P-methoxy benzyl carbocation is more stable than p-nitrobenzyl carbocation? why?

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Stability of p-methoxy benzyl carbocation vs p-nitrobenzyl carbocation

Introduction: Carbocations are positively charged ions with an empty p orbital. The stability of a carbocation depends on the stability of the positive charge. In this case, we will compare the stability of p-methoxy benzyl carbocation and p-nitrobenzyl carbocation.

Explanation: The stability of a carbocation depends on various factors such as inductive effect, resonance effect, and hyperconjugation effect. Let's analyze each of these factors for both the carbocations.

Inductive effect: The inductive effect is the polarity of the substituent attached to the carbocation. In this case, methoxy group (-OCH3) is an electron-donating group while the nitro group (-NO2) is an electron-withdrawing group. As a result, the p-methoxy benzyl carbocation will be more stable than the p-nitrobenzyl carbocation due to the electron-donating nature of the methoxy group.

Resonance effect: The resonance effect is the delocalization of the positive charge through pi bonds. In this case, both carbocations have a resonance structure. However, the p-methoxy benzyl carbocation has a more stable resonance structure due to the electron-donating nature of the methoxy group. The resonance structure of p-methoxy benzyl carbocation involves the delocalization of the positive charge to the oxygen atom of the methoxy group. In contrast, the resonance structure of p-nitrobenzyl carbocation involves delocalization of the positive charge to the nitro group, which is an electron-withdrawing group.

Hyperconjugation effect: The hyperconjugation effect is the delocalization of the positive charge through the sigma bonds. In this case, both carbocations have the same number of hyperconjugation structures. Therefore, the hyperconjugation effect does not significantly contribute to the stability of the carbocations.

Conclusion: In conclusion, the p-methoxy benzyl carbocation is more stable than the p-nitrobenzyl carbocation due to the electron-donating nature of the methoxy group and the more stable resonance structure.