Huckel rule explains the aromatic properties of monocyclic organic compounds. According to this rule, a planner ring would be practically more stable if it has 4n + 2 𝜋 electrons. For example, an aromatic molecule is stable if it has a delocalized ring of alternating single and double bonds.
In 1931, German physicist Erich Huckel gave this rule. According to him, the cyclic compounds having planner ring and (4n+2) delocalized pi electrons exhibit aromatic character. The Huckel rule predicts the stability of aromatic molecules such as benzene, which has six carbon atoms in a ring with alternating single and double bonds. It can also be used to explain the stability of other aromatic molecules, such as naphthalene and pyridine.
Moreover, it can be used to determine whether a compound is aromatic, antiaromatic, or non-aromatic using molecular orbital theory (MOT) calculations. This is one of the important conditions for a molecule to have an aromatic system.
Conditions for Huckle rule: It can be applied only on monocyclic compounds having; a planner ring and 4n + 2 delocalized pi electrons.
Huckle rule and Aromaticity
Initially, the benzene ring was the main criterion for aromaticity. But there are many other factors that are also required for a compound to be aromatic. In fact, there are many systems other than benzene rings that have aromatic characteristics.
Characteristics of aromatic compounds
Aromaticity is a symbol of stability and it depends on the following factors:
- Cyclic compounds
- Unhybridized p-orbitals
- Delocalization of electron
- 4n + 2 electron system (Huckle rule)
- Resonance
- Planner ring
- Sp2 hybridization (not always)
The molecules having these characteristics are aromatic in nature. The Huckel rule is one of the important requirements for a molecule to have aromaticity. Aromatic compounds are more stable than non-aromatic compounds.
4n + 2 rule
4n + 2 is the number of pi electrons of a compound. Here, ‘n’ may be zero or any positive integer i.e (1,2,3,4…). If a molecule has a number of pi electrons according to this rule then it has aromatic characters. That is the reason why these compounds are more stable than non-aromatic compounds.
Examples of Huckle rule
There are different systems that follow the huckle rule. For example, the 2π electron system, 6π electron system, 14π electron system, and 18π electron system. However, some 10π electron system, as in 10-annulene is an exception to the huckel rule.
Two pi-electrons system
When n=0 then a molecule has two 2 electrons in a ring.
Putting the value of n in the formula
4n + 2
4 (0) + 2
0 + 2 = 2
So,
Number of pi bonds = 1
Number of pi electrons = 02
For example, a three-member ring with a positive charge on the third carbon atom is an example of a two-pi-electron system. As it obeys the huckel rule so it is an aromatic system. It is a stable system because of resonance.
Six-pi (6π) electron system
When n=1, a molecule has six (6) electrons in a ring.
Putting value in the formula:
4n + 2
4 (1) + 2
4 + 2 = 6
Number of pi bonds = 3
Number of pi electrons = 6
For example, the benzene ring is a common example of an aromatic compound. But this system is not only limited to benzene rings, there are many heterocyclic compounds that also have six pi electrons and they show aromaticity. These compounds also show resonance and are stable as compared to non-aromatic compounds.
The common examples of six-membered heterocyclic compounds that have six pi electrons system are given below:
Examples of five-membered heterocyclic compounds that have six pi (6π) electrons are as follows:
These are planner molecules. The lone pair of the heteroatom is parallel to the p orbitals of the ring, so these are in conjugation. These compounds have resonance energies and hence they are stable. They have two pi bonds and a lone pair.
Ten pi (10π) electron system
When n=2, a molecule has ten (10π) electrons in a ring.
Putting the value of n in the formula:
4n + 2
4 (2) + 2
8 + 2 = 10
so,
Number of pi bonds = 5
Number of pi electrons = 10
For example, azulene is the isomer of naphthalene. It obeys the huckle rule as it has 10 delocalized pi electrons, and exhibits aromaticity.
More than ten pi (10π) electron system
There are also certain examples of compounds having more than ten electrons that also have aromatic character. For example,
When n=3, a molecule has 14π electrons in a ring.
Putting the value of n in the formula;
4n + 2
4 (3) + 2
12 + 2 = 14
So,
Number of pi bonds = 7
Number of pi electrons = 14
14π electron system
This is similar to the [10]-Annulene. It has angle strain that’s why it is a non-planar molecule. It is an unstable molecule. However, a number of stable bridges [14]-Annulene have been prepared.
18π electron system
When n=4, a molecule has eighteen pi (18π) electrons in a ring.
Putting the value of n in the formula;
4n + 2
4 (4) + 2
16 + 2 = 18
Number of pi bonds = 9
Number of pi electrons = 18
[18]-Annulene is comparatively more stable than [10]-Annulene and [14]-Annulene. It also has an aromatic character.
Exceptions to the Huckel rule
[10]-Annulene is the expectation of the huckle rule. Although 10-annulene has a huckle number, it is not stable. In addition, it is a non-aromatic compound because it has non-planar geometry.
Related resources
- Organic Compounds: Introduction, Applications, Examples
- Types of Organic Reactions and their Examples
- Carbocation Stability
Concepts Berg
What is n in the huckle number?
The n is any positive integer (0,1,2,3,4…) including 0.
What are aromatic compounds?
The cyclic compounds having 4n+2 delocalized pi electrons in conjugation are called aromatic compounds. They are very stable and don’t break down easily. They prefer substitution reactions.
What happens when n=0?
When n = 0 then a molecule has two pi electrons and it shows aromaticity.
Why are aromatic compounds more stable?
The stability of aromatic compounds depends upon the following factors,
- Delocalization of pi electrons.
- Conjugation
- High Resonance energies
- Planarity of a ring, etc.
What are huckle magic numbers?
Huckle’s magic numbers are, 2, 6, 10, 14, 18, 22, etc.
Why [10]-Annulene is a non-aromatic compound?
[10]-Annulene has a small ring size. The hydrogen atom present at the center of the ring possesses steric hindrance to disturb the planarity of molecules. That’s why it is a non-aromatic compound.Why huckle’s rule is applicable to azulene?
As azulene is a bicyclic compound but huckle rule is applicable to azulene because it is cyclic, plane, and has considerable resonance energy and a dipole moment.
Reference
- 2nd edition of Organic Chemistry by Joseph M. Hornback
- 12th edition of Organic Chemistry by T.W Graham Solomons, Craig B. Fryhle, and Scott A. Synder.
- 7th edition of Organic Chemistry by John Mcmurry.
