Proton transfer from water or other acid to pyridine does not involve the electrons of the aromatic sextet. Why, then, is pyridine a considerably weaker base than aliphatic amines? The answer is that the unshared pair of electrons on the pyridine nitrogen lies in a relatively electronegative \(x f^{2}\) hybrid orbital, whereas in aliphatic amines, the unshared pair lies in an spr hybrid orbital. This effect decreases markedly the basicity of the electron pair on an sp²-hybridized nitrogen compared with that on an \(s p^{3}\) hytvridized nitrogen. There are two nitrogen atoms in imidazole, each with an unshared pair of electrons. One unshared pair lies in a \(2 p\) orbital and is an integral part of the \((4 n+2)\) \(\pi\) electrons of the aromatic system. The other unshared pair lies in an spr hybrid orbital and is mot a part of the aromatic sextet; this pair of electrons functions as the proton acceptor.

Understanding the difference in basicity between pyridine and aliphatic amines starts with a look at their structures. Pyridine is an aromatic compound, notable for its ring structure that includes a nitrogen atom. Unlike aliphatic amines, which have an sp3 hybridized nitrogen atom that provides a lone pair of electrons for accepting protons, pyridine's nitrogen is sp2 hybridized.

The lone pair on the sp2 hybridized nitrogen occupies an orbital that is part of the aromatic ring system, specifically a 2p orbital. This orbital is more electronegative, and as a result, the electrons are held more tightly to the nitrogen. Consequently, these electrons are less readily available to engage with protons, which is a key factor in determining basicity.

Aliphatic amines, which often contain an sp3 hybridized nitrogen, have their lone pair in an orbital that is not involved in aromaticity. This makes the lone pair more available and reactive to accept a proton, thereby making aliphatic amines generally stronger bases than pyridine.

The concept of hybridization explains how atoms in a molecule blend their atomic orbitals to form new hybrid orbitals, which shapes the molecule's geometry and influences its reactivity. In sp2 hybridization, one s orbital mixes with two p orbitals to form three sp2 hybrid orbitals. These orbitals lie in a single plane, creating a trigonal planar arrangement with 120-degree angles between them. This is typically seen in aromatic rings like benzene and in the nitrogen of pyridine.

In contrast, sp3 hybridization occurs when one s orbital mixes with three p orbitals, forming four sp3 hybrid orbitals. These orbitals arrange themselves in a tetrahedral shape, pointing in four unique directions and offering a greater volume of space for the electrons. This difference profoundly affects the properties of molecules. Sp2 hybridized orbitals are part of conjugated and aromatic systems, giving stability but less reactivity, while sp3 hybridized orbitals are more exposed and thus more chemically reactive, making compounds with sp3 hybridized atoms generally more basic.

Imidazole is an aromatic heterocycle with a distinct structure containing two nitrogen atoms within its five-membered ring. Its aromaticity arises from a network of alternating double and single bonds, forming a conjugated pi system. Each nitrogen atom in imidazole is different regarding its hybridization and participation in the ring's aromatic system.

The first nitrogen is part of this system; the lone pair occupies a 2p orbital that is delocalized across the ring. This delocalization contributes to the ring's aromatic stability and diminishes the basicity of that lone pair. On the other hand, the second nitrogen holds a lone pair in an sp2 hybridized orbital, which is not involved in the ring's aromaticity. This lone pair is more available to react with acids, making this nitrogen atom the site of basicity in imidazole.

Therefore, even though both nitrogens have lone pairs, only the sp2 hybridized nitrogen's lone pair is effectively basic and acts as a proton acceptor. This asymmetry is key to understanding the reactivity and chemical properties of imidazole.