alcohol in the presence of an acid catalyst. They consist of a carbon atom bonded to two alkoxy groups (–OR) and one hydrogen atom. Understanding the structure of acetals is crucial for predicting their reactivity, particularly in hydrolysis reactions.

Acid-Catalyzed Hydrolysis

Acid-catalyzed hydrolysis is a reaction where an acetal reacts with water in the presence of an acid, leading to the regeneration of the original aldehyde or ketone and alcohol. The acid protonates the acetal oxygen, making it more susceptible to nucleophilic attack by water, facilitating the cleavage of the C–O bond. This mechanism is essential for understanding how acetals can revert to their parent carbonyl compounds.

Mechanistic Steps in Organic Reactions

Understanding the mechanistic steps in organic reactions involves identifying the sequence of bond-breaking and bond-forming events. In the case of acid-catalyzed hydrolysis of acetals, the mechanism typically includes protonation of the acetal, nucleophilic attack by water, and subsequent deprotonation. Familiarity with these steps is vital for accurately proposing a detailed mechanism for the reaction.

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Related Practice

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 how you would accomplish the following syntheses. You may use whatever additional reagents you need.

(c)

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Textbook Question

Show how you would accomplish the following syntheses. You may use whatever additional reagents you need.

(b)

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Textbook Question

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

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Textbook Question

Show how you would accomplish the following syntheses. You may use whatever additional reagents you need.

(a)

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