Question: Which of the following carbocations (A or B) is more stable? Explain your choice.
Carbocation B is more stable than carbocation A. The lone pair on the nitrogen atom is able to stabilize the positive charge on the carbon atom due to +I effect.
The stability of the carbocations depends upon the following factors:
It implies that the tertiary carbocations are more stable due to the presence of the three alkyl groups.
The tertiary carbocation has three alkyl groups attached to the positively charged carbon atom, due to which the charge can be delocalized on more than one carbon atom.
Among the carbocations, B is more stable as compared to carbocation A. In the case of B, the positively charged carbon is next to a nitrogen atom which can donate its lone pair of electrons (+I inductive effect) to stabilize the positive charge.
The size of the empty orbitals of the carbocations is comparable to the orbital of the nitrogen atom. As a result, the overlapping of the orbitals will be effective between the nitrogen and the carbon atom, due to which carbocation B will be more stable than A.
On the other hand, the oxygen atom is more electronegative as compared to the nitrogen atom. The lone pair of the oxygen atom on carbocation A does not completely stabilize the positive charge of the carbon atom.
Question: As you will learn in Chapter 9, the epoxide is an ether with an oxygen atom in a three-membered ring. Epoxides can be made by intramolecular SN2 reactions of intermediates that contain a nucleophile and a leaving group on adjacent carbons, as shown.
Assume that each of the following starting materials can be converted to an epoxide by this reaction. Draw the product formed (including stereochemistry) from each starting material. Why might some of these reactions be more difficult than others in yielding nucleophilic substitution products?
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