Aromatics

Learn More About Magoosh MCAT
This is MCAT Organic Chemistry 1. We'll be covering Aromatics. Now, to determine if something is aromatic, we need to consider three important factors. First, it must have alternating double and single bonds, these atoms must then be arranged into a cyclical structure or a ring.

And if there are multiple rings they need to be coplanar, or in other words they need to be flat. And the last consideration is that they must obey Huckel's Rule. Was says they will have 4n+2 pi electrons. This means that they can have 2, 6, 10,14,18, etc., electrons.

Now, the two compounds we have here are benzene and methalene. Benzene has alternating double and single bonds. It's a single ring so it's coplanar with itself. And it has 2, 4, 6, 6 pi electrons. Naphthalene has alternating double and single bonds.

It's coplanar. And it also follows Huckel's Rules because it has 2, 4, 6, 8, 10 pi electrons. So both of these are aromatic compounds. Now, there are a couple of common aryl compounds you want to have memorized. Toluene, which is a benzene ring with a methyl group attached to it.

Phenol, which is a benzine ring with a hydroxyl group attached to it. An aniline, which is a benzine ring with a NH2, or amino group attached to it. Now, there's another one that you don't have to memorize for the test, but that you might wanna keep tucked away. This is one of the most expensive Aryl compounds. This is Mercedes Benzing.

This is just a bit of organic chemistry humor. It's such a dry subject, we have to find the laughs where we can. For a couple of common aryl substituents, a phenyl group is when we have direct attachment of the benzene ring. If there's a methyl group between the attachment, it's a benzyl group. So this right here would be a CH2.

So this right here will be a one, two, three, four, five, six, seven, eight, nine, ten. A decane and a point of potassium is in the 1, 2, 3, 4, 5, so this is a 5 phenyl decane. If this group was on the same thing, it would be a 5-benzyl decane.

So having this methylene group in here makes it a benzyl substituent as opposed to the phenyl substituent. Now, in the case of disubstituted rings, there are several prefixes you'll want to memorize. First is ortho, this is when we have substitution on adjacent carbons. Next, we have meta on a meta substitution we have one empty between the substituted carbons and last para, indicates the substitution are on opposite sides of the ring.

So let's draw a few examples using the o, m and p forms of xylene. Now, xylene is a dimethyl benzene. So if we draw benzene, we put one methyl group here in this position, this is the ortho-xylene because they are on adjacent carbons, draw benzine again. One meta group, second meta group. This is meta because there is an empty carbon in-between.

Last, we have one meta group and we put the other one on the opposite side of the ring. This is going to be the paraxylene, ortho-xylene, meta-xylene, para-xylene. Now, let's take a moment and name a few dissents to two benzine compounds. In our first compound we have 2 chlorines, 1 is at the 1 position, the other is at the 3 position, so this could either be a 1,3-dichlorobenzene, or these 2 are meta to each other.

So we could also call this meta-dichlorobenzene. And meta is often abbreviated as just m. Now, the next one we have here is benzene with a hydroxyl group and a methyl group. We said earlier that benzene with a hydroxyl group is a phenol. Benzene with a methyl group is a toluene, so we can name this either ortho methylphenol or 2-hydroxytoluene and there's even a common name for this one, ortho-O-cresol.

The last one here, these guys are Para to each other so we call this one a para-nitro aniline or 4 nitro aniline or 1 amino for nitrobenzene. So multiple ways to name these substituents on the rings.

Here, using the meta orthopedic system or by numbering the ring or most of the time they are just common names. Now, let's name a few more bonus compounds. On the left here we have an orthodox benzene. Which is a structurally rigid benzene. In the middle we have our paradigm benzene.

Which is an ideal benzene for us to have. And the last one on the right here, this is a meta physical benzine. This is a compound that we're just not certain if it exists or not. Now, let's do a few reactions doing aromatics. An important point to note here, Aromatics, this benzene rings are very stable which means if that we want to get them to react we kind of have to force them.

We could do that using heat, catalysts, strong acids, etc., but under the normal circumstances they don't react. So something has to be done to force that ring to react. So for the case of halogenation if we don't do anything but just trying add our chlorine here, we'll get no reaction. So instead what we need to do is add in Iron three chloride strongly as acid and heat this thing up.

If we do that we won't get no reaction instead we will haloginate the ring. And so, now we have mono chlorobenzene. So again, aromatics require a little bit of a push actually react. So in nitration we will take benzene, nitric acid, a little bit of sulfuric acid. We can get the to be nitrated, Like this, again, we had to force this to happen by using some strong acids.

To sulfonate benzene, again, we will use sulfuric acid and some heat to force this on, we get the sulfinated product. Friedel-Crafts Acylation, this is another one where since there is a name on this, you'll want to remember it by name, what's involved, any reagents or conditions. So here we have benzene plus an acyl halide. We'll use a Lewis acid like aluminum trichloride.

And now we can acylate the benzene ring. So far, we've only talked about a single substitution to a ring, but if we have multiple substituents, each substituent will direct subsequent editions. We have three basic types. We have the activating ortho/para. These are electron donating groups.

This includes NH2, NR2, And R alcohol groups. Essentially these are all groups with lots of electrons that they can donate. Deactivating ortho/para substituents are those that are weakly electron withdrawing. This includes the halogens, this are electronegative species that have some affinity to electrons and so pull it away from the rim.

And last thing we have the deactivating meta substituents, now these are strong electron withdrawing that include things like NO2, SO3H, Carboxylic acid, Ester, Ketone and Aldehyde. So these things tend to have strong resonance structures so they can deal localize electrons. For example, the carboxylic acid.

We have this delocalized electron. Because they can delocalize, they're much stronger electron withdrawing because they can support this electrons under the resonance structures. Then know this much strongly withdraw electrons, the default Is to be an Ortho/Para director. Once you withdraw electrons above a certain threshold, then you start orthing things into the meta position, but until then you default to Ortho/Para.

What is the product of three successive nitrations of toluene? Let's bring up the toluene. We said the toluene is benzine with the methyl group on it. And in nitration we're going to add HNO3 is sulfuric acid and we'll put an NO2 group onto toluene here. So first question is this CH3, is this methyl group an ortho/para or a meta director?

Well, this is essentially just an alkyl group. It has lots of electrons it can donate, so if it donates them in it's going to be an ortho/para director. So either here, here, or here, one of these three spots, the two ortho or the para will be the most likely for this nitrous group to add into. So let's add this first one into the ethyl position.

And if we do a second nitration. Now, this point a metho group is still on ortho/para director. What about the nitrates groups? In O2 is a strong oxygen resonant structure, so it's going to be electron withdrawing and deal with direct substituents to the matter position. Well, meta is going to be one, two away or here.

Or one, two away here. Well, that meta position just happens to line up with the methyl groups ortho/para, so we can put the next nitrous group, Right there. And now for our third and last one. Again, let's look at these.

This nitro group wants to push things into a one, two meta position, this one wants to go onto one, two meta position. And the meta group wants to go into in or self position. So everything is directing to this position right here. And then, our final product here is a trinitrotoluene or TNT. Now, luckily it gets harder and harder to add each of these successive nitrogens, otherwise you would be in trouble once we made this.

But if you are working in the OCHEM lab and you are using lots of toluene, you might want to avoid the Nitric acid and the Sulfuric acid. Last reaction, this section is a hydrogenation or a reduction. Remember that a reduction is either the gain of electrons or hydrogens. So in this case, we want to reduce this benzene ring. Well, we said that benzene rings are fairly stable, so to do this we're gonna add hydrogen, but we're gonna need to use a catalyst like rhodium and some heat.

If we do this we can reduce benzene down to cyclohexane. The fully hydrogenated cyclic C6 compound. Again, remember, benzene rings are stable and they are essentially inert unless we push them. And that concludes this section of organic chemistry.

Show Transcript

Next Lesson


Organic Chemistry 1