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In 1931, a German chemist and physicist, Erich Huckel, gave a theory that could help identify if a compound had aromatic properties and differentiate between the three categories of a compound, aromatic, non-aromatic, and antiaromatic. This proposed theory was called Huckel’s Rule, named after the very person who gave birth to it.
Huckle’s rule is applied to organic compounds. Any chemical compound with one or more carbon atoms covalently linked to the atoms of other elements such as hydrogen, oxygen, or nitrogen is called an organic compound. Studying these compounds forms the basis for a branch of chemistry called organic chemistry. Before delving deeper into the statement of Huckel’s rule, let us understand what aromatic compounds are?
Aromatic Compounds
When electrons in the pi orbitals of a conjugated cycloalkene compound can move around or delocalize, thus enhancing its stability, it is called aromaticity. Aromatic compounds are organic compounds made up of carbon and hydrogen atoms in continuous rings like structures with delocalised pi electrons.
The presence of the pi electrons in aromatic compounds contributes to them being highly stable. The name aromatic is given as the compound emits a pleasant aroma.
Criteria for Aromaticity
For a compound to be aromatic, it should fulfil all the given criteria.
The compound should be cyclic, meaning its structure should form continuous rings.
The compound should consist of fully conjugated p orbitals at every atom in the ring.
The geometry of the compound should be planar, which means all the molecules should lie on the same plane.
The compound must follow Huckle’s rule.
Non-aromatic Compounds
We now know what criteria should be fulfilled to identify a compound to be aromatic. So, what are non-aromatic compounds?
Non-aromatic compounds are those compounds that do not fulfil one or most of the requirements that are discussed above. Hence, all aliphatic compounds are examples of non-aromatic compounds.
Huckel’s Rule
We learned that one of the criteria for aromaticity is that it should obey Huckel’s rule. But what is Huckel’s rule exactly, and how does it help determine if a compound is aromatic or not? Let us learn the answer to these questions.
Statement
Huckel’s rule states that a cyclic and planar compound consists of 4n+2π electrons.
Explanation
Huckel’s rule is a group of algorithms to be used, keeping in mind the physical structure of the compound. The rule takes note of the pi electrons in the conjugated pi orbitals and if it satisfies the following condition.
(4n+2)π
Where n is an integer.
The conditions to determine antiaromatic compounds are slightly different. For a compound to be identified as antiaromatic, it should fulfil the above criteria and the following condition
4n.
What is so Special About (4n+2) π Electrons?
According to Huckel’s Molecular Orbital Theory, if the bonding of the molecular orbitals is filled with paired electrons, then that compound becomes particularly stable. Since aromatic compounds are highly stable, it is natural to speculate that the statement might be true for them. In the case of aromatic compounds, 2 electrons will fill the lowest energy orbit. In comparison, 4 electrons fill the outer subsequent energy orbits, thus filling all bonding orbits and leaving no antibonding orbitals occupied. This, in total, gives electrons.
How to Apply Huckel’s Rule?
The application of Huckel’s rule is very simple. You only have to follow the following steps.
Calculate the number of pi electrons of the compound.
Compare the number to the series of values given by the function (4n+2) π (e.g. 6, 10, 14, 18, etc.)
If the criteria for the aromatic compound are fulfilled, and the number of pi elements matches a number on the series, then the compound can be identified as an aromatic compound.
Conclusion
This article briefly discussed what aromatic compounds are and how to identify them. We also learned the difference between aromatic compounds and non-aromatic compounds. We delved a bit deeper into understanding the statement behind Huckel’s rule and how it helps in identifying aromatic compounds.
