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Which of the following carbocations would be likely to rearrange? Draw each rearranged structure below.
Molecule III
6. Thermodynamics and Kinetics
Carbocation Intermediate Rearrangements
3:25 minutesProblem 45c
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) 
Verified step by step guidance1
Identify the structure of the given carbocation. Determine whether it is a primary, secondary, or tertiary carbocation, as this will influence its stability and likelihood of rearrangement.
Recall the concept of carbocation rearrangement: Carbocations can undergo hydride shifts or alkyl shifts to form a more stable carbocation. Stability increases in the order: primary < secondary < tertiary < allylic < benzylic.
Examine the neighboring carbon atoms to the carbocation. Check if a hydride (H⁻) or an alkyl group (R⁻) can shift to the carbocation center to form a more stable carbocation.
If a rearrangement is possible, draw the structure of the new carbocation that forms after the shift. Ensure that the new carbocation is more stable than the original.
Illustrate the mechanism of the rearrangement. Use curved arrows to show the movement of electrons during the hydride or alkyl shift, and ensure the final structure reflects the rearranged carbocation.
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3mKey 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 undergo rearrangement to form more stable structures. The stability of a carbocation is influenced by factors such as the degree of substitution (primary, secondary, tertiary) and resonance. Tertiary carbocations are generally the most stable due to hyperconjugation and inductive effects from surrounding alkyl groups.
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Determining Carbocation Stability
Rearrangement Mechanisms
Carbocation rearrangements typically occur through two main mechanisms: hydride shifts and alkyl shifts. A hydride shift involves the movement of a hydrogen atom with its bonding electrons to the positively charged carbon, while an alkyl shift involves the migration of an alkyl group. These shifts allow the carbocation to achieve a more stable configuration, often leading to the formation of a more substituted or resonance-stabilized carbocation.
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Reaction Pathways
Understanding the reaction pathways is crucial for predicting carbocation rearrangements. The pathway includes the initial formation of the carbocation, the rearrangement step, and the final product formation. Analyzing the energy changes and intermediates involved helps in determining whether a rearrangement is favorable and what the resulting carbocation will be, guiding the drawing of the mechanism.
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