Aromatic rings

[zvezda]

First of all, I see a lot of benzene structural formulas that either show that the electrons are delocalised or show 3 C-C double bonds. My question is, why is this the case? Isn't it more correct to show the delocalised electrons with the circle in the centre of the carbon ring?

Second, with rings that involve Nitrogen and such, are electrons still delocalised? I just don't understand why they would be, as Nitrogen has a full outer shell.

thanks in advance

[Hancock]

While it is more correct to show a circle of delocalized electrons, this is not favourable because then you don't know how many delocalised electrons you have in the aromatic ring. If we draw the 3 double C-C bonds, we can clearly see that there are 6 delocalized electrons orbiting in the p orbitals above the surface of the ring.

If there are rings that have nitrogen atoms included (whether that be a nitrogen in place of a carbon, or a nitro-functional group), we can observe that the nitrogen atoms MAY be delocalized, depending on the arrangement of the atoms. If the nitrogen has no atom attached to it, it will have a lone pair which will act as if it is a delocalized pair of electrons. If there is a hydrogen, for example, attached to the nitrogen group, then it it's electrons will not contribute to the delocalized sea of electrons in the plane above the aromatic ring.

[zvezda]

But we know that each carbon donates one electron to the sea of electrons, so wouldn't it make sense that because there's 6 carbons there'll be six delocalised electrons?

Ok thanks for that.

And by the way, this is just a question out of curiosity, why do the carbons donate an electron? Why aren't there just 3 C-C double bonds in the ring?

[Hancock]

It's kind of a mystery why there aren't just 3 C-C double bonds in a benzene ring. The way it was explained to us in first year chem was that each carbon in fact has a bond order of 1.33 instead of 2 (for a double bond), so in effect, they are kind of all sharing the 3 double bonds that are there.

Edited note: You will not need to know this for Year 12 Chemistry. This is strictly Uni Chem and I'm sure even some people at uni found this concept confusing.

And yes, you are correct in saying there would be six delocalized electrons (explained a bit in my previous post). The reasons carbons 'donate' an electron, in the case of a benzene ring, is due to the atomic orbital structure of each carbon. You may not have come across this yet, but there is something called 'hybridisation' in which electronic orbitals 'mix' together to form new structured orbitals.

So, there are 2 orbitals you need to know for this to make sense. The s orbital, which is circular, and the p orbitals, which are dumbell shaped and have 3 in the direction of the x, y and z axis.



However, for carbons in the benzene ring, they have what is called an sp2 hybridized orbitals, which is the mixing of the s orbital, and 2 p orbitals.



While these sp2 orbitals are used to bond each carbon to each other, there is still a lone electron in the p orbitals of each carbon. This creates the 6 delocalized electrons in the benzene ring. I'm presuming that is what you were enquiring about 'donating' electrons, and I hoped it helped a bit.

[mark_alec]

It's kind of a mystery why there aren't just 3 C-C double bonds in a benzene ring.

I'd argue that symmetry requires this to be the case.

[Hancock]

Agreed. ^^

[zvezda]

Yeah that's helped, thanks. Good luck with your exchange

[Aurelian]

It's kind of a mystery why there aren't just 3 C-C double bonds in a benzene ring. The way it was explained to us in first year chem was that each carbon in fact has a bond order of 1.33 instead of 2 (for a double bond), so in effect, they are kind of all sharing the 3 double bonds that are there.

Not really... It's pretty obvious when you see the conjugated, side on overlap of pi orbitals...

Anyway, @ OP, another reason I would suggest for the prevalence of the "less correct" three double-bond representation of benzene is that it makes it a lot easier to draw what are called reaction mechanisms. Reaction mechanisms are given by diagrams depicting the flow of electrons and bond formations which occur during a reaction.

That probably wont make much sense until you do first year chemistry at uni, but yeah.