Huckel’s Rule

Nov 16, 2022, 16:45 IST

Huckel's rule estimates the aromatic properties of any planar ring molecule in the field of organic chemistry. The supporting quantum mechanics needed for the formulation of this rule was first solved by the German physical chemist and physicist Erich Armand Arthur Joseph Huckel in 1931.

Huckel 4n + 2 Pi Electron Rule

A cyclic ring molecule is said to obey Huckel's rule when the total number of pi electrons belong to the molecule can be compared to the given formula "4n + 2", where n is positive integer (including zero).

According to Huckel's rule, examples of molecules have only been determined for the value ​​of "n" ranging from zero to six. The total no. of pi electrons in the benzene molecule shown below can be found to be 6, under the 4n+2 pie electron rule, where n=1.

Therefore, the benzene molecule's aromaticity is determined because it obeys Huckel's rule. This rule is also justified using the Pariser-Parr-Pople method and the linear combination of atomic orbitals (LCAO) method.

Aromatic compounds generally are relatively stable due to the resonance energy or delocalized electron cloud. For a molecule to exhibit aromatic properties, it must meet the following conditions:

• 4n + 2 pie electrons must be present in the system of connected p orbitals (where the electrons are delocalized) belonging to the molecule.
• To fulfill the first condition, the molecule must have an planar structure where the p orbitals are more or less parallel and also can interact with each other.
• The molecule have a cyclic structure and a ring of p orbitals with no sp3 hybridized atoms.

Other aromatic compounds that satisfy Huckel's rule include pyrrole, pyridine, and furan. All three of these examples have six pi electrons each; therefore, the value of n for them will be one.

Monocyclic Hydrocarbons Stability

The stability of a monocyclic hydrocarbons, their cations, and their anions can be understood using Huckel's rule. A perfect example of such a monocyclic hydrocarbon is benzene.

It can be noted that benzene tends to undergo substitution reactions, with the number of pi electrons remaining the same in the product. Benzene does not tend to participate in addition reactions that would cause the loss of pi electrons. However, catalysts are generally a prerequisite for the benzene molecule to participate in the substitution reaction. This stability of the pi electron system belonging to benzene is often called "aromaticity."

See the example of cyclopentadiene, its corresponding anion (cyclopentadienyl anion) is easy to make because it has six pi electrons and is relatively stable. On the other hand, the cyclopentadiene cation has only four pi electrons, which means it does not exhibit aromaticity according to Huckel's rule. This cation is quite difficult to form, especially compared to its acyclic counterpart - the acyclic pentadienyl cation.

Huckel's rule is, therefore, very useful in estimating the aromaticity (and thus the stability) of ring molecules of planar structures.

Frequently Asked Questions (FAQs)

Q1. Which compounds does not follow Huckel rule?

Ans. Aromatic Compound

Q2. Do non aromatic compounds follow Huckel's rule?

Ans. Yes, if a compound does not have a continuous ring of the conjugated p orbitals in the planar conformation, then it is nonaromatic. Huckel's Rule is a useful in evaluating the potential for a ringed molecule to be aromatic.

Q3. Are aromatics polar or nonpolar?

Ans. nonpolar

Q4. Which compound is not aromatic?

Ans. Ethyl ethanoate is an ester having the functional group -COO. It is not an aromatic compound.

Q5. What type of bond is aromatic?

Ans. Covalent bonds