The key difference between alternant and nonalternant hydrocarbons is that alternant hydrocarbons have no rings constructed with an odd number of carbon atoms, whereas nonalternant hydrocarbons are compounds having at least one ring constructed with an odd number of carbon atoms.
Alternant hydrocarbons and nonalternant hydrocarbons are two major types of hydrocarbon compounds containing ring structures. Alternant hydrocarbons are conjugated hydrocarbon systems with no odd-membered ring. Nonalternant hydrocarbons are compounds featuring at least one ring containing an odd number of carbon atoms.
CONTENTS
1. Overview and Key Difference
2. What are Alternant Hydrocarbons
3. What are Nonalternant Hydrocarbons
4. Alternant vs Nonalternant Hydrocarbons in Tabular Form
5. Summary – Alternant vs Nonalternant Hydrocarbons
What are Alternant Hydrocarbons?
Alternant hydrocarbons are conjugated hydrocarbon systems that have no odd-membered ring. These systems can undertake a starring process where the carbon atoms are divided into two sets. One set contains carbon atoms with stars in a way that no two starred or unstarred atoms are bonded to each other. The starred set contains the highest number of atoms. Upon obtaining this condition, the secular determinant in the Huckel approximation gets a simpler form. This is because cross-diagonal elements between atoms in the same set tend to be necessarily zero. Some examples of alternant hydrocarbon compounds include cyclobutadiene, naphthalene, benzyl radical, etc.
Figure 01: The Chemical Structure of Cyclobutadiene, an Alternant Hydrocarbon Compound
There are three major properties of alternant hydrocarbons, including the pairing of the molecular orbital energies for the pi system, similar coefficients for the two paired molecular orbitals at the same site and equality of the population or electron density of all sites.
If there is an odd number of atoms in the alternant hydrocarbon, this means it contains an unpaired orbital with zero bonding energy. When considering this orbital, we can write the coefficients on the atomic sites without calculation. There, the coefficient on all of the orbitals belonging to the smaller set is zero, and the sum of the coefficients of the surrounding orbitals that are starred should also be zero.
What are Nonalternant Hydrocarbons?
Nonalternant hydrocarbons are compounds featuring at least one ring containing an odd number of carbon atoms. Azulene, Pentalene, and Heptalene are examples of classical nonalternant hydrocarbon compounds.
Figure 02: The Chemical Structure of Azulene, a Nonalternant Hydrocarbon Compound
They are bicyclic nonalternant hydrocarbon compounds. These compounds have a fascinating pi-framework with a consecutive arrangement of odd-membered carbocycles.
What is the Difference Between Alternant and Nonalternant Hydrocarbons?
Alternant hydrocarbons are conjugated hydrocarbon systems that have no odd-membered ring. Nonalternant hydrocarbons are compounds featuring at least one ring containing an odd number of carbon atoms. The key difference between alternant and nonalternant hydrocarbons is that alternant hydrocarbons have no rings constructed with an odd number of carbon atoms, whereas nonalternant hydrocarbons are compounds having at least one ring constructed with an odd number of carbon atoms. Cyclobutadiene, naphthalene, and benzyl radical are examples of alternant hydrocarbons, while Azulene, Pentalene, and Heptalene are examples of nonalternant hydrocarbons.
Below is a summary of the difference between alternant and nonalternant hydrocarbons in tabular form for side by side comparison.
Summary – Alternant vs Nonalternant Hydrocarbons
Alternant hydrocarbons and nonalternant hydrocarbons are two major types of hydrocarbon compounds containing ring structures. The key difference between alternant and nonalternant hydrocarbons is that alternant hydrocarbons have no rings constructed with an odd number of carbon atoms, whereas nonalternant hydrocarbons are compounds having at least one ring constructed with an odd number of carbon atoms.
Reference:
*1, Akihito Konishi, and Makoto Yasuda. “Breathing New Life into Nonalternant Hydrocarbon Chemistry: Syntheses and Properties of Polycyclic Hydrocarbons Containing Azulene, Pentalene, and Heptalene Frameworks” CSJ Journals.
2. “Alternant Hydrocarbons.” Hydra.pharm.monash.edu.au.
Image Courtesy:
1. “Cyclobutadiene-3D-vdW” By Edgar181 – Own work (Public Domain) via Commons Wikimedia
2. “Azulene molecule ball” By Jynto (talk) – Own work. Created with Discovery Studio Visualizer (CC0) via Commons Wikimedia