Aromaticity: Lone Pairs

Bottom line for today: for purposes of determining aromaticity, each atom can only contribute one p orbital, whether it be part of a Pi bond or lone pair.

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Figuring out when an atom’s lone pair is involved in aromaticity is pretty straightforward in certain situations, like that of pyrrole and the cyclopentadiene anion.

But what do you do in a situation like that of pyridine, which has 3 pi bonds, but also a lone pair? At first glance it might seem like it has 8 Pi electrons, which would make it anti-aromatic.

Here’s where being able to figure out the structure of a molecule becomes crucially important.

Let’s look at the structure of pyridine (below).  Note how its lone pair is pointing 90 degrees away from the p orbitals that make up the double bonds in the structure? This means that the lone pair is not involved in aromaticity. 

So the shortcut rule is: for the purposes of aromaticity, if an atom is involved in a pi bond, ignore the lone pair. 

Now what about furan (below)? It has two lone pairs, which seem to give it 8 electrons. But let’s look at the structure here.

Note that at any one time, only one lone pair can align itself with the other p orbitals, while the other lone pair is pointing off at 90 degrees.