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. 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. 

Eric Huckel, a German Physicist and Chemist, in the year 1931 proposed a theory that helped us determine the presence of Aromatic properties in a Planar Ring Molecule. Huckel's Rule states that a Cyclic, Planar molecule is considered to be Aromatic if it has 4n + 2π Electrons. 

Huckels Rule is a set of Algorithms that determines whether the molecule is Aromatic, AntiAromatic, or NonAromatic by combining the number of π Electrons and the physical structure of the Ring System. 

In order to determine the number of π Electrons in the case of an Aromatic System the following algorithm is used:

N= 4n + 2

where n is an integer

In order to determine the number of π Electrons in the case of an AntiAromatic System the following algorithm is used:

N= 4n

where n is an integer

A Compound is said to be NonAromatic when it does not have a continuous ring of the conjugated p orbital in a planar conformation.

Let’s understand it by taking the example of Benzene and Cyclo octa-tetraene. These both Compounds possess ring structure. Although one of them possesses 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] = 6π 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≠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.)

Why 4n + 2 Electrons?

Huckel’s Molecular Orbital theory states that when all the Bonding Molecular Orbitals of a Compound are filled with paired Electrons, then it is said to be particularly stable. Since Aromatic Compounds are particularly stable therefore this theory is true for them. There are no orbitals filled and no anti-bonding orbitals are occupied in the case of Aromatic Compounds only 2 Electrons fill the lowest energy molecular orbital and 4 Electrons fill each subsequent energy level which is denoted by n giving a total of 4n +2π Electrons. 

Few Examples of Huckel’s Rule

• Cyclopropenyl Anion

4π Electrons having a planer conjugated system (4n,n=1)

2 Electrons- one double bond

2 Electrons- lone pair of carbon with a negative charge

Therefore, it is AntiAromatic.

• Cyclopropenyl Cation

2π with a planer conjugated system(neither 4n nor 4n+2)

2 Electrons-one double bond

Therefore, it is Non-Aromatic

• Naphthalene

10π Electrons with planer conjugated system (4n+2,n=2)

10 Electrons- five double bond

Therefore, it is Aromatic.

• Pentalene

8π Electrons with planer conjugated system (4n,n=2)

8 Electrons- four double bonds

Therefore, it is AntiAromatic.

• Furan

6π Electrons with planer conjugated system (4n+2,n=1)

4 Electrons- two double bonds

2 Electrons- lone pair of O

Therefore, it is Aromatic.

• Pyrrole

6π Electrons with planer conjugated system (4n+2,n=1)

4 Electrons- double bonds

2 Electrons- lone pair of N

• Pyridine

6π Electrons with planer conjugated system (4n+2,n=1)

6 Electrons- 3 double bonds

0 Electrons-lone pair of N as it is associated with a double bond.

Applications of Huckel’s Rule 

Some other applications of Huckel’s Rule are listed below-

• 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. 

• 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 fulfills 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 –

(image will be uploaded soon)

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 fulfills other conditions of Aromaticity as well. Imidazole is an Aromatic Compound. 

Exceptions of Huckel’s Rule 

1. Cyclobutadiene C₄H₄ is stable below 35 K, while usually planer ring molecules having 4n are stable.

2. Some of the Compounds such as pyrene which is a Polycyclic Compound and is Aromatic do not follow Huckel's Rule.

3. Trans-bicalicene has 8 π bonds and is an Aromatic Polycyclic Compound but does not follow Huckel’s Rule.

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.