What is the Difference Between Alternant and Nonalternant Hydrocarbons?

The main difference between alternant and nonalternant hydrocarbons lies in the structure and the number of carbon atoms in their rings.

Alternant hydrocarbons are conjugated hydrocarbon systems that do not possess any odd-membered rings. These hydrocarbons display interesting properties, such as paired molecular orbital energies for the π system, equal electron density at all sites in the ground state, and a simpler form of the secular determinant in the Hückel approximation.

Nonalternant hydrocarbons are compounds featuring at least one ring containing an odd number of carbon atoms. Examples of classical nonalternant hydrocarbon compounds include azulene, pentalene, and heptalene. These compounds have a fascinating π-framework with a consecutive arrangement of odd-membered rings.

In summary, alternant hydrocarbons are characterized by their lack of odd-membered rings and display unique properties in their molecular orbitals and electron density, while nonalternant hydrocarbons have at least one ring containing an odd number of carbon atoms, resulting in a different structural arrangement.

Comparative Table: Alternant vs Nonalternant Hydrocarbons

Alternant and nonalternant hydrocarbons are two major types of hydrocarbon compounds containing ring structures. The key difference between them lies in the number of atoms in the ring structure. Here is a summary of their differences in tabular form:

Alternant hydrocarbons have at least one ring constructed with an odd number of atoms, while nonalternant hydrocarbons have at least one ring constructed with an even number of atoms. Examples of alternant hydrocarbons include cyclobutadiene, naphthalene, and benzyl radical, while examples of nonalternant hydrocarbons include azulene, pentalene, and heptalene.