Alternant hydrocarbons

Aromatic and conjugated hydrocarbons can be classified into two groups by a process of trying to label atoms into two sets. We normally label one set with a star (*) and leave the other set unlabeled. The objective is to see whether we can divide the atoms into two sets in such a way that all members of one set are neighboured by atoms of the other set. If this is possible, we call the hydrocarbon an alternant hydrocarbon.

Two examples are benzene and the allyl radical:

 

[Benzene]
 
[Allyl]
 

Note that the two sets may not be the same size.

If there is no way to divide the atoms into two sets in this way, the hydrocarbon is non-alternant. An example is fulvene, where we start the starring process from the end of the side-chain:

 

[Fulvene]
 

Here we see that two adjacent atoms in the ring are unstarred.

A simple rule is that all aromatic and conjugated hydrocarbons consisting of even membered rings and any kind of chain are alternant hydrocarbons. An odd membered ring makes the system a non-alternant. Check this out with the systems listed below.

Alternant hydrocarbons give some very interesting features in their Huckel theory results. Run the molecules above with the Hückel calculator and see if you can find some of these features.  Look carefully at the orbital energies, the coefficients, the charge density and the spin densities.

When you have spent some time carefully thinking about the results of these molecules, check this summary of the features.

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