Question

Rank the following compounds in order of increasing basicity and explain the order you chose.

Short answer

Heterocyclic compounds denote the five-membered or six-membered rings comprising of heteroatoms like nitrogen, oxygen sulfur, etc. There are several heterocyclic compounds, and some of them include pyridine, pyrrole, and thiophene.

Step-by-step solution

Heterocyclic compounds 

Heterocyclic compounds denote the five-membered or six-membered rings comprising of heteroatoms like nitrogen, oxygen sulfur, etc. There are several heterocyclic compounds, and some of them include pyridine, pyrrole, and thiophene.

Basicity of nitrogen-containing compounds

The compound containing a weaker conjugate acid will have a higher $p{K}_a$ value, and such compounds will possess a stronger base. The existence of electron-donating groups in amines increases electron density, and such amines are regarded as basic.

Ranking the compounds based on the order of increasing basicity

The resonance structures of pyrrole can be given as:

Resonance structures of pyrrole

The structures provided above indicate that the lone pair of electrons on the nitrogen atom of pyrrole take part in resonance because of which they are not available for donation. Hence, pyrrole is less basic.

The protonation of imidazole can be given as:

Protonation of imidazole

The protonation of thiazole can be given as:

Protonation of thiazole

Among the nitrogen atom and sulfur atom, the nitrogen atom is regarded to be more electronegative. So the positive charge on the nitrogen atom is less stable as compared to the positive charge on the sulfur atom. Hence thiazole is more stable and functions as a strong base. The basicity order of the compounds can be given as: