Unlike hydroboration–oxidation, the addition of H2O catalyzed by H3O+ is not stereospecific. Thinking carefully about the mechanism of the reaction, give two reasons why.

Ch. 8 - Alkenes 1: Properties and Electrophilic Additions

Mullins - Organic Chemistry: A Learner Centered Approach 1st Edition

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Ch. 8 - Alkenes 1: Properties and Electrophilic Additions

Problem 56

Chapter 7, Problem 56

Unlike hydroboration–oxidation, the addition of H₂O catalyzed by H₃O⁺ is not stereospecific. Thinking carefully about the mechanism of the reaction, give two reasons why.

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Understand the mechanism of H₂O addition catalyzed by H₃O⁺: This reaction proceeds via an electrophilic addition mechanism where the alkene reacts with H₃O⁺ to form a carbocation intermediate. The carbocation is planar and sp² hybridized, which is key to understanding the lack of stereospecificity.

Reason 1: The carbocation intermediate formed during the reaction is achiral and planar. This allows the nucleophile (H₂O) to attack from either side of the plane with equal probability, leading to the formation of both enantiomers or diastereomers if applicable. This contrasts with hydroboration–oxidation, which involves a concerted mechanism that is stereospecific.

Reason 2: The reaction does not involve a concerted mechanism like hydroboration–oxidation. Instead, the reaction proceeds stepwise, with the formation of an intermediate carbocation. The lack of a concerted mechanism means there is no control over the stereochemistry of the product.

Compare with hydroboration–oxidation: In hydroboration–oxidation, the addition of H and OH occurs in a single step (concerted mechanism), ensuring syn addition and stereospecificity. The absence of such a concerted mechanism in H₃O⁺-catalyzed hydration explains the lack of stereospecificity.

Conclude: The planar nature of the carbocation intermediate and the stepwise mechanism are the two main reasons why the addition of H₂O catalyzed by H₃O⁺ is not stereospecific.

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Key Concepts

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

Mechanism of Electrophilic Addition

Electrophilic addition involves the attack of an electrophile on a nucleophile, leading to the formation of a carbocation intermediate. In the case of H₂O addition catalyzed by H₃O⁺, the reaction proceeds through a planar carbocation, allowing nucleophilic attack from either side. This lack of preference for one face over the other results in a mixture of stereoisomers, making the reaction non-stereospecific.

Carbocation Stability and Rearrangement

Carbocations are positively charged species that can undergo rearrangements to form more stable structures. During the addition of H₂O in the presence of H₃O⁺, the carbocation can rearrange to a more stable form before the nucleophilic attack occurs. This potential for rearrangement further complicates the stereochemical outcome, as different pathways can lead to different products.

Stereospecificity vs. Stereoselectivity

Stereospecificity refers to reactions where the configuration of the reactant directly determines the configuration of the product, leading to a single stereoisomer. In contrast, stereoselectivity allows for the formation of multiple stereoisomers, with one being favored. The addition of H₂O in the presence of H₃O⁺ is stereoselective rather than stereospecific, as it can yield both enantiomers due to the non-preferential attack on the planar carbocation.

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Related Practice

Textbook Question

The first step of oxymercuration–reduction is stereospecific, and yet this fact wasn't emphasized in that discussion. Show the stereospecificity of the first step for the following alkene and then explain why the stereospecificity becomes unimportant after the second step.

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Textbook Question

Suggest an alkene to undergo hydroboration–oxidation (1. BH₃ 2. NaOH, H₂O₂) to give exclusively the alcohols shown. Pay close attention to the relative (but not absolute) stereochemical outcome.

(d)

(c)

Predict the major product(s) of the following elimination reactions, paying close attention to the stereochemical outcome of the reactions.

(e)

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Textbook Question

Calculate the index of hydrogen deficiency for the following molecular formulas and structures.

(a)