What is a planar molecule
When looking at a structural formula, it is often not that easy to decide whether the molecule is an aromatic or some other compound.
Are the following molecules aromatic?
Fortunately, there are a few rules by which an aromatic molecule can be clearly identified as such.
Detection criteria of aromatics
- Are aromatics plane ring systems.
- Aromatics have an odd number of π-electron pairs in conjugated double bonds: 1, 3, 5, 7, 9,
- The C-C bond lengths in the ring system all are the same.
- The number of π electrons corresponds to H ckel's rule.
The H ckel rule
The HeckelRule says that an unsaturated, ring-shaped substance is only an aromatic if for the number of π electrons in their conjugated ring system the following relationship applies.
(4n + 2) π electrons with n = 0, 1, 2, 3, ...
The symbol n is a positive, integer number. (By the way, n is not the number of rings! This is often inferred because this applies to benzene, naphthalene and anthracene, of all places.)
An example for n = 0 is an exotic compound, the cyclopropenylium cation.
There is no system of conjugated double bonds here. The HeckelHowever, the rule applies for n = 0; then namely 4 x 0 + 2 = 2. These two π-electrons are actually delocalized in the ring. This makes the cyclopropenylium cation an aromatic.
So aromatics are planar carbocyclic compounds with 2, 6, 10, 14, 18 (etc.) π electrons in the ring. Only these can be mesomeric stabilized.
What is an anti-aromatic?
A carbocyclic compound with an even number of π-electron pairs is called an anti-aromatic.
Such connections do not correspond to HeckelRule. Antiaromatics can, but do not have to be, planar.
One example is cyclobutadiene.
Its molecule is planar, its double bonds are conjugated. This speaks for the presence of an aromatic.
The number of its π-electrons is 4. So let's turn the HeckelRule on. Is that possible: (4n + 2) => 4? No. So there is no number n for which, if you put it into the HeckelFormula inserts, 4 comes out!
Cyclobutadiene is actually not an aromatic. Because the double bonds in the ring are firmly fixed. You even have to put energy into it so that they change their position in the ring. Ultimately, cyclobutadiene is an alkene - exactly an alk-diene.
Pseudo or quasi-aromatics
Many think that the concept of aromaticity only applies to organic chemistry. However, there are also examples in inorganic chemistry of aromatic compounds that can do without the element carbon.
A well-known example is the sulfur cation S.42+. It has 6 delocalized electrons. It thus formally fulfills the H ckel 4n + 2 rule and can therefore be described as aromatic. In order to distinguish "classic" (i.e. organic) aromatics such as benzene from the inorganic ones, the terms pseudo or quasi-aromatic were introduced.
Here are two important boundary structures of the mesomeric states of the sulfur cation.
There is a less likely co-ionic structure in which one S atom carries two positive charges.
Ralf Alsfasser, H. J. Meyer: Modern inorganic chemistry, de Gruyter, 2007
Equipped with this information, we can take a closer look at the fabric examples above
- benzene is the Aromat absolutely. Benzene is cyclic planar and has a conjugated ring system with 6 π electrons.
-  -Annulen, the famous "stop sign" molecule Cyclooctatetraene. After HeckelRule is Annulen a Anti-aromaticbecause it has 8 π electrons. Because no Hckel-n can be found that leads to the electron number 8 via (4n + 2). The annulene molecule is also not planar, but takes the shape of a tub.
But there is one Annulene dianion. That has 10 π-electrons. Here n is equal to 2. The annulene dianion is thus an aromatic - and it is also built planar.
- naphthalene has 10 π electrons. So it follows the HeckelRule with n = 2.
- Cyclobutadiene is (as already explained) the typical one Anti-aromatic, so no aromatic. Even though its molecule is planar.
- The Anthracene is a polycondensed Aromat from three benzene rings with a total of 14 π electrons. The HeckelThe rule applies here with n = 3.
Like all rules in chemistry, this rule has its limits
Do you want to z. For example, to clarify whether corannulene or pyrene or 3,4-benzopyrene are aromatics, the o. A. formulated recognition criteria are not particularly helpful. Pyrene has 16 π electrons. Corannulene and benzopyrene have π-electron numbers 20, i.e. 10 electron pairs. The HeckelUsually says "no". In addition, corannulene is not even planar. Nevertheless, the three compounds have the spectroscopic and chemical properties of aromatic compounds. They are also called polycondensed aromatics (PAH).
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