Posted by: Mark Foreman | February 14, 2010

Spot the aromatics

OK, a goblin has found out the password for the blog and before I could chase him out of the house he drew some “interesting” cyclic compounds which he uploaded. We now have the task of sorting the aromatics out from the non aromatics. let us start.

Benzene or not benzene ? (sounds like Shakespeare)

We have here a series of different things all have a C6H6 molecular formula. We will start with Dewar benzene.

Is it aromatic ?

I would vote for no, the reason is this if the middle (long) bond is a sigma bond then the pi system will only contain four electrons. This is clearly not 4n+2 electrons which is the right number required for an aromatic compound. Also this is a very strained compound which will convert with a half life of days into normal benzene.

Next one is a prism, is it aromatic or not ?

I would say no, the molecule is only made up of sigma bonds. So it has no pi system. As it has no pi system then it can not be a aromatic compound. But it is an interesting molecule.

If you replace one of the hydrogens in this molecule with something else then you can only get one isomer of the product. Also if you replace two of the hydrogens in this molecule then in common with benzene you can get three isomers of the product.

Next one is shown below.

Again I would vote for it not being aromatic, it is a molecule with only sigma bonding. No pi bonding, no pi cloud, no aromatic nature.

Then we come to our next version of benzene.

This is a molecule with a single pi bond, it has a pi cloud which contains 2 electrons. 4n+2 =2 when n = 0 but it would be best no to think of this molecule as being aromatic because it only has one canonical form. In all the aromatics I have seen it is possible to draw at least two canonical forms.

Last of all we have another cyclic version of benzene

Both these canonical forms have 6 pi electrons which is good when we consider the 4n + 2 rule. Also they are cyclic, and the molecules are planar which is good as it gives us good orbital overlap. The two forms in this diagram are for a real aromatic compound benzene.

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