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PROBLEM 7.57

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Organic Chemistry
Found in: Page 292

Short Answer

Question: Which of the following carbocations (A or B) is more stable? Explain your choice.

A.

B.

Answer

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.

See the step by step solution

Step by Step Solution

Stability of the carbocations

The stability of the carbocations depends upon the following factors:

  • Inductive effect: The +I effect, i.e., the electron releasing effect of the groups attached to the positively charged carbon, affects the stability of the carbocations.

It implies that the tertiary carbocations are more stable due to the presence of the three alkyl groups.

  • Hyperconjugation: The tertiary carbocations are also stabilized by hyperconjugation.

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.

The stability of the given carbocations

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.

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