Chemistry: Aromatic Hydrocarbons

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So far we have talked about three functional groups, alkanes, alkene, alkynes. We see that the alkenes and the alkynes both contain pi bonds. Basically, they contain carbon-carbon, double bonds and in the case of alkynes, carbon-carbon triple bonds.
Now, what we are going to talk about is another functional group called aromatic hydrocarbons. Aromatic hydrocarbons also contain carbon-carbon pi bonds, but they have different reactivity from the ordinary carbon-carbon, pi bond that we have seen in alkenes.
First of all, the word aromatic hydrocarbons has historical context. In the olden days aromatic compounds had a certain kind of smell to them. That is where they got their name. Nowadays the smell of the hydrocarbon is not really what dictates whether or not you can call it aromatic. It is dictated by something very different. Basically, the electronics of the system.
Now, if you will recall, we took an alkene and treated it with bromine and we got the addition product when the bromine like bromine added across the alkene. Aromatic Hydrocarbons, on the other hand, behave differently. If you take benzene, which is an aromatic hydrocarbon, and treat it with bromine, you get no reaction. Basically, the benzene molecule is much more stable, and much less reactive than the ordinary alkene. If you force the molecule, by adding a catalyst and force it to react with bromine, you get a reaction, but a reaction where only one bromine is incorporated, instead of two, and a reaction where the pi bond remains. The pi bond doesn't go away like it does up here, with this reaction. So basically, what happens is that the substitution reaction, where the H on the benzene molecule get substituted for bromine.
What we are going to do, is talk a little bit about the nomenclature of aromatic hydrocarbons. Then talk about the bonding, the reactivity and try to understand reactivity in the context that they are bonding.
Some simple aromatic hydrocarbons are given on this sheet over here. We have benzene, toluene, naphthalene and phenol. Benzene is the simplest one. It has six carbons and pi bonds between all the carbons and three different pi bonds. Toluene is a benzene molecule with a CH[3] attached to it. Naphthalene is essentially two benzene molecules fused together into one ten-member molecule. Phenol is a benzene molecule with an OH bound to the carbon of the benzene. So these are some common molecules.
Benzene in the old days, was used for people to wash their hands when they were tarring roofs and had lots gunky, greasy, oily stuff of their hands, but that is now known to be carcinogen. It causes cancer, so no one uses that anymore.
Toluene is a replacement for benzene. Toluene is what is found in model airplane glue and oftentimes, when kids sniff glue these days, what they are sniffing is the toluene.
Naphthalene is the active ingredient in mothballs.
Phenol is an antiseptic and it is used in various antiseptic and sore throat remedies.
You can substitute around the benzene ring and you can put constituents in various places. Here we have chlorines on adjacent carbons and that is called ortho. So, whenever you have two constituents on carbons that are next to each other they are called ortho. Here we have chlorines with a carbon in between. Whenever they are like this, they are called meta. Here we have chlorines on opposing carbons of aromatic ring and when they are like this, they are called para. So if you have them right next to each other, they are called ortho. If you have one carbon in between them, they are meta, and if you have two carbons in between them they are para. This would be ortho chlorobenzene, meta chlorobenzene and para chlorobenzene.
Now the bonding of the benzene is a little bit different than the bonding in simple alkenes. The difference has to do with the fact that all the electrons are part of the ring - all the pi electrons. When the pi electrons are like this, you can imagine doing is taking this pair of electrons and putting them over here between this carbon-carbon bond. Take that pair of electrons over there and put them between this carbon-carbon bond. Take a pair of electrons over there and put them between this carbon-carbon bond and what you end up with is what is known as a resonance structure, where we have the electrons in different positions around the benzene ring.
Resonance structures, as you will recall, are molecules that have the same atoms spatial arrangements, but different electronic arrangements. In other words, the atoms are in the same positions, but the electrons have new ground in the molecule and that is what this would be.
Now the pi system is still composed of p orbitals. Each of these carbons has a p orbital on it, and here it is represented down here. Here are the p orbitals. Each carbon has a p orbital and the top loaves all overlap, that is shown in red. And the bottom loaves also overlap and that is shown in green. Basically what we have is an extended pi system where the carbons overlap with their neighbors all around in a ring like this.
So what would be the best representation of the molecule if it is not with isolated bonds like you have over here in these two, but one ring in the middle that indicates that all the pi electrons are delocalized and are shared among the six membered-ring.
If you interrupt this circular array of pi electrons, you would no longer have an aromatic molecule. So here is the benzene molecule with two H's added to it, and in this case it is not aromatic. The pi electrons can't go all the way around the ring, they are interrupted by these two and so it doesn't have this unusual stability that benzene has.
Now the definition of an aromatic molecule is a molecule, which contains a certain number of electrons in the pi system. That number is 4n + 2, where n is a whole number. So molecules that have the pi electrons circulating in a ring system and have 4n + 2 electrons total in the pi system are called aromatic. So think about this, N has to be a whole number and n can be equal to 0, in which case you would have two electrons in the pi system. N can be equal to 1, then 4 + 2 would be equal to 6. So 4 x 1 = 4 + 2 = 6. That is this situation over here and that is what you have in benzene. The 6 pi electrons would be aromatic. Also n can be 2, whole number. 4n +2 = 10, 4 x 2 = 8 + 2 = 10 and that is the situation when you have naphthalene. Ten pi electrons circulating around the naphthalene ring.
So, in order to be aromatic the molecule must fill the 4n + 2, known as Huckel's rule, set number of electrons in the pi system.
What if you don't have it? Well, if n is not a whole number, then you end up with an anti-aromatic molecule. So here are two compounds cyclobutadiene and cyclooctatetraene, where n is not a whole number. This is a four-electron total in the pi system. This is a very reactive molecule. This is much more reactive than a normal alkene would be and it is very difficult to even isolate the molecule. This compound is cyclooctatetraene, is not as reactive as cyclobutadiene, but it is still more reactive than ordinary alkene. Both of these compounds are anti-aromatic and have high energy. They don't enjoy this aromatic stabilization.
Since the aromatic compounds are extra stable, that means they are not reactive. So benzene, when you treat it with bromine, if it were to react, it would give you an additional product. You would get this product, and the problem with doing this, you have lost the aromatic stabilization you used to have, by interrupting the pi system, by having the two bromines, you would no longer have the pi system on the right and this compound is no longer aromatic.
So if you force the reaction to occur, instead of getting the additional product, instead you get the substitution product where the bromine substitutes one of the hydrogens and gives you this product and you maintain the pi character, the six pair pi electron, aromatic molecule.
So to recap what we have seen. Aromatic molecules are unusually stable. In order to be aromatic, they must fulfill the 4n + 2 rule. They are less reactive than ordinary alkenes and when they do react instead of giving you addition products, they give substitution products.
Organic Chemistry
Hydrocarbons
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