Animated Solution for Chemistry - Organic Chemistry: The order of basicity among the following compounds is

Guanidine, Amidines, and Imidazoles

When evaluating the basicity of nitrogenous compounds, the golden rule is simple but profound: a base is only as strong as the stability of its conjugate acid.

When a base accepts a proton ($H^{+}$), it transforms into a positively charged cation. If this newly formed cation can effectively disperse its positive charge through resonance or inductive effects, the original compound will be highly eager to accept that proton, making it a strong base. Let's apply this principle to our four contenders.

Guanidine (Compound IV)

Let's start with compound IV, Guanidine. When guanidine accepts a proton at its imine nitrogen ($=NH$), it forms the guanidinium ion.

This cation is a masterpiece of symmetry. The positive charge can be perfectly delocalized over all three nitrogen atoms, resulting in three equivalent resonance structures. Because equivalent resonance structures contribute equally to the resonance hybrid, they provide massive stabilization energy. This makes guanidine an exceptionally strong base—often referred to as a superbase—and easily the strongest among our options.

Acetamidine vs 2-Imidazoline

Next, we compare the two amidines: Acetamidine (I) and 2-Imidazoline (II).

When acetamidine is protonated, it forms the acetamidinium ion, which is stabilized by two equivalent resonance structures. However, acetamidine has a secret weapon: the methyl group ($-C{H}_3$) attached to the central carbon. This methyl group provides electron density through hyperconjugation ($+H$) and the inductive effect ($+I$). By pumping electrons toward the electron-deficient center, it further stabilizes the positive charge.

On the other hand, 2-imidazoline (compound II) also forms a conjugate acid with two equivalent resonance structures. But if we look closely at the central carbon between the two nitrogens, it only has a hydrogen atom attached to it. It completely lacks the electron-donating $+H$ effect that acetamidine enjoys.

Therefore, while both are strong bases, the extra boost from the methyl group makes Acetamidine (I) slightly more basic than 2-Imidazoline (II).

Imidazole (Compound III)

Finally, we arrive at compound III, Imidazole.

Protonating imidazole yields the imidazolium ion, which, like the amidines, is stabilized by two equivalent resonance structures. You might think its aromatic nature would make it highly stable. However, there is a catch.

The double bond in the imidazole ring means that the carbon atoms are $s{p}^2$ hybridized. Because $s{p}^2$ carbons have more s-character than $s{p}^3$ carbons, they are more electronegative. They exert an electron-withdrawing inductive effect ($-I$), which pulls electron density away from the already positive nitrogen system. This destabilizes the conjugate acid, making imidazole the weakest base of the group.

Putting it all together, the order of basicity is dictated first by the number of equivalent resonance structures, and then fine-tuned by inductive and hyperconjugative effects:

Thus, the correct order is IV > I > II > III, which corresponds to option (D).