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Why Aniline is Weaker Base than Benzylamine: A Detailed Analysis

In the field of organic chemistry, understanding the basicity of amines is crucial for predicting their behavior in chemical reactions. Two commonly discussed amines are aniline and benzylamine. Although both contain an amino group (-NH2), their basicities differ significantly. The question "why aniline is weaker base than benzylamine" can be explored by examining the electronic effects of the substituents attached to the nitrogen atom in each compound.

Resonance Effect in Aniline

Aniline (C6H5NH2) consists of an amino group directly attached to a benzene ring. The lone pair of electrons on the nitrogen atom can interact with the π-electrons of the benzene ring, leading to resonance. In this resonance structure, the lone pair on the nitrogen becomes delocalized into the benzene ring, decreasing the electron density on the nitrogen. As a result, the nitrogen in aniline is less available to donate its lone pair to a proton, making aniline a weaker base.

The resonance effect is a key factor in answering why aniline is weaker base than benzylamine. In aniline, the delocalization of the lone pair reduces the electron density on the nitrogen, thus reducing its ability to accept protons (H+), which is a defining characteristic of basicity.

Lack of Resonance in Benzylamine

Benzylamine (C6H5CH2NH2), on the other hand, has an amino group attached to a -CH2- group, which in turn is connected to a benzene ring. Unlike aniline, the lone pair of electrons on the nitrogen in benzylamine is not involved in resonance with the benzene ring. Therefore, the electron density on the nitrogen atom remains high. This higher electron density allows the nitrogen atom to more readily accept a proton, making benzylamine a stronger base compared to aniline.

The absence of resonance in benzylamine explains why aniline is weaker base than benzylamine. Without the electron-withdrawing effects of resonance, the nitrogen in benzylamine is more nucleophilic, meaning it is better able to donate its lone pair to a proton.

Inductive Effect

While resonance plays the most significant role, the inductive effect also contributes to the basicity difference between aniline and benzylamine. In aniline, the electron-withdrawing nature of the benzene ring further reduces the electron density on the nitrogen through an inductive effect, albeit to a lesser extent compared to resonance. Conversely, in benzylamine, the -CH2- group acts as an electron-donating group through the inductive effect, which slightly increases the electron density on the nitrogen. This minor inductive contribution, along with the lack of resonance, reinforces why benzylamine is a stronger base than aniline.

Conclusion

In conclusion, the main reason why aniline is weaker base than benzylamine lies in the resonance effect, where the lone pair on the nitrogen in aniline is delocalized into the benzene ring, reducing its availability to accept protons. Additionally, the inductive effects further contribute to this difference in basicity. Benzylamine, lacking such resonance, retains a higher electron density on its nitrogen, making it more basic. Understanding these electronic effects is essential for predicting the behavior of these compounds in various chemical environments.