CSIR NET Chemical Science: Short Notes on Huckel Theory for Conjugated π-electron Systems

By Neetesh Tiwari|Updated : January 24th, 2024

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Are you looking for some short and reliable notes during your CSIR NET preparations? Then, you have come to a perfect place!

Candidates preparing for their CSIR NET exam might need to get some short study notes and strategies to apply while preparing for the key exam of their life. At this point, We at BYJU'S Exam Prep come up with short notes on the Huckel Theory for Conjugated π-electron Systemswhich comes under the part, i.e., Physical Chemistry of the Chemical Science syllabus. 

Our experienced subject-matter experts have meticulously designed this set of short notes for Huckel Theory for Conjugated π-electron Systems to give you the most standard set of study materials to focus upon. In this cut-throat competitive world, students need to prepare themselves with the best study materials to help them learn and for their future. So, here we are offering the best study notes that are reliable and can be used by the students during their preparations for the upcoming CSIR-NET 2024 exam.


Huckel Molecular Orbital Theory

In conjugated systems, the Hückel approximation is used to derive the energies and forms of the π molecular orbitals. The covalent bonding in these hydrocarbons may be divided into two different "frameworks" using the Hückel approximation: the σ -bonding framework and the π -bonding framework. Different combinations of atomic orbitals produce the wave functions used to characterize the bonding orbitals in each framework.

The approach is limited to conjugated hydrocarbons, and only π electron molecular orbitals are taken into account because they govern the general features of these compounds; sigma electrons are omitted. The orthogonality of and orbitals in planar molecules justifies this phenomenon, which is known as sigma-pi separability. As a result, the Hückel approach is only applicable to planar systems. The Hückel approximation assumes that the electrons in the π bonds "experience" an electric potential due to the complete
σ -bonding framework in the molecule (i.e., concentrates solely on bond creation because the σ bonding framework has already been constructed).

Conjugated System

A conjugated system has an area of overlapping p-orbitals that spans the interjacent single bonds and allows electrons to delocalize across all of the aligned p-orbitals. Thus, these π electrons belong to a group of atoms rather than a single bond or atom.


Before considering the Hückel treatment for ethylene, it's a good idea to analyse the molecule's overall bonding image. In ethylene, bonding occurs when the 2s, 2px, and 2py atomic orbitals on each carbon atom are sp2 hybridized, leaving the 2pz orbitals unaffected.


The σ-bonding framework is formed by mixing electrons in the sp2 hybrid orbitals on carbon and the electrons in the 1s atomic orbitals of the four hydrogen atoms. The unhybridized 2pz orbitals give rise to the -π bonding framework. The resulting molecular orbital diagram demonstrates the independence of these two frameworks.


The HMO theory is an approximation method for treating planar conjugated hydrocarbons that simplifies the variation method.

  • This theory distinguishes between σ electrons and π electrons.
  • π -electrons play a major role in determining the properties of conjugated compounds.
  • The variation approach and the LCAO(π)-MO approximation is used to calculate HMO.

Download PDF for Short Notes on Huckel Molecular Orbital Theory


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