5.11 Aromaticity

In the case of benzene, the number of conjugated electrons unlocks a special set of properties. Each double bond contributes 2 electrons to the pi orbitals, so the total number of electrons is 6. That's 2 more than a multiple of 4; it satisfies a mathematical formula called Hückel's Rule, namely 4n + 2, which is just the right number for the ring to be aromatic.

This term doesn't mean that the compound has a smell, though many aromatic compounds do, and that's why that name was chosen. Aromaticity refers to an unusual stability of the ring, possible only when the ring is planar, fully conjugated, and satisfies Hückel's Rule. We've already seen that aromatic groups are called aryl groups, and now we know why.

Aromatic rings are difficult to break apart. They are more resistant to attack by compounds that would react with their functional groups. Aniline C₆H₅HN₂, for instance, is less basic than the saturated equivalent cyclohexylamine C₆H₁₁NH₂. The former compound's aromaticity makes it less inclined to ionize, compared to the latter compound which is basically ammonia with a tail.

Some heterocycles are aromatic, including pyridine, pyrazine, pyrrole, imidazole, furan, and thiophene. Chalcogen and pnictogen heterocycles can satisfly Hückel's Rule even without the right number of double bonds, because they can contribute lone pairs to the formula.

Pyrrole, for example, has only 2 double bonds, resulting in only 4 pi electrons, failing to satisfy the rule. But the nitrogen atom can contribute its lone pair to the pi orbitals, making a total of 6 and resulting in aromaticity.

Furan is built similarly, with only two double bonds, but its oxygen atom contributes one of its lone pairs, making 6 pi electrons. (Its other lone pair lies coplanar with the ring, as if it were a hydrogen atom.)

The stability of aromatic rings can be observed in thiophene, a sulfur compound that doesn't smell like sulfur and doesn't act like a sulfide in reactions. Instead, thiophene resembles benzene in chemistry and odor.

Generally, if a ring can be aromatic, it will be, and if a reaction can result in an aromatic ring, it will.

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