Ch.11 - Reactions of Alcohols

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

Wade 9th Edition

Ch.11 - Reactions of Alcohols

Problem 63a

Chapter 11, Problem 63a

Alcohols combine with ketones and aldehydes to form interesting derivatives, which we will discuss in Chapter 18. The following reactions show the hydrolysis of two such derivatives. Propose mechanisms for these reactions.
(a)

Verified step by step guidance

Step 1: Analyze the reaction. The given reaction involves the hydrolysis of an acetal under acidic conditions (H⁺ and H₂O). Acetals are derivatives formed from aldehydes or ketones reacting with alcohols. Hydrolysis reverses this process, regenerating the original aldehyde or ketone and releasing the alcohol.

Step 2: Protonation of the acetal oxygen. Under acidic conditions, the oxygen atom in the acetal is protonated, increasing its electrophilicity and making the molecule more susceptible to nucleophilic attack.

Step 3: Cleavage of the first bond. Water acts as a nucleophile and attacks the carbon atom bonded to the protonated oxygen, leading to the cleavage of the bond between the acetal oxygen and the carbon. This forms a hemiacetal intermediate.

Step 4: Protonation of the hemiacetal intermediate. The hydroxyl group in the hemiacetal is protonated, making it a better leaving group. This facilitates the cleavage of the bond between the carbon and the hydroxyl group, forming the aldehyde.

Step 5: Release of methanol. The methoxy group (-OCH₃) is protonated and leaves as methanol (CH₃OH), completing the hydrolysis reaction.

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Key Concepts

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

Nucleophilic Addition

Nucleophilic addition is a fundamental reaction mechanism in organic chemistry where a nucleophile attacks an electrophilic carbon atom, typically found in carbonyl groups of aldehydes and ketones. This process leads to the formation of a tetrahedral intermediate, which can further undergo protonation or elimination to yield alcohols or other derivatives. Understanding this mechanism is crucial for analyzing reactions involving alcohols and carbonyl compounds.

Hydrolysis Reactions

Hydrolysis reactions involve the breaking of chemical bonds through the reaction with water, resulting in the formation of new products. In the context of alcohols and carbonyl compounds, hydrolysis can convert derivatives back into their original forms or into different functional groups. Recognizing the conditions and products of hydrolysis is essential for proposing accurate reaction mechanisms.

Reaction Mechanism

A reaction mechanism is a detailed step-by-step description of the process by which reactants are converted into products. It outlines the sequence of bond-breaking and bond-forming events, including intermediates and transition states. Proposing a mechanism for the hydrolysis of alcohol-derivative reactions requires a solid understanding of the involved species and their interactions, which is critical for predicting the outcome of the reaction.

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Related Practice

Textbook Question

Chromic acid oxidation of an alcohol (Section 11-2A) occurs in two steps: formation of the chromate ester, followed by an elimination of H⁺ and chromium. Which step do you expect to be rate-limiting? Careful kinetic studies have shown that Compound A undergoes chromic acid oxidation over 10 times as fast as Compound B. Explain this large difference in rates.

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Textbook Question

Alcohols combine with ketones and aldehydes to form interesting derivatives, which we will discuss in Chapter 18. The following reactions show the hydrolysis of two such derivatives. Propose mechanisms for these reactions.

(b)

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Textbook Question

(b) Under the same conditions, an optically active sample of trans-2-bromocyclopentanol reacts with concentrated aqueous HBr to give an optically inactive product, (racemic) trans-1,2-dibromocyclopentane. Propose a mechanism to show how this reaction goes with apparently complete retention of configuration, yet with racemization. (Hint: Draw out the mechanism of the reaction of cyclopentene with Br₂ in water to give the starting material, trans-2- bromocyclopentanol. Consider how parts of this mechanism might be involved in the reaction with HBr.)

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Textbook Question

(a) The reaction of butan-2-ol with concentrated aqueous HBr goes with partial racemization, giving more inversion than retention of configuration. Propose a mechanism that accounts for racemization with excess inversion.

Textbook Question

Under normal circumstances, tertiary alcohols are not oxidized. However, when the tertiary alcohol is allylic, it can undergo a migration of the double bond (called an allylic shift) and subsequent oxidation of the alcohol. A particularly effective reagent for this reaction is Bobbitt's reagent, similar to TEMPO used in many oxidations. (M. Shibuya et al., J. Org. Chem., 2008, 73, 4750.)

Show the expected product when each of these 3° allylic alcohols is oxidized by Bobbitt's reagent.

(a)

(b)

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Textbook Question

Show the expected product when each of these 3° allylic alcohols is oxidized by Bobbitt’s reagent

(c)

(d)

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Alcohols combine with ketones and aldehydes to form interesting derivatives, which we will discuss in Chapter 18. The following reactions show the hydrolysis of two such derivatives. Propose mechanisms for these reactions.(a)

Verified video answer for a similar problem:

Key Concepts

Nucleophilic Addition

Hydrolysis Reactions

Reaction Mechanism

Alcohols combine with ketones and aldehydes to form interesting derivatives, which we will discuss in Chapter 18. The following reactions show the hydrolysis of two such derivatives. Propose mechanisms for these reactions.
(a)