Acenaphthylene: Uses & Aromaticity

Instructor: Laura Foist

Laura has a Masters of Science in Food Science and Human Nutrition and has taught college Science.

Acenaphthylene is a polycyclic aromatic hydrocarbon that at first doesn't appear to follow all of the rules of aromaticity. We will learn why it is aromatic and how it is used.

Uses of Acenaphthylene

Chances are you are wearing acenaphthylene right now! Acenaphthylene is used in making dyes and pigments that may have been used to dye your clothes. Since it isn't soluble in water it is useful in making dyes because it won't wash out when you wash your clothes.

It is also used in making soaps, pesticides, and plastics. Acenaphthylene can be toxic, which is why it is a useful pesticide. And since it isn't soluble in water, it can act as the oil-binding molecule in soaps, which need to have one hydrophobic and one hydrophilic end.

Aromaticity

Let's review the rules of aromaticity:

  • Cyclic
  • Every atom in the ring is conjugated
  • Flat planar molecule
  • Huckel's rule, or 4n+2 pi electrons, where n is any whole number

Now let's look at acenaphthylene. It is cyclic: we have a pentagon fused to two hexagons. Every atom is conjugated -- each double bond is separated by only one single bond. This means that pi electrons can be shared along the entire ring. The molecule is flat.

Now let's count how many pi electrons there are:


There are 12 pi electrons
Counting pi electrons


There appear to be 12 pi electrons. This does not fit the 4n+2 rule (we can't have 'n' be a whole number and have the equation equal 12). So according to this rule it would appear to not be aromatic! Yet experimentally we have seen that it is aromatic. How is this possible? Why is Huckel's rule being broken?

Real aromatic ring on the acenaphthylene molecule

Huckel's rule is a bit of an oversimplifcation, which becomes clear when we have 3 or more fused rings. People have developed several hypthoses to explain the fact that Huckel's rule may not apply to these molecules. One is that the center pi bond isn't actually part of the aromatic ring. The aromatic ring is just the outside ring. That would mean there are only 10 pi electrons, which does fit Huckel's rule: 4(2)+2 = 10.

Experimental Aromaticity

Molecules with multiple aromatic rings fused together are called polycyclic aromatic hydrocarbons (PAHs). Some of these molecules follow the typical 4 rules of aromaticity. Others need to be tested experimentally. Since aromatics are much more stable than similar non-aromatic molecules, aromaticity can be tested experimentally by comparing the stability of the compound in question with a similar, known, aromatic compound.

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