Why is aniline less basic than methylamine?

Aniline and methylamine are two common organic amine compounds, which play an important role in chemical reactions. Although they all belong to amine compounds, the basicity of aniline is lower than that of methylamine, which is a question worth pondering. This article will analyze the molecular structure, electronic effect, and electron density of nitrogen atoms to help us better understand this phenomenon.

1. Molecular structure on alkaline effect

The chemical structure of aniline contains a benzene ring (C6H5) and an amino group (NH2), while the molecule of methylamine consists of a methyl group (CH3) and an amino group (NH2). From the structural point of view, the amino group of aniline is directly connected to the benzene ring, while the amino group of methylamine is connected to a carbon atom, and a methyl group is connected to the carbon atom. Due to the existence of the benzene ring, the nitrogen atom of aniline can have a resonance effect with the benzene ring, and part of the electron density is attracted by the benzene ring, resulting in a decrease in the electron density on the nitrogen atom of aniline, thereby reducing its basicity.

In contrast, the methyl group in methylamine donates electrons to the nitrogen atom by an electron donating effect (I effect), which makes the nitrogen atom in methylamine relatively more electron-rich, thereby enhancing its basicity. In short, the structure of aniline makes it less basic, while methylamine exhibits a stronger basicity due to the influence of the methyl group.

2. Resonance effect on aniline alkalinity

The resonance effect plays a key role in the basicity of aniline. As a conjugated system, benzene ring has strong stability. When the amino group of aniline is combined with the benzene ring, the lone pair of electrons of the nitrogen atom may participate in the π electron cloud of the benzene ring, forming a resonance structure. This process leads to a decrease in the electron density on the nitrogen atom, which reduces the attraction of the nitrogen atom to the proton, thereby weakening the basicity of aniline.

In contrast, methylamine has no such resonance effect. The methyl group cannot form a similar electron cloud sharing with the nitrogen atom, so the nitrogen atom in methylamine has a higher electron density, can more effectively combine with the proton, and exhibits a stronger alkalinity.

3. Electronic effect: benzene ring of the electron-withdrawing effect

In addition to the resonance effect, the electron-withdrawing effect (-I effect) of the benzene ring is also an important reason for the low basicity of aniline. Due to the particularity of its electronic structure, the benzene ring has a certain electron-withdrawing effect on the connected amino group, which further weakens the affinity of the nitrogen atom to the proton. This electron-withdrawing effect can not be ignored in aniline molecules, which aggravates the loss of electron density of nitrogen atoms and reduces the alkalinity of aniline.

The methyl group in methylamine is different. The methyl group not only does not have an electron-withdrawing effect, but enhances the electron density of the nitrogen atom by providing electrons, thereby enhancing the basicity of the nitrogen atom. This is an important factor in the stronger basicity of methylamine.

4. Conclusion: aniline alkaline low reason summary

The reason why the basicity of aniline is lower than that of methylamine is mainly due to the resonance effect and electron withdrawing effect brought by the benzene ring in its molecular structure. These effects reduce the electron density of nitrogen atoms in aniline, making it less affinity for protons, thereby reducing its basicity. Methylamine, on the other hand, enhances the electron density of the nitrogen atom due to the electron supply effect of the methyl group and exhibits a stronger alkalinity.

Therefore, the phenomenon that the basicity of aniline is lower than that of methylamine can be explained by the difference of electronic effect and molecular structure. This difference has an important guiding significance for the design and synthesis of chemical reactions, especially in the selection of suitable amine compounds as catalysts or reagents.