Question

Which compound do you think is more basic, \(p\) -aminobenzaldehyde or aniline? Explain.

Short answer

Aniline is more basic than p-aminobenzaldehyde due to the absence of an electron-withdrawing aldehyde group.

Step-by-step solution

Identify the Basic Site

In order to determine which compound is more basic, we must first identify the nitrogen atom as the basic site since it's the site where a proton (H+) would be accepted.

Compare Structures

Examine the structures of both p-aminobenzaldehyde and aniline. Both have an amino group attached to a benzene ring, but p-aminobenzaldehyde also has an aldehyde group. The aldehyde group is at the para position relative to the amino group.

Analyze Electron Donating and Withdrawing Effects

The amino group is an electron-donating group, which increases electron density on the nitrogen, increasing basicity. However, in p-aminobenzaldehyde, the electron-withdrawing effect of the aldehyde group at the para position reduces the electron-donating effect of the amino group, decreasing the basicity.

Assess Effects in Aniline

In aniline, there is no electron-withdrawing group like the aldehyde in p-aminobenzaldehyde. The full electron-donating ability of the amino group is retained, making the nitrogen more electron-rich and basic.

Conclusion

Comparing the two compounds, the presence of the electron-withdrawing aldehyde group in p-aminobenzaldehyde decreases its basicity relative to aniline, which has no such electron-withdrawing group.

Key concepts

Electron-donating Groups

Electron-donating groups (EDGs) are molecules or parts of molecules that can release electrons into another part of the structure, increasing electron density. For aromatic amines, such as aniline, the amino group (\(\text{-NH}_2\)) acts as an electron-donating group. It releases electrons into the benzene ring, particularly through resonance or the mesomeric effect.
This increased electron density makes the nitrogen atom in the amino group more likely to accept a proton (\(\text{H}^+\)), thereby increasing the compound's basicity. The electron-donating capability of the amino group in aniline helps maintain its strong basic character, as there are no additional substituents that disrupt this electron flow.

• By enhancing the electron density at the nitrogen site, EDGs make the compound more basic.
• In aniline, there are no competing groups, so the nitrogen remains highly electron-rich.

Electron-withdrawing Groups

Electron-withdrawing groups (EWGs) are groups that pull electron density away from other areas of a molecule, typically reducing electron density and basicity. When an electron-withdrawing group, like an aldehyde, is present on a benzene ring, it can delocalize electrons away from the aromatic ring and any attached groups.
In \(p\)-aminobenzaldehyde, the aldehyde group is para to the amino group and acts as an electron-withdrawing group. This means that the overall electron density is reduced even at the nitrogen atom, which decreases its ability to attract protons (\(\text{H}^+\)).

• EWGs decrease the electron density of nearby sites, reducing basicity.
• The aldehyde group's position at the para site reduces the electron-donating effect of the amino group in \(p\)-aminobenzaldehyde.

Aromatic Compounds

Aromatic compounds are a class of compounds characterized by their ring-like structure and delocalized electrons, which contribute to stability. Benzene is the most common example of an aromatic structure, with its alternating double and single bonds that create a stable aromatic ring.
In aromatic amines like aniline and \(p\)-aminobenzaldehyde, the aromatic system plays a vital role when determining basicity. The ring itself can influence electron transfer due to its resonance properties.

• Aromatic rings can either enhance or reduce the effects of substituent groups based on their electronic properties.
• Understanding the interplay between aromaticity and substituent effects is crucial for predicting the behavior, such as basicity, of these compounds.