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.

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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.

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

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 –

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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 –

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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 –

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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 –

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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 –

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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 –

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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 –

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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 –

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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 –

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

Some compounds possess aromaticity or are aromatic compounds still they do not follow Huckel’s rule. Generally, these compounds are poly cyclic. For example, Pyrene is a polycyclic aromatic compound and has 4 fused benzene rings. It has 8 pi – bonds, thus 16 pi electrons. For pyrene, 4n + 2 16. Thus, it does not follow Huckel’s rule but still an aromatic compound. Other exceptions are trans – bicalicene, coronene. Both of these are also polycyclic aromatic compounds which contain 8 and 12 pi – bonds respectively and do not follow Huckel’s rule. But still show aromaticity. Structure of all these compounds are given below –

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This ends our coverage on the topic “Huckel’s Rule”. We hope you enjoyed learning and were able to grasp the concept. We hope after reading this article you will be able to solve the problems based on the topic. If you are looking for solutions of NCERT Textbook problems based on this topic, then log on to Vedantu website or download Vedantu Learning App. By doing so, you will be able to access free PDFs of NCERT Solutions as well as Revision notes, Mock Tests and much more.