Organic Chemistry : Identifying Carbocations
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Example Question #1771 : Mcat Biological Sciences
Which of the following carbocation intermediates requires the least activation energy?
Cannot be determined
The more stable the carbocation, the lower the activation energy for reaching that intermediate will be. The more substituted a carbocation is, the more stable it is. The carbocation bonded to three alkanes (tertiary carbocation) is the most stable, and thus the correct answer.
Secondary carbocations will require more energy than tertiary, and primary carbocations will require the most energy.
Example Question #2 : Organic Intermediates
Rank the following carbocations from least to most stable.
I < V < IV < II < III
V < IV < I < III < II
I < V < IV < III < II
V < I < IV < II < III
V < IV < III < II < I
I < V < IV < III < II
You many know that primary carbocations are less stable than secondary, which are in turn less stable than tertiary carbocations. This holds true in this question, and as such, V < IV. To evaluate compounds I, II, and III, however, we will need to determine their resonance capabilities.
We can compare the two special secondary carbocations, II and III—a benzyl carbocation and an allyl carbocation, respectively. The structure of these cations allow for the positive charge to be shared over several atoms via resonance as shown below:
We can see that the benzyl carbocation (II) is more stable than the allyl carbocation (III), as all its resonance forms have a secondary carbocation, and there are more resonance forms. As both share charge over multiple atoms, both are more stable than IV and V, which place charge on one atom only. Thus far, we can say: V < VI < III < II.
Lastly, we must consider I, a vinylic carbocation. This cation puts its negative charge on an sp2-hybridized carbon, and is the only example of this situation. Because an sp2 orbital has more overall s- character than an sp3 orbital, electrons are held closer to the nucleus. As the electrons feel more positive charge from the nucleus, the atom can accommodate more negative charge in general. We can think of this as a shift towards more electronegative behavior. As such, a positive charge will be more unstable on an sp2-hybridized carbon than on an sp3-hybridized carbon, and we can conclude than I is the least stable of all choices.
Thus, the correct answer is I < V < IV < III < II.
Example Question #243 : Organic Chemistry
Under which reaction mechanism can rearrangements occur?
Rearrangements generally require carbocations. Carbocations are only formed under mechanisms with unimolecular rate-limiting steps (i.e. carbocation formation). 
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