Complex chemicals possess names almost equally complex. Occasionally, a simpler nomenclature can be employed. For basic aromatic compounds, composed of a ring of alternating single and double bonded carbon atoms in a ring, the name [n]annulene has been is commonly used. Examples, including annulene are given below.
We won’t go into the theory behind aromaticity. What we will do is lay out the basic factors that students use to determine if a compound is aromatic.
1. The ring is composed of conjugated single and double bonds (…−C=C−C=…).
2. The molecule is relatively flat.
3. The number of available π-electrons equals 4n + 2 (a Hückel number), where n is
generally a small positive integer.¹
4. Crowding does not severely limit or prevent aromaticity.
5. Ring size affects aromaticity, but does not automatically prohibit it (see ref. 1).
For instance, the benzene molecule is composed of 6 conjugated single and double bonds. It is flat. And the number of π-electrons is 2 from each double bond, giving a total of 6 – a good number, since 4(1) + 2 = 6.
This compound crystallizes out as dark, brick-red crystals that are air-sensitive. It decomposes at 96°C. Due to the triple bonds, it possesses 18 π-electrons. Six of these, however, do not contribute to any potential aromaticity, since their orbitals are orthogonal² to those of the other π-electrons.
Contributing π-electrons amount to 12 in number. This is not a Hückel number, however, since 12 = 4n with n=3. Since it is not 4n + 2, the molecule is not aromatic. If the 6 other π-electrons did add in, the molecule would have been aromatic with n=4!
Annulene Derivatives: Answering Our Initial Inquiry
While we do not have test results to prove our two compounds illustrated in Figure 1 are aromatic, they seem to meet the following criteria:
1. Both molecules consist completely of conjugated double bonds, even if
more than one ring.
2. The two molecules are flat.
3. Both molecules possess the required 4n + 2 number of π-electrons.
4. Crowding does not appear to be a problem.
5. Ring size falls well within reasonable range.
We thus state, until proven otherwise, that the two annulene derivatives we introduced in our discussion are, indeed, aromatic organic compounds.
¹ Erich Hückel developed this equation in connection with molecular orbital theory.
² Orthogonality is a mathematics/physics concept describing how two properties act
independently, in effect, “perpendicular” to each other, “ignoring” each other.
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