Aromatic hydrocarbons, also called aromatic compounds, are compounds that contain benzene as a part of their structure. Benzene is a cyclic hydrocarbon with the formula C6H6.
Alright. So let's talk about aromatic compounds. Aromatic compounds are compounds containing benzene as part of its structure and actually as you might think, aromatic compounds actually are exactly that. Aromatic. Meaning that they have an aroma, a scent to them. And so, we're going to call them aromatic. They might have scent of like different fruits or perfumes are very aromatic and before they [IB] understood the structure of it they will call it aromatic because they have this like great scent along with it.
So there was this one, one structure that always came up when dealing with aromatic structures and that is the structure benzene. And it was first isolated by Faraday in 1825 from coal. And so he discovered this compound is very unreactive. A very stable, very stable structure. And they actually analyzed it and found it to be C6H6. They were able to find them like it was structure of this. However, they were unable to find like what it actually looked like, how it looked like in a structural form. So several years later, several decades later, they came up with this formula. And this is C6H6 and this is suggested in 1860. However, this was actually not, they weren't satisfied with this as being the structure and they weren't able to check it either. And the reason they weren't satisfied with it is because they knew that this structure benzene was very unreactive, and it, and this has very dense electron, electron dense places within its structure meaning that it has, there are lots of cites where actually reactions will take place as we've learned about when you're deali- when you're talking about alkenes or alkynes.
And so they have very electron dense areas, so this is very actually highly reactive. And not only that. This behaves, this structure behaves as other alkanes do but this did not. This didn't behave as alkenes. As the alkenes do. So they knew they had to scratch that. So even like a couple of years later, this guy Kekule actually had what he refers to as a dream and while he was falling asleep one night he was thinking of specific structures and how this could be. And this actually popped up in his head. And it 's a cyclical structure having all the double bonds in every other carbon and it actually fit. But the only problem with this, and this actually was accepted for a number of years. But this was still highly reactive and they weren't quite satisfied with it but it actually fulfilled a lot of other things that they were looking for when trying to figure out the structure of Benzene, which is the backbone of all these aromatic compounds.
So, Pauling was doing other things within Chemistry and he actually discovered 1930, in 1930 he proposed this idea of hybrid orbitals and resonance structures and resonance structures is when electrons are actually located are actually not fixed, they're actually delocalized amongst the structure. And so he actually took that idea and put it towards benzene. And so instead of, I'm going to actually draw this out for you. Instead of it being a fixed structure, where it looks like that. What he is going to do is he's going to delocalize those and actually make it so this goes here, this goes here and this goes here. And so it's actually, what's hapenning is the double bond is switching around and constantly circling around this. So if you put this on a flat surface instead of on a board, if you put this on a flat surface, the electrons are continously floating on top, not having a particular home. So each carbon atom is not necessarily having its own electrons, they're actually sharing all the valence electrons amongst all the six carbons making this ring like structure and that's what we're going to use. We're going to use this circle to denote that [IB] they're delocalized. So this is actually the true structure of benzene, the backbone of all aromatic compounds.
Alright. So let's look at some examples of some aromatic compounds that you might come across. So we have naphthalene which is two benzene structures fused together and this is what you've used mothballs if you've had mothballs they actually do give off a funny scent in they're actually used to kill different bugs within its preserved different clothing structures. So this is a structure that was mothballs within the active ingredient in moth balls.
Then you have p xylene which looks very, which looks like this. And you do name them benzene or aromatic structures the same exact way you'd name anything else. So you'd want to give the sustituent groups the numbers appropriately so it's going to be one, two, three. One six. Actually it should be one four. Not one six. So this is carbon one, two, three, four dimethyl benzene. Two methyls and it's a benzene ring. So it was actually named the same way you would name any other kind of alkene and then there's this structure here. It's benzopyrene. This is used in chimney soot and actually benzenes are extremely cancernogenic and so you might want to, they reuse quite often and sometimes, not all benzenes are like. There's not all aromatic compounds are cancernogenic but benzenes do have a [IB] of being cancernogenic or creating cancer, I mean the cause of cancer, within humans and animals.
So this, these are this is the basic backbone of all aromatic compounds is benzene. Very important structure.