Ch. 21 - Carboxylic Acid Derivatives

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

Wade 9th Edition

Ch. 21 - Carboxylic Acid Derivatives

Problem 60

Chapter 21, Problem 60

Explain this curious result. What does this reaction tell you about the relative reactivity of esters and ketones?

Verified step by step guidance

Step 1: Analyze the reaction. The reaction involves an ester (CH3COOCH3) reacting with a Grignard reagent (PhMgBr) in the presence of water. The products are 0.5 equivalents of the original ester and 0.5 equivalents of a tertiary alcohol (Ph-C(CH3)(OH)-CH3).

Step 2: Understand the mechanism. Grignard reagents are nucleophiles that attack electrophilic carbonyl groups. In this case, the ester has two reactive sites: the carbonyl carbon and the alkoxy group. The Grignard reagent first attacks the carbonyl carbon, forming a tetrahedral intermediate.

Step 3: Consider the reactivity of esters versus ketones. After the first attack, the intermediate collapses, expelling the alkoxy group and forming a ketone. Ketones are more reactive than esters because their carbonyl carbon is more electrophilic, making them more susceptible to a second attack by the Grignard reagent.

Step 4: Explain the curious result. The reaction stops after the formation of 0.5 equivalents of tertiary alcohol because the Grignard reagent is consumed in the first attack on the ester and the subsequent attack on the ketone. The remaining ester does not react further due to the limited amount of Grignard reagent.

Step 5: Conclude the relative reactivity. This reaction demonstrates that ketones are more reactive than esters toward nucleophilic attack by Grignard reagents. The Grignard reagent preferentially reacts with the ketone formed after the first attack on the ester.

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Key Concepts

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

Reactivity of Esters vs. Ketones

Esters and ketones exhibit different reactivities due to their structural features. Esters, which have an alkoxy group (-O-R) attached to a carbonyl, are generally more reactive than ketones because the electron-donating alkoxy group stabilizes the carbonyl carbon, making it more susceptible to nucleophilic attack. In contrast, ketones have two alkyl groups that can provide steric hindrance and electron-donating effects, but they do not enhance reactivity to the same extent as esters.

Nucleophilic Addition Reactions

Nucleophilic addition is a fundamental reaction in organic chemistry where a nucleophile attacks an electrophilic carbon atom, typically found in carbonyl compounds. In the reaction shown, phenylmagnesium bromide (a Grignard reagent) acts as a strong nucleophile, attacking the carbonyl carbon of the ester. This results in the formation of a tetrahedral intermediate, which can then be hydrolyzed to yield an alcohol and regenerate the ester.

Grignard Reagents

Grignard reagents are organomagnesium compounds that are highly reactive and serve as powerful nucleophiles in organic synthesis. They are formed by reacting alkyl or aryl halides with magnesium metal. In the context of the reaction, phenylmagnesium bromide reacts with methyl acetate, demonstrating the ability of Grignard reagents to add to carbonyl compounds, leading to the formation of alcohols after hydrolysis.

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Related Practice

Textbook Question

In each part, rank the compounds in order of increasing rate of nucleophilic attack at C=O by a strong nucleophile like methoxide.

(b)

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Textbook Question

In each part, rank the compounds in order of increasing rate of nucleophilic attack at C=O by a strong nucleophile like methoxide.

(a)

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Textbook Question

Methyl p-nitrobenzoate has been found to undergo saponification faster than methyl benzoate.

(b) Would you expect methyl p-methoxybenzoate to undergo saponification faster or slower than methyl benzoate?

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Textbook Question

The structures of four useful polymers are shown, together with some of their best-known products. In each case,

(i) Determine the kind of polymer (polyamide, polyester, etc.).

(ii) Draw the structures of the monomers that would be released by complete hydrolysis.

(iii) Suggest what monomers or stable derivatives of the monomers might be used to make these polymers.

(a)

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Textbook Question

In Section 21-16, we saw that Sevin insecticide is made by the reaction of 1-naphthol with methyl isocyanate. A Union Carbide plant in Bhopal, India, once used this process to make Sevin for use as an agricultural insecticide. On December 3,1984, either by accident or by sabotage, a valve was opened that admitted water to a large tank of methyl isocyanate. The pressure and temperature within the tank rose dramatically, and pressure-relief valves opened to keep the tank from bursting. A large quantity of methyl isocyanate rushed out through the pressure-relief valves, and the vapors flowed with the breeze into populated areas, killing about 2500 people and injuring many more.

(a) Write an equation for the reaction that took place in the tank. Explain why the pressure and temperature rose dramatically.

(b) Propose a mechanism for the reaction you wrote in part (a).

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Textbook Question

The structures of four useful polymers are shown, together with some of their best-known products. In each case,

(i) Determine the kind of polymer (polyamide, polyester, etc.).

(ii) Draw the structures of the monomers that would be released by complete hydrolysis.

(iii) Suggest what monomers or stable derivatives of the monomers might be used to make these polymers.

(b)

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