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Why Aniline Is Less Basic Than Ethylamine: A Detailed Analysis

When comparing the basicity of aniline and ethylamine, it becomes evident that aniline is less basic than ethylamine. This difference in basicity can be attributed to the structural and electronic factors associated with each molecule. Understanding why aniline is less basic than ethylamine requires a closer look at the molecular structure, electron delocalization, and the impact of these factors on the availability of the nitrogen lone pair for protonation.

The Molecular Structure of Aniline and Ethylamine

Aniline and ethylamine both contain an amine group (-NH2), which is responsible for their basicity. However, the key difference lies in the group attached to the nitrogen atom. In ethylamine, the nitrogen is bonded to an ethyl group (-CH2CH3), while in aniline, the nitrogen is bonded to a phenyl ring (a benzene ring). This structural difference plays a crucial role in their relative basicity.

Electron Delocalization in Aniline

One of the primary reasons why aniline is less basic than ethylamine is due to electron delocalization in the benzene ring. In aniline, the lone pair of electrons on the nitrogen atom is partially delocalized into the aromatic ring through resonance. This delocalization reduces the electron density on the nitrogen, making it less available to accept a proton (H+). As a result, aniline exhibits lower basicity compared to ethylamine, where the nitrogen’s lone pair is more localized and readily available for protonation.

Inductive and Resonance Effects

The inductive effect also contributes to the reduced basicity of aniline. The phenyl group in aniline exerts an electron-withdrawing effect through the sigma bond, further decreasing the electron density on the nitrogen atom. This inductive withdrawal contrasts with the electron-donating inductive effect of the ethyl group in ethylamine, which increases the electron density on the nitrogen, enhancing its basicity.

Moreover, the resonance effect in aniline, where the nitrogen lone pair is involved in the delocalization with the benzene ring, is a significant factor. The resonance structure where the nitrogen lone pair is shared with the ring leads to a decrease in the nitrogen's nucleophilicity and, consequently, its basicity. Ethylamine lacks this resonance stabilization, so its nitrogen lone pair remains more localized and available, making ethylamine more basic.

Solvent Effects and Protonation

Solvent effects can also influence the basicity of amines, but the intrinsic electronic factors discussed above are the dominant reasons why aniline is less basic than ethylamine. When these molecules are in an aqueous solution, the solvent can stabilize the protonated forms. However, because aniline's nitrogen is less nucleophilic, it is less likely to be protonated compared to ethylamine.

Conclusion

In summary, the question "why aniline is less basic than ethylamine" can be explained by the resonance and inductive effects associated with the aromatic ring in aniline. The delocalization of the nitrogen lone pair into the benzene ring significantly reduces the availability of the lone pair for protonation, leading to a lower basicity in aniline compared to ethylamine. Understanding these electronic and structural factors is essential for grasping the fundamental differences in the chemical behavior of these two amines.