An aromatic organic compound is a flat structure generally consisting of carbon atoms in a ring with conjugated (alternating) double bonds about the entire ring. A section of the ring, if drawn as a straight chain, would look like -C=C-C=C-C=C-.
It is not essential that all the ring atoms are carbon. Certain other atoms can replace carbon and be aromatic. Most notably among these are nitrogen and sulfur. See the Khan Academy videos on aromatic heterocycles for more information.
Strictly speaking, it is not the number of conjugated double bonds, but the number of conjugated electrons that are of importance here. This is because most aromatic species are neutral molecules. However, in certain ionic species, including those discussed in this article, these numbers differ.
Curiously, it is the total number of conjugated double bonds that is of special significance. We refer to the mathematical expression dictating that number as Hückel’s Rule.
What Is Hückel’s Rule?
Single carbon-to-carbon bonds are so-called sigma bonds. They are in-line, so writing the bonds as -C-C-C-C- fairly well represents the geometry of the bonds. However, double bonds include a pi bond. That pi bond resides above and below the two conjoined carbon atoms.
If a ring of carbon atoms has alternating double bonds, those pi-electrons are not blocked between neighboring bonds. This makes it possible to create and sustain a flowing diatropic ring current.
Hückel recognized a simple mathematical relationship determined whether a ring compound could exhibit aromaticity or not.
If the ring contains 4n + 2 pi electrons, the molecule should be aromatic.
Consider a few ring sizes that fill the bill. For n = 1, we calculate that a ring of six carbon atoms should be aromatic.
Benzene matches the description. It has six carbon atoms and six pi electrons. Those electrons form three conjugated double bonds. The molecule is flat and, significantly, exhibits a strong ring current.
Typical Aromatic Hydrocarbons
Hückel’s Rule does what it is intended to do. We can derive a list of aromatic hydrocarbons, based on it, though of course, the other factors including ring flatness are also requisite. A couple of lovely discussions on the topic may be found in the UCLA aromaticity tutorial.
Atypical Aromatic Structures
It is the number of pi bonds, rather than the number of carbon atoms, that determines what are Hückel Aromatic Structures. It is true that the pi bonds must form a closed loop of conjugation. What structures can deviate in number of carbon atoms and yet be Hückel Aromatic Structures? Certain ions, including the cyclopropenyl cation, the cyclopentadienyl anion and the cycloheptatrienyl cation.
The chemical formula for the cyclopropenyl cation is C₃H₃⁺.
Since the ring is basically a triangle in this instance, and three points determine a plane, the cyclopropenyl cation is flat. With n = 0, the Hückel value is 2. It has two pi electrons, hence only one double bond. It is an exception to the rule of conjugation; it is aromatic.
The cyclopropenyl cation possesses a ring current. The stabilization of aromaticity is weakened by the extreme ring strain. The cyclopropenyl cation is most decidedly atypical.
The chemical formula for the cyclopentadienyl anion is C₅H₅⁻.
Without the extra electron, this species is non-aromatic.
There are only two conjugated double bonds, and a total of 5 pi electrons. Charged, the extra electron resides in the p orbital of the carbon atom that has no double bond. This electron brings the number of pi electrons up to six. It is stable, conjugated, flat, and possesses a ring current. The cyclopentadienyl anion is aromatic.
Cycloheptatrienyl Cation a.k.a. the Tropylium Ion
The chemical formula for the cycloheptatrienyl cation is C₇H₇⁺.
This structure has one carbon too many for all of them to be conjugated. But the total pi electron count in the tropylium ion is six. The ring closure for the conjugation is a bit less than perfect, due to the seventh carbon. As to flatness, some computer models suggest the ion is completely flat, while others that the seventh carbon has a slight pucker out of the plane.
If the species is completely flat, likely the stabilization due to aromaticity is responsible.