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Why would concentrated HBr be an inappropriate catalyst for the dehydration of alcohols?

Why would concentrated HBr be an inappropriate catalyst for the dehydration of alcohols?

Chemistry
Chemistry
10th Edition
ISBN:9781305957404
Author:Steven S. Zumdahl, Susan A. Zumdahl, Donald J. DeCoste
Publisher:Steven S. Zumdahl, Susan A. Zumdahl, Donald J. DeCoste
Chapter1: Chemical Foundations
Section: Chapter Questions
Problem 1RQ: Define and explain the differences between the following terms. a. law and theory b. theory and...
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### Why would concentrated HBr be an inappropriate catalyst for the dehydration of alcohols? 1. **The conjugate base Br− is a good nucleophile and would attack the carbocation to form an alkyl bromide** 2. **HBr is strongly acidic, so the water molecule would not be a good leaving group after alcohol protonation** 3. **HBr would be more likely to promote rearrangement of the carbocation intermediate** 4. **HBr is too weakly acidic to protonate alcohols** --- **Explanation**: This multiple-choice question discusses the reasons why concentrated Hydrobromic Acid (HBr) would not be suitable as a catalyst for the dehydration of alcohols. Here’s an overview of the choices: 1. **Conjugate Base Nucleophilicity**: Br−, the conjugate base of HBr, is a strong nucleophile. In the presence of a carbocation intermediate, Br− will readily attack the carbocation, leading to the formation of an alkyl bromide instead of facilitating the desired dehydration reaction. 2. **Acidity and Leaving Group**: Despite HBr being strongly acidic, its strong acidity means that the water molecule formed after the protonation of the alcohol might not be an effective leaving group, hindering the dehydration process. 3. **Carbocation Rearrangement**: HBr may promote rearrangements of the carbocation intermediate. Such rearrangements can complicate the reaction pathway and lead to unexpected products instead of straightforward dehydration. 4. **Protonation Ability**: This choice suggests the idea that HBr might be too weakly acidic to effectively protonate the alcohol, though this is less likely considering the strong acidic nature of HBr. This content could be incorporated into an educational module on dehydration reactions, focusing on the selection of appropriate acids and the influence of nucleophilicity and acidity on reaction pathways.
Transcribed Image Text:### Why would concentrated HBr be an inappropriate catalyst for the dehydration of alcohols? 1. **The conjugate base Br− is a good nucleophile and would attack the carbocation to form an alkyl bromide** 2. **HBr is strongly acidic, so the water molecule would not be a good leaving group after alcohol protonation** 3. **HBr would be more likely to promote rearrangement of the carbocation intermediate** 4. **HBr is too weakly acidic to protonate alcohols** --- **Explanation**: This multiple-choice question discusses the reasons why concentrated Hydrobromic Acid (HBr) would not be suitable as a catalyst for the dehydration of alcohols. Here’s an overview of the choices: 1. **Conjugate Base Nucleophilicity**: Br−, the conjugate base of HBr, is a strong nucleophile. In the presence of a carbocation intermediate, Br− will readily attack the carbocation, leading to the formation of an alkyl bromide instead of facilitating the desired dehydration reaction. 2. **Acidity and Leaving Group**: Despite HBr being strongly acidic, its strong acidity means that the water molecule formed after the protonation of the alcohol might not be an effective leaving group, hindering the dehydration process. 3. **Carbocation Rearrangement**: HBr may promote rearrangements of the carbocation intermediate. Such rearrangements can complicate the reaction pathway and lead to unexpected products instead of straightforward dehydration. 4. **Protonation Ability**: This choice suggests the idea that HBr might be too weakly acidic to effectively protonate the alcohol, though this is less likely considering the strong acidic nature of HBr. This content could be incorporated into an educational module on dehydration reactions, focusing on the selection of appropriate acids and the influence of nucleophilicity and acidity on reaction pathways.
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