Q15-38E - Organic Chemistry (Mcmurry)

Question

Ribavirin, an antiviral agent used against hepatitis C and viral pneumonia, contains a 1,2,4-triazole ring. Why is the ring aromatic?

Ribavirin

Step-by-Step Solution

Answer

1,2,4-triazole

Step 1: Aromaticity

Aromatic compounds should have the following characteristics such as:

• It should be planar.
• It should be cyclic.
• It should obey $(\mathbf{4}\mathbf{n}\mathbf{+}\mathbf{2})\mathbf{\pi }$ electrons.
• It should be conjugated.

Step 2: Ribavirin

In the ribavirin structure, there is a 1, 2, 4-triazole ring which is a five-membered ring. It has three nitrogen atoms and two carbon atoms in the ring. The two nitrogen atoms are ${\mathrm{sp}}^2$ hybridized and they contribute one pi-electrons from each nitrogen atom. Even though these are having lone pair of electrons on nitrogen, they won’t participate in the resonance. Because these orbitals are in the plane of the ring.

The other nitrogen atom is ${\mathrm{sp}}^3$ hybridized and contributes one lone pair of electrons to resonance which is in a p-orbital and perpendicular to the ring.

The other electrons from the two carbon atoms are ${\mathrm{sp}}^2$ hybridized. So, there are six pi-electrons in the 1,2,4-triazole ring. Thus, the ring is aromatic.

1,2,4-triazole