Is pyridine aromatic? Explain its aromaticity and the role of the nitrogen atom.

Hückel's rule states that a compound is aromatic if it is cyclic, planar, fully conjugated, and has $(4n+2)$ $\pi$ electrons, where $n$ is a non-negative integer.

• Cyclic and Planar: Pyridine is a 6-membered heterocyclic ring that is planar.
• Fully Conjugated: All 6 atoms (5 carbons and 1 nitrogen) are $sp^2$ hybridized, each contributing a p-orbital to the cyclic $\pi$ system.
• Number of $\pi$ electrons: Pyridine has three double bonds within the ring, contributing $3 \times 2 = 6$ $\pi$ electrons. The lone pair on the nitrogen atom is in an $sp^2$ orbital, perpendicular to the $\pi$ system, and does not contribute to the aromaticity.
• Hückel's Rule: With 6 $\pi$ electrons, we can set $4n+2 = 6$. $$4n = 4$$ $$n = 1$$ Since $n=1$ is an integer, pyridine is aromatic.

The nitrogen atom in pyridine is $sp^2$ hybridized. It contributes one electron to the $\pi$ system through its p-orbital, forming part of a double bond. Its lone pair of electrons is localized in an $sp^2$ orbital that lies in the plane of the ring and is not part of the aromatic $\pi$ system. This lone pair is available for protonation, making pyridine a base. The electronegativity of nitrogen withdraws electron density from the ring, making pyridine less reactive towards electrophilic aromatic substitution compared to benzene.

• Cyclic and Planar: Furan is a 5-membered heterocyclic ring that is planar.
• Fully Conjugated: All 5 atoms (4 carbons and 1 oxygen) are $sp^2$ hybridized, each contributing a p-orbital.
• Number of $\pi$ electrons: Furan has two double bonds within the ring, contributing $2 \times 2 = 4$ $\pi$ electrons. The oxygen atom has two lone pairs. One lone pair is in an $sp^2$ orbital, but the other lone pair occupies a p-orbital that aligns with the p-orbitals of the carbon atoms, contributing 2 electrons to the cyclic $\pi$ system.
• Hückel's Rule: Total $\pi$ electrons = $4 + 2 = 6$. We can set $4n+2 = 6$. $$4n = 4$$ $$n = 1$$ Since $n=1$ is an integer, furan is aromatic.

The oxygen atom in furan is $sp^2$ hybridized. It contributes two electrons from one of its lone pairs to the aromatic $\pi$ system, which occupies a p-orbital. The other lone pair is in an $sp^2$ orbital and is not part of the $\pi$ system. The contribution of these two electrons from oxygen makes furan an electron-rich aromatic system, which is more reactive towards electrophilic aromatic substitution compared to benzene.

The final answers are:

• Pyridine is aromatic because it is cyclic, planar, fully conjugated, and has 6 $\pi$ electrons ($4n+2$ with $n=1$). The nitrogen's lone pair is not part of the $\pi$ system.
• Furan is aromatic because it is cyclic, planar, fully conjugated, and has 6 $\pi$ electrons ($4n+2$ with $n=1$). One of the oxygen's lone pairs contributes to the $\pi$ system.