# Huckel's Rule

Contribution of aromatic compounds is huge in the development of mankind and they still have more potential to do the same. Computer parts, DVDs and linchpin components of automotive parts are made up of aromatic compounds. Drugs such as Aspirin and paracetamol which we are using since ages are aromatic compounds. A branch of chemistry which deals with aromatic compounds is called organic chemistry. Aromatic molecules do not react easily with other compounds and remain stable. Aromatic compounds possess the property of aromaticity. Huckel’s rule helps to decide whether an aromatic compound possesses aromaticity or not. Huckel’s rule was given by German physicist and physical chemistry Erich Huckel in 1931. Although its succinct formulation as 4n + 2 was given by Professor William von Eggers Doering. It is also known as Huckel’s 4n + 2 pi- electrons rule.

Before understanding the Huckel’s rule, you need to have a good understanding of aromaticity. In organic chemistry, the term aromaticity is used to describe a property of a cyclic, planar molecule with a ring of resonance bonds that exhibits more stability than other geometric or connective arrangements with the same set of atoms. To be an aromatic compound or possess aromaticity, compounds must fulfil following all four conditions –

• The molecule must be cyclic.

• Every atom in the cyclic ring must be conjugated. As it will provide the cyclic ring delocalized pi-electron system. Thus, we can say every atom in the cyclic ring must have an empty p orbital and must be capable of participating in resonance.

• All compounds must follow Huckel’s rule.

• The molecule should be planar or flat. Those compounds which follow the above 4 rules of aromaticity, they are generally flat as in that condition they possess large enough potential energy.

Thus, Huckel’s rule is one of the criteria which should be fulfilled by aromatic compounds to possess aromaticity. Now, let us understand what Huckel’s rule is.

### What is Huckel’s Rule?

According to Huckel’s rule, all planar aromatic compounds must have [4n+2] pi-electrons where n is an integer (i.e. n= 0, 1, 2, 3, 4…etc.). This rule estimates whether a planar ring compound will possess aromatic properties or not.

Let’s understand it by taking the example of Benzene and Cyclo octa-tetraene. These both compounds possess ring structure. Although one of them possess aromaticity and another doesn’t. Which one will have aromaticity can be decided by Huckel’s rule. As you can see in the given structure of Benzene below that it has 6 pi – electrons. In 4n + 2, if we put n = 1 then [(4×1)+2] = 6, thus it obeys Huckel’s Rule. Benzene is an aromatic compound and possesses aromaticity.

n = 1

[(4×1)+2] = 6$\pi$ electrons

Aromatic Compound

On the other hand, if you see the structure of Cyclooctatetraene, given below. Then you calculate that it has 8 pi – electrons. In 4n + 2, if we put n = any integer, then [(4n) + 2] $\neq$ 8, thus it does not obey Huckel’s Rule. Cyclooctatetraene is a non -aromatic compound and does not possess aromaticity.

(Number of pi- electrons = 8 as it has 4 pi- bonds, So, for any value of ‘n’, [4n+2] cannot be equal to 8.)

### Applications of Huckel’s Rule

• Stability of Mono Cyclic Hydrocarbons – We can explain the stability of conjugated monocyclic hydrocarbons by Huckel’s rule. Most common example of it is benzene. It has 6 pi – electrons and follows Huckel’s rule and is stable in nature. The planar cyclopentadienyl anion (C5H5-) also has 6 pi – electrons and is stable in nature. While its cation has 4 pi – electrons and is not stable in nature. planar ring molecules which do not obey Huckel’s rule and have 4n pi – electrons are less stable.

• Thiophene – It has a planar ring structure with two pi bonds and two pi – electrons on sulfur atom which are denoted by red color in the structure given below –

Thus, thiophene has 6 pi – electrons and follows Huckel’s Rule as [41+2] = 6. It obeys all other conditions of aromaticity as well and is an aromatic compound.

• Cyclopropenyl Ion - It has a planar ring structure with one pi bond which is shown in the structure given below –

Thus, it has two pi – electron and if we keep n = 0 then [40+2] = 2. So, it obeys Huckel’s rule and all other conditions of aromaticity as well and is an aromatic compound.

• Furan – It has a planar ring structure with two pi – bonds in the ring and two pi – electrons on the oxygen atom in the ring which are denoted by red color in the structure given below –

Thus, furan has 6 pi – electrons and if we keep n= 1 in Huckel’s 4n + 2 rule then [41+2] = 6. So, furan follows Huckel’s rule and fulfils other conditions of aromaticity as well. Furan is an aromatic compound.

• Pyrrole - It has a planar ring structure with two pi – bonds in the ring and two pi – electrons on the nitrogen atom in the ring which are denoted by red color in the structure given below –

Thus, pyrrole has 6 pi – electrons and if we keep n= 1 in Huckel’s 4n + 2 rule then [41+2] = 6. So, pyrrole follows Huckel’s rule and fulfils other conditions of aromaticity as well. Pyrrole is an aromatic compound.

• Pyridine - It has a planar ring structure with three pi – bonds in the ring which are shown in the structure given below –

Thus, pyridine has 6 pi – electrons and if we keep n= 1 in Huckel’s 4n + 2 rule then [41+2] = 6. So, pyridine follows Huckel’s rule and fulfils other conditions of aromaticity as well. Pyridine is an aromatic compound.

• Pyrimidine - It has a planar ring structure with three pi – bonds in the ring which are shown in the structure given below –

Thus, pyrimidine has 6 pi – electrons and if we keep n= 1 in Huckel’s 4n + 2 rule then [41+2] = 6. So, pyrimidine follows Huckel’s rule and fulfils other conditions of aromaticity as well. Pyrimidine is an aromatic compound.

• Double Ring Compound – Naphthalene is a double ring compound. It has a total of 5 pi – bonds in two rings which are shown in the structure given below –

As naphthalene has 5 pi – bonds, so it contains 10 pi – electrons. If we keep n= 2 in the Huckel’s 4n + 2 rule, then 42 + 2 = 10. Thus, it follows Huckel’s rule and fulfils other conditions of aromaticity as well. Naphthalene is an aromatic compound.

• Oxazole - It has a planar ring structure with two pi – bonds in the ring and two pi – electrons on the oxygen atom in the ring which are denoted by red color in the structure given below –

Thus, oxazole has 6 pi – electrons and if we keep n= 1 in Huckel’s 4n + 2 rule then [41+2] = 6. So, oxazole follows Huckel’s rule and fulfils other conditions of aromaticity as well. Oxazole is an aromatic compound.

• Imidazole - It has a planar ring structure with two pi – bonds in the ring and two pi – electrons on the nitrogen atom in the ring which are denoted by red color in the structure given below –