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Ch. 8 - Alkenes I: Properties and Electrophilic Additions
Mullins - Organic Chemistry: A Learner Centered Approach 1st Edition
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All textbooksMullins 1st EditionCh. 8 - Alkenes I: Properties and Electrophilic AdditionsProblem 45b
Chapter 7, Problem 45b

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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1
Analyze the given carbocation structure: The carbocation is located on a secondary carbon atom, which is less stable than a tertiary carbocation.
Consider the possibility of carbocation rearrangement: Carbocations tend to rearrange to form a more stable structure, typically moving from secondary to tertiary carbons.
Identify the potential rearrangement mechanism: A hydride shift (movement of a hydrogen atom with its bonding electrons) or an alkyl shift can occur to stabilize the carbocation.
Determine the new carbocation structure: If a hydride shift occurs, the positive charge will move to the tertiary carbon, resulting in a more stable tertiary carbocation.
Draw the mechanism: Illustrate the movement of the hydride or alkyl group and the resulting carbocation structure, ensuring the new carbocation is more stable than the original.

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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 be classified based on their stability: primary, secondary, and tertiary. Tertiary carbocations are the most stable due to hyperconjugation and inductive effects from surrounding alkyl groups. Understanding the stability of carbocations is crucial for predicting their behavior, including whether they will rearrange to form more stable structures.
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Determining Carbocation Stability

Carbocation Rearrangement

Carbocation rearrangement occurs when a carbocation shifts to a more stable form, often through hydride or alkyl shifts. This process is driven by the stability of the resulting carbocation, which can lead to the formation of a more stable tertiary carbocation from a less stable primary or secondary one. Recognizing potential rearrangements is essential for predicting reaction pathways in organic chemistry.
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Understanding why carbocations shift.

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 process can be illustrated through curved arrows in reaction mechanisms, showing the movement of electrons. Understanding these mechanisms helps in visualizing how carbocations evolve during reactions and aids in predicting the products formed.
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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

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)

856
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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)

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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)

1432
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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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