use resonance theory to account for the fact that aniline(pKb 9.37) is a weaker base than cyclohexanamine(pKb 3.34)

To explain why aniline (pKb 9.37) is a weaker base than cyclohexanamine (pKb 3.34) using resonance theory, we need to consider the differences in the resonance structures of these two organic compounds.

Resonance theory suggests that compounds with more stable resonance structures will exhibit greater conjugation and have increased stability. Therefore, the relative stability of the resonance structures can provide insight into the acidity or basicity of a compound.

1. Aniline: Aniline is an aromatic compound with an amino group (-NH2) attached to a benzene ring. In the case of aniline, the lone pair of electrons on the nitrogen atom can delocalize through resonance with the pi electrons of the benzene ring. However, this resonance is not as effective as in other aromatic amines like pyrrole or pyridine due to the presence of electron-withdrawing groups such as the attached benzene ring. This electron-withdrawing effect deactivates the benzene ring, making it less electron-rich. Consequently, the resonance stabilization of the nitrogen lone pair is reduced, leading to a weaker basicity.

2. Cyclohexanamine: Cyclohexanamine is an aliphatic amine that lacks the aromatic benzene ring present in aniline. As a result, it does not experience the electron-withdrawing effects of an aromatic ring. In cyclohexanamine, the lone pair of electrons on the nitrogen atom is only involved in sigma bonds and does not delocalize extensively through resonance. The absence of resonance stabilization makes the lone pair of nitrogen electrons more readily available to act as a base, resulting in stronger basicity compared to aniline.

In summary, aniline is a weaker base than cyclohexanamine because the presence of an electron-withdrawing aromatic ring in aniline decreases the resonance stabilization of the nitrogen lone pair, leading to reduced basicity. On the other hand, cyclohexanamine lacks an electron-withdrawing aromatic ring, allowing the nitrogen lone pair to be more available as a base, resulting in stronger basicity.