Why is aniline a weaker base than ethylamine

Why is aniline a weaker base than ethylamine?

In chemical reactions, the strength of bases is determined by their ability to accept protons. For the two amine compounds, aniline and ethylamine, aniline is significantly less basic than ethylamine. Why is aniline a weaker base than ethylamine? This paper will analyze this problem in depth through molecular structure, electronic effect and conjugate effect.

1. Aniline and ethylamine molecular structure difference

Aniline (C6H5NH2) and ethylamine (C2H5NH2) are both amine compounds with similar amino (-NH2) structure, but their molecular structure differences have an important impact on their basicity.

The amino group in the aniline molecule is attached to a benzene ring, while ethylamine is a simple ethyl (C2H5) substituted amine. The benzene ring is an electron-dense structure, which attracts and consumes the electron density of the amino group through the conjugate effect, reducing the basicity of the amino group. In contrast, ethyl-substituted ethylamine does not have a similar conjugation effect, so its amino group can retain more electron density, making ethylamine more basic.

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

The weak basicity of aniline can be mainly attributed to the electron withdrawing effect of benzene ring. When the benzene ring is connected to the amino group through a covalent bond, the benzene ring attracts the electron cloud of the amino group, reducing the affinity of the amino group to the proton. This electron-withdrawing effect makes the amino group in aniline unable to accept protons as efficiently as ethylamine.

In contrast, ethylamine is more basic because ethyl (-C2H5), as an electron donor, can push the electron cloud to the amino group through an inductive effect, thereby enhancing the attraction of the amino group to the proton and improving the basicity of ethylamine.

3. E-donation effect and alkaline

Due to the electron donation effect of ethyl, the electron density of the amino group of ethylamine is higher than that of aniline, which makes ethylamine more receptive to protons than aniline. An amino group having a higher electron density is more likely to bind to a proton, and exhibits a stronger basicity. The amino group in aniline has a low electron density due to the electron withdrawing effect of the benzene ring, which leads to its weak basicity.

4. Conjugate effects

The π electrons on the benzene ring can be conjugated with the lone pair electrons of the amino group to form a conjugated system, which not only reduces the activity of the lone pair electrons of the amino group, but also reduces the basicity of aniline. Ethylamine is not affected by such conjugation effects, so it is relatively alkaline.

5. Summary: Why is aniline a weaker base than ethylamine?

The weaker base of aniline than ethylamine is mainly due to the electron withdrawing effect and conjugation effect of the benzene ring, which leads to the lower electron density of the amino group of aniline and the difficulty of accepting protons. However, ethylamine does not have these effects. The electron donation effect of ethyl group enhances the alkalinity of amino group, making the alkalinity of ethylamine relatively strong.

This difference is very important for the understanding of chemical reactions, especially when designing reaction mechanisms and selecting appropriate catalysts. Understanding the influence of molecular structure on the strength of alkalinity can help us better grasp the laws of chemical reactions.