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

Bruice8th EditionOrganic ChemistryISBN: 9780135213711Not the one you use?Change textbook

Bruice 8th Edition

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

Problem 27

Chapter 17, Problem 27

A ketone can be prepared from the reaction of a nitrile with a Grignard reagent. Describe the intermediate formed in this reaction, and show how it can be converted to a ketone.

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The reaction begins with the nucleophilic attack of the Grignard reagent (R-MgX) on the carbon atom of the nitrile group (R'-C≡N). This carbon is electrophilic due to the polarization of the triple bond between carbon and nitrogen.

The nucleophilic attack results in the formation of an intermediate imine salt. The structure of this intermediate can be represented as R'-C=N⁻-MgX, where the nitrogen atom carries a negative charge and is bonded to the magnesium ion.

The imine salt intermediate is then hydrolyzed in the presence of water or an acidic workup. During hydrolysis, the imine group (C=N⁻) is converted into a carbonyl group (C=O).

The hydrolysis step involves protonation of the nitrogen atom, followed by the cleavage of the C=N bond and the addition of water to form a ketone (R'-C=O-R).

The final product of the reaction is a ketone, where the alkyl group from the Grignard reagent (R) is attached to the carbonyl carbon of the original nitrile (R'-C≡N).

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Key Concepts

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

Grignard Reagents

Grignard reagents are organomagnesium compounds that act as nucleophiles in organic reactions. They are formed by the reaction of an alkyl or aryl halide with magnesium metal in anhydrous ether. In the context of the reaction with nitriles, Grignard reagents attack the electrophilic carbon of the nitrile, leading to the formation of a new carbon-carbon bond.

Nitrile Intermediate

When a Grignard reagent reacts with a nitrile, it forms an intermediate known as an imine or a ketimine, depending on the structure of the nitrile. This intermediate contains a carbon-nitrogen double bond and is crucial for the subsequent transformation into a ketone. The imine can be hydrolyzed to yield the corresponding ketone upon treatment with water or an acid.

Hydrolysis of Imines

Hydrolysis of imines involves the addition of water to the imine intermediate, leading to the cleavage of the carbon-nitrogen double bond. This reaction typically requires an acid catalyst to facilitate the process. The result is the formation of a ketone and the release of ammonia, completing the conversion from the nitrile to the ketone through the imine intermediate.

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Imine Mechanism

Related Practice

Textbook Question

What are the products of the following reactions?

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Textbook Question

Imines can exist as stereoisomers. The isomers are named using the E,Z system of nomenclature (Section 4.2 ). The lone pair has the lowest priority.

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Draw the structure of each of the following compounds:

a. the (E)-hydrazone of benzaldehyde

b. the (Z)-oxime of propiophenone

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Textbook Question

Why is the pK_a value of protonated hydroxylamine (6.0) so much lower than the value of a protonated primary amine such as protonated methylamine (10.7)?

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Textbook Question

How would you make the following compounds from N-benzylbenzamide?

c. benzyl alcohol

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Textbook Question

The pK_a of protonated acetone is about –7.5 and the pK_a of protonated hydroxylamine is 6.0.

a. In a reaction with hydroxylamine at pH 4.5 (Figure 16.2), what fraction of acetone is present in its acidic, protonated form?

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