Ch. 18 - Ketones and Aldehydes

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

Wade 9th Edition

Ch. 18 - Ketones and Aldehydes

Problem 17

Chapter 18, Problem 17

Propose mechanisms for the three imine-forming reactions just shown.

Verified step by step guidance

Step 1: Protonation of the carbonyl group - The reaction begins with the protonation of the oxygen atom in the carbonyl group (C=O) by the acid catalyst (H⁺). This increases the electrophilicity of the carbonyl carbon, making it more susceptible to nucleophilic attack.

Step 2: Nucleophilic attack by the amine - The amine (e.g., ammonia, aniline, or methylamine) attacks the carbonyl carbon, forming a tetrahedral intermediate. This step involves the lone pair of electrons on the nitrogen atom attacking the electrophilic carbon.

Step 3: Proton transfer - A proton transfer occurs within the intermediate to stabilize the structure. The hydroxyl group (-OH) formed in the intermediate is protonated, making it a better leaving group.

Step 4: Elimination of water - The protonated hydroxyl group leaves as water (H₂O), resulting in the formation of a double bond between the carbon and nitrogen atoms. This step forms the imine product.

Step 5: Deprotonation of the imine - The final step involves the deprotonation of the nitrogen atom in the imine to yield the neutral imine product. This completes the reaction mechanism.

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Key Concepts

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

Imine Formation

Imine formation is a reaction between a carbonyl compound (like an aldehyde or ketone) and an amine, resulting in the creation of an imine. This process typically involves the nucleophilic attack of the amine on the carbonyl carbon, followed by the elimination of water. The reaction is often acid-catalyzed to enhance the electrophilicity of the carbonyl carbon.

Mechanism of Nucleophilic Addition

The mechanism of nucleophilic addition involves the attack of a nucleophile (the amine) on the electrophilic carbon of the carbonyl group. This step forms a tetrahedral intermediate, which then collapses to release a water molecule, leading to the formation of the imine. Understanding this mechanism is crucial for proposing the correct steps in the imine-forming reactions shown.

Acid-Base Catalysis

Acid-base catalysis plays a significant role in imine formation by protonating the carbonyl oxygen, making the carbonyl carbon more electrophilic. This increases the reactivity of the carbonyl compound towards nucleophiles. In the reactions depicted, the presence of an acid (H+) facilitates the formation of the imine by stabilizing the transition state and promoting the elimination of water.

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Related Practice

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Show how you would accomplish the following syntheses.

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Give the structures of the carbonyl compound and the amine used to form the following imines.

(a)

(b)

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Show how you would accomplish the following syntheses.

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Depending on the reaction conditions, two different imines of formula C₈H₉N might be formed by the reaction of benzaldehyde with methylamine. Explain, and give the structures of the two imines.

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Show how you would accomplish the following syntheses.

b. cyclopentanecarbaldehyde → 2-cyclopentyl-2-hydroxyacetic acid

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