Question

Aniline (conjugate acid p \(K_{\mathrm{a}} 4.63\) ) is a considerably stronger base than diphenylamine \(\left(\mathrm{p} K_{\mathrm{a}} 0.79\right)\). Account for these marked differences.

Short answer

Answer: Aniline is a stronger base than Diphenylamine. This is because Aniline has only one phenyl group attached to the nitrogen atom, which weakens the electron-withdrawing effect compared to Diphenylamine, which has two phenyl groups. The higher pKa value of the conjugate acid of Aniline (4.63) compared to Diphenylamine (0.79) also confirms that Aniline is a stronger base.

Step-by-step solution

Molecular Structures of Aniline and Diphenylamine

Aniline has the molecular formula C6H5NH2, and Diphenylamine has the molecular formula (C6H5)2NH. The difference between these two compounds lies in the number of phenyl groups (C6H5) attached to the nitrogen atom in the molecule. Aniline has one phenyl group attached to a nitrogen atom, while Diphenylamine has two phenyl groups attached to a nitrogen atom. Step 2: Analyzing the pKa values

The relationship between pKa and Base Strength

In the exercise, we are given the pKa values of the conjugate acids of both compounds. It is important to remember that the lower the pKa, the stronger the acid; since the conjugate base is more stable, it is weaker. In this case, Aniline has a higher pKa (4.63) than Diphenylamine (0.79), meaning that Aniline is a stronger base than Diphenylamine. Step 3: Understanding the Effect of Phenyl Groups on Base Strength

Effect of Phenyl Groups on Lone Pair Electrons and Base Strength

The basic strength of amines (such as Aniline and Diphenylamine) depends on the availability of the lone pair electrons on the nitrogen atom to accept a proton. Phenyl groups are known to have a "withdrawing" effect on the lone pair electrons due to the resonance within the aromatic ring since the electrons in the double bonds can interact with the lone pair of nitrogen. In Aniline, the presence of one such phenyl group slightly reduces the electron density around the nitrogen, slightly decreasing its base strength as this resonance effect makes the lone pair of electrons less available for protonation. In Diphenylamine, there are two phenyl groups, and therefore the withdrawing effect is greater, further reducing the electron density around the nitrogen atom. Consequently, the lone pair of electrons in Diphenylamine is even less available for protonation, making it a weaker base than Aniline. Step 4: Conclusion

Comparing the Basic Strengths of Aniline and Diphenylamine

Based on the molecular structures and pKa values, we can conclude that Aniline is a stronger base than Diphenylamine due to the presence of only one phenyl group, which weakens the electron-withdrawing effect compared to Diphenylamine. With the given pKa values for their conjugate acids (4.63 for Aniline and 0.79 for Diphenylamine), we confirmed that Aniline is a considerably stronger base.