Ch. 8 - Alkenes I: Properties and Electrophilic Additions

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

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Mullins 1st Edition

Ch. 8 - Alkenes I: Properties and Electrophilic Additions

Problem 45a

Chapter 7, Problem 45a

Which of the following carbocations would you expect to rearrange? If you expect rearrangement, draw the carbocation you expect to form and the mechanism by which it will form.
(a)

Verified step by step guidance

Step 1: Analyze the given carbocation structure. The carbocation is located on a secondary carbon atom, which is less stable compared to a tertiary carbocation.

Step 2: Recall the concept of carbocation rearrangement. Carbocations tend to rearrange to form a more stable structure, typically moving from secondary to tertiary carbons via hydride or alkyl shifts.

Step 3: Identify the possibility of rearrangement. In this structure, a hydride shift from the adjacent tertiary carbon can occur, resulting in the formation of a tertiary carbocation.

Step 4: Draw the rearranged carbocation. After the hydride shift, the positive charge will relocate to the tertiary carbon, creating a more stable carbocation.

Step 5: Illustrate the mechanism. Show the movement of the hydride (H⁻) from the tertiary carbon to the secondary carbon, and indicate the resulting rearranged carbocation structure.

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Key Concepts

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

Carbocation Stability

Carbocations are positively charged carbon species that can rearrange to form more stable structures. Stability is influenced by factors such as the degree of substitution (primary, secondary, tertiary) and resonance. Tertiary carbocations are the most stable due to hyperconjugation and inductive effects from surrounding alkyl groups, while primary carbocations are the least stable.

Carbocation Rearrangement

Carbocation rearrangement occurs when a less stable carbocation transforms into a more stable one, often through hydride or alkyl shifts. This process is driven by the stability of the resulting carbocation. For example, a primary carbocation can rearrange to a secondary or tertiary carbocation, which is more stable, thus favoring the formation of the latter.

Mechanism of Rearrangement

The mechanism of carbocation rearrangement typically involves the migration of a hydrogen atom or an alkyl group to the positively charged carbon. This shift can be represented in a stepwise mechanism, where the initial carbocation undergoes a transition state before forming a new, more stable carbocation. Understanding this mechanism is crucial for predicting the products of reactions involving carbocations.

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Related Practice

Textbook Question

When combining isoprene equivalents, IPP is never the electrophile. Why might this be? What is special about DPP that allows it to function better as an electrophile? [Draw the carbocation that each becomes.]

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Textbook Question

Ethers can be synthesized by substituting ethanol for water in the acid-catalyzed hydration reaction. Suggest an arrow-pushing mechanism which accounts for the formation of the ethyl ether under these conditions.

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Textbook Question

Suggest a mechanism for the synthesis of farnesol beginning with IPP and DPP.

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Textbook Question

Which of the following carbocations would you expect to rearrange? If you expect rearrangement, draw the carbocation you expect to form and the mechanism by which it will form.

(c)

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Textbook Question

Which of the following carbocations would you expect to rearrange? If you expect rearrangement, draw the carbocation you expect to form and the mechanism by which it will form.

(b)

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Textbook Question

Which of the following carbocations would you expect to rearrange? If you expect rearrangement, draw the carbocation you expect to form and the mechanism by which it will form.

(e)

780

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Which of the following carbocations would you expect to rearrange? If you expect rearrangement, draw the carbocation you expect to form and the mechanism by which it will form.(a)

Verified video answer for a similar problem:

Key Concepts

Carbocation Stability

Carbocation Rearrangement

Mechanism of Rearrangement

Which of the following carbocations would you expect to rearrange? If you expect rearrangement, draw the carbocation you expect to form and the mechanism by which it will form.
(a)