# Is pyrene aromatic despite failing Hückel's rule?

Pyrene doesn't seem to be aromatic. However, sources claim that it is aromatic.

Considerations:

1. Pyrene is cyclic. ✓
2. Pyrene is flat (planar). ✓
3. Pyrene has 16 π electrons.
4. Every atom in the ring structure of pyrene is $\ce{sp^2}$ hybridized. ✓

The problem is that pyrene fails the $4n+2$ rule. $4n + 2 \neq 16$ where $n$ is an integer.

Something leads me to suspect that Hückel's rule is an oversimplification here. Or is pyrene indeed non-aromatic?

• while pyrene is formally aromatic (i.e. has ring current), it reacts mostly like diene with isolated double bonds. – permeakra Sep 22 '14 at 7:01
• I guess the better defining characteristic of an aromatic compound is its response to the magnetic field which is used in NMR spectroscopy. If a compound produces an aromatic ring current as a result of induction due to the applied magnetic field, then it can be categorised as aromatic. – Kaustubh Tiwari Jan 18 '19 at 9:22

Pyrene is aromatic. The Hückel $4n+2$ rule works best with monocyclic ring systems. If you look at the following resonance structure for pyrene with a central double bond, the monocyclic periphery has 14 π electrons (ignoring the greyed-out central double bond), but that is a rationalization.

Nonetheless, pyrene undergoes reactions characteristic of aromatic systems and has ring currents expected from aromatic systems.

• Is that implying that the conjugated pi system does not involve the central double bond? @ron – Dissenter Sep 22 '14 at 0:41
• Correct, in other words, pyrene becomes 14-annulene. But as I said above, this is an after the fact rationalization. The 4n+2 rule was really meant to apply to monocyclic pi systems. You run into problems when it is extended beyond its originally intended use. – ron Sep 22 '14 at 0:47
• What if you drew resonance structures? The central bond then vanishes. – vs_292 Dec 28 '17 at 9:46

To clarify @ron's point, the general "cheminformatics" rule for deducing aromatic fused-ring systems is whether there is one path (usually the peripheral one) that satisfies $4n+2$:

So for naphthalene:

The bold bonds give you a ring path of $\mathrm{sp^2}$ carbons with 10 π electrons, so it's aromatic. There are some arguments about whether the central bond is aromatic or not. (I say baloney, but it's fair to say aromaticity generates lots of arguments.)

So yes, the bold path gives you 14 π electrons for a $4n+2$ path. Again, I've heard people say the central two carbons aren't aromatic. (IIRC, the $\ce{^13C}$ NMR disagrees.)

I wrote the code for Open Babel and aside from some tricky nitrogen-heteroaromatic rings, this rule generally matches chemical reality. (Setting aside the question of the interior carbons.)

I think another fairly safe rule is "if a fused aromatic system is made up of individually aromatic rings, aromaticity is retained."

• What's the debate about the central bond? – Dissenter Sep 22 '14 at 0:54
• @Dissenter it's outside the mainstream of chemistry, but since including the central bond would make 16 $\pi$ electrons, some cheminformatics people question of that bond is truly "in" the aromatic system. As I said above, I think it's baloney - most experimental or computational definitions of aromaticity would indicate aromatic carbons there. Some people are just too tied to Hückel - but ask them about $\ce{C60}$. Heh. – Geoff Hutchison Sep 22 '14 at 0:57
• C60 is aromatic? @GeoffHutchinson – Dissenter Sep 22 '14 at 0:57
• I'd say so, but that's a whole different debate. – Geoff Hutchison Sep 22 '14 at 1:00
• I absolutely love the statement you give in the comment and I am herewith encouraging you to include it into your answer. I believe, that there is a pretty common misconceptions, that only Hückel aromats are actually aromatic systems. But you are of course right, aromaticity generates a lot of arguments. – Martin - マーチン May 18 '15 at 4:20

While the previous answers are correct in indicating that pyrene has aromaticity, they are often correct for the wrong reason. The proper way to break down the aromaticity in pyrene is to make the most Clar sextets possible. Clar sextexts are sets of six electrons that are cyclically delocalized. There are two unique ways to draw the Clar sextets in pyrene.

The first drawing shows two Clar sextets, while the second only has one, and thus should be less favored. If we perform NICS computations to determine where the delocalization happens, we see more delocalization in rings A and D than in rings B and C.

This can be further expanded to other systems such as pentacene, which shows a much lower singlet triplet gap than many other organic molecules thanks to the triplet having two stabilizing Clar sextets.

For a more complete description of Clar sextets and aromaticity, I would recommend an article by Portella et al.1 Also, as others have mentioned, when moving to 3D, things can get weird.

1. Portella, G.; Poater, J.; Solà, M. Assessment of clar’s aromatic π-sextet rule by means of PDI, NICS and HOMA indicators of local aromaticity. J. Phys. Org. Chem. 2005, 18 (8), 785–791. DOI: 10.1002/poc.938.
• I realized in my answer that I didn't give my stock comment about "many definitions of aromaticity." I do like Clar's sextets with aromatic hydrocarbons. My one problem is that when I've brought them up in classes, it reinforces the false connection between "benzene" and aromaticity. – Geoff Hutchison Sep 21 '15 at 1:40
• Nice, but can Mr. Clar do his sextets in cyclooctateraenediide? – mykhal Nov 9 '18 at 13:49

These are both aromatic, but only one is flat.