![cyclodecapentaene](https://www.quirkyscience.com/wp-content/uploads/2016/05/Cis-1.jpg)
There are aromatic hydrocarbons and there are non-aromatic hydrocarbons. For basic mono-ring structures that have alternating carbon-carbon double bonds, there is a rule called Hückel’s rule that defines whether a given hydrocarbon can be aromatic or not. It might seem, at first that cyclodecapentaene (C10H10 or [10]-annulene) should be aromatic. Yet, cyclodecapentaene is not an aromatic hydrocarbon. Let’s find out why not.
A Few Simple Rules for Aromatic Behavior
- The double bonds in the ring must be conjugated (alternating). This allows for the electrons to be delocalized [in conceptualization be flipped in either a clockwise or counterclockwise direction]. This allows for the presence of what is called a ring current.
- The ring must be flat.
- The mono-ring must feature 4n + 2 π-electrons (Hückel’s rule). For example, benzene, with its n value of 1, has 4(1) + 2 = 6 π-electrons. Since it also meets requirements 1, 2, and 4, it is aromatic.
- Bond angles and atom proximities should not be so severe they nullify aromatic energy stabilization.
Why Cyclodecapentaene is not an Aromatic Hydrocarbon
![cyclodecapentaene](https://www.quirkyscience.com/wp-content/uploads/2016/05/Trans-Top-1.jpg)
There are four basic structural shapes for cyclodecapentaene. Two of these, however, are obviously not flat. So we dismiss those two forms right off. However, the other two structures fulfill a number of the four requirements.
The rings of both the all cis- isomer (Figure 1) and the trans-cis-trans-cis-cis isomer (Figures 2 & 3) of cyclodecapentaene possess completely conjugated double bonds. Check!
![cyclodecapentaene](https://www.quirkyscience.com/wp-content/uploads/2016/05/Trans-1.jpg)
Are these two forms flat? At first glance, they would seem to be. Check!
Both possess 5 double bonds or 10 π-electrons. This fulfills Hückel’s rule for n = 2. Check!
So far, based on requirements 1, 2, and 3, it looks like our hydrocarbon should be aromatic in both its isomer forms.
No. 4 Straw Breaks the Camel’s Back
Uh-oh. Both cis- and trans- isomers break rule qualification number 4.
The cis-isomer has exaggerated carbon-carbon bond angles of approximately 144°. The ideal angle for those bonds is 120°.
The trans-isomer has two hydrogen atoms that come so close together, ring flatness, though seemingly possible with a molecular model kit, is, in fact, prohibited.
Note: You might also enjoy The Aromatic Cyclopentadienyl Anion
References:
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