Benzene structure: Can Kekulé's actual idea work?

[RyanJ]

Hey everyone!

 

I've missed a bit of school and I'm reading up on what I've missed (turns out it was a lot of organic chemistry stuff) and one of these topics was about benzene.

 

Kekulé's original idea was alternating double and single bonds within the carbon loop but this was shown to be inaccurate because the bond lengths were identical, and that benzene under went substitution rather than addition (Now what you'd expect if there were double bonds in the molecule) and the molecule was more energetically stable than was expected.

 

My question is this; could Kekulé's idea for benzene actually exist - eve if for a short period of time?

 

-- Ryan Jones

[Tartaglia]

If you study chemistry at university you will study group theory and molecular orbital theory and all will become clear. It is a little difficult to explain but aromatic systems with 4n+2 pi electrons delocalise, whereas those with 4n pi electrons are localised

[RyanJ]

If you study chemistry at university you will study group theory and molecular orbital theory and all will become clear. It is a little difficult to explain but aromatic systems with 4n+2 pi electrons delocalise, whereas those with 4n pi electrons are localised

 

Unfortunately I haven't made it that far yet - just working ahead, even though its not essential to know this I just find it useful to ask the questions anyway.

 

Considering it would be very unstable I'd say it couldn't exist for very long if at all but I don't know for shure which is why I'm asking and can't seem to find out much about it.

 

-- Ryan Jones

[woelen]

Phrased in a more simple way: You can regard the 6 electrons, which would be used for making the three additional bonds, as being distributed over all 6 C-atoms.

 

So, the structure of benzene is that all 6 C-atoms are connected to each other with a single bond, there also is a single bond with the H-atoms, and then 6 electrons remain (one from each C-atom). Each of these 6 electrons contributes to a bond, connecting all 6 C atoms. Plastically, think of each of these 6 electrons, giving each other hands. Because all 6 electrons contribute to all 6 C atoms in the same way, there is no difference in bond length, and all C atoms are equivalent. Schematically, this usually is depicted by means of a ring in the hexagon for the benzene-ring, instead of drawing three double bonds.

 

This delocalization in fact is very common, and many molecules, but also ions, contain delocalized electrons. Another nice example is the nitrate ion, NO3(-). This sometimes is presented as (-)-O-N(=O)2, with two O-atoms bonded to the N with a double bond, and one O-atom bonded with a single bond, and the remaining electron attached to the single-bonded O-atom.

 

This structure is not correct, however. It would make one O-atom very different from the others, but in reality, in NO3(-), all O-atoms are equivalent. The real structure is that of three O-atoms, all bound to the nitrogen atom with a single bond, and then there are 6 electrons remaining, which together are distributed over the entire ion. So, each O-atom has 1/3 of the -1 charge, and each O-atom can be regarded as being bonded for 5/3.

[RyanJ]

Thanks for the reply Woelen, very interesting!

 

-- Ryan Jones

[jdurg]

I believe the same holds true for ozone which has three oxygen atoms bound in a manner that results in a delocalized bond.