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 HydrocarbonThere 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!
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 BackUh-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.
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