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  • We've talked up carbon quite a bit here on Crash Course Chemistry,

  • but we really haven't justified the lofty position I put it in.

  • Just chains so far and while chains are good, they're not enough reason on their own for this high level of esteem.

  • But carbon atoms, they do more than just form chains.

  • Sometimes, they form rings, or as chemists call them: cyclic structures,

  • and if the rings have the right number and placement of double bonds they start acting weird and wonderful.

  • The compounds made from them are called aromatic,

  • literally because they have aromas, they can smell amazing.

  • Basil, vanilla, cloves, tarragon, lemon, cinnamon.

  • When we smell these things, it's because aromatic compounds, compounds with carbon rings,

  • are binding to receptors in our noses that tell our brains "OMG tasty stuff nearby".

  • Of course, moth-balls and modelling glue and lots of other stuff are also aromatic and don't smell so good...

  • But if you've ever smelled an herb, taken medicine, worn clothes or enjoyed a meal,

  • you probably owe that experience, at least in part, to aromatic hydrocarbons.

  • [Theme Music]

  • But before we talk about aromatics, let's talk about the most basic cyclic structures, the cycloalkanes.

  • You recognize the -alkane ending there, so there must be no double bonds, right?

  • And you can't make a ring with one or two carbons, so the simplest cyclic hydrocarbon must be cyclopropane.

  • But those tight bond angles of that triangular cyclopropane make it very fragile.

  • This instability makes it highly reactive and it would much rather just be a straight chain.

  • But as more carbons join into the ring cyclic structures get more stable.

  • Cyclobutane with its four carbon atoms is still fairly unstable and reactive,

  • while in cyclopentane the bonds are even more stable

  • because they form something close to those tetrahedral bond angles,

  • while the angles in cyclohexane are basically perfect for the tetrahedral orbitals to overlap without strain.

  • Cycloalkenes also start at three carbons and get increasingly stable as more are added,

  • though their double bonds make their molecules plainer instead of tetrahedral.

  • Finally, cycloalkynes do exist, but they are crazy hard to form because the triple bond is linear,

  • so it hates bending into rings, and that is why the simplest cycloalkyne has eight carbons.

  • Of course these rings can have all sorts of things stringing off of them

  • and their names work almost exactly the same way as straight chains do.

  • Example: This ring has five carbon atoms and a double bond, so it's a cyclopentene.

  • It has two branches, an ethyl group and a methyl group.

  • We list these in alphabetical order in the name.

  • And we number the carbons in the ring so we get the lowest possible numbers on the branches and on the double bond.

  • If we number counterclockwise from the beginning of the double bond, we get the branches at the 2 and 5 positions,

  • but if we number clockwise from the double bond the branches are at the 1 and 3 positions.

  • So, this must be 1-Ethyl-3-methylcyclopent-1-ene.

  • But since it's only one double bond and it's in the first position,

  • the second one is often left out as being understood.

  • So we simply call it 1-Ethyl-3-methylcyclopentene.

  • Incidentally all of the identifying parts of an organic molecule,

  • the double and triple bonds, the carbon chains that branch off the main chain,

  • those are called substituents because they substitute for hydrogen atoms.

  • So far we've just been looking at the cyclic hydrocarbons, so what's up with the aromatics?

  • How are they different?

  • Well this is one of the times when carbon starts doing some of its spectacular dances.

  • Cyclic hydrocarbons that contain resonant structures.

  • Things are getting interesting, and by interesting, I mean--what?

  • Resonance occurs when electrons are distributed around the molecule

  • in a way that makes it impossible to draw with a single Lewis structure.

  • It's a limitation of our tools to represent this beauty and complexity of reality.

  • In a resonance structure, the real world structure of a molecule is essentially

  • an average of all the possible structures that we can draw.

  • The simplest aromatic hydrocarbon (and one of the most common) is benzene (C6H6).

  • It's the pungent compound that gives gasoline its strong sweet smell.

  • It contains 3 double bonds, so that each carbon has a total of 4 bonds.

  • We normally write this and other organic structures without the hydrogens in order to simplify things,

  • and we often don't even write the c's for the carbons.

  • Every corner is assumed to be a carbon atom, and since we know carbon needs four bonds,

  • it's easy to figure out where the hydrogens belong even when they are not written.

  • Here's the thing though.

  • Those double bonds could just as easily be in these positions and they are in those positions -- sort of.

  • Remember there are two types of bonds in double bonds.

  • There's those sigma bonds that happen linearly,

  • and then there's those p-orbitals that stick up above and below the plane of the molecule.

  • Those p-orbitals in an aromatic compound all merge together in a ring

  • that stretches above and below the whole molecule.

  • It's a distribution of electrons that's like a donut on top of and below a molecule-it's extremely stable.

  • Overall, the bonds are in both places so the actual structure is essentially an average

  • of the two draw-able Lewis structures.

  • That's resonance, it's reality poking us in the nose and saying,

  • "oh you thought you were so clever with your Lewis structures, but in this situation, they are useless."

  • To show that the double bonds aren't in any specific place, we often just show the structure with a circle in the middle

  • signifying that the double bonds are averaged in a ring throughout the benzene group.

  • Technically the number of bonds between each pair of carbons is defined as 1.5.

  • It's not a double bond, it's a one-and-a-half bond.

  • Now aromatics can have all sorts of substituents as well,

  • and the naming system is pretty much the same as with other cyclics.

  • Another example: So this is a benzene with two methyl groups on it.

  • As always, we number the rings so that the branches have the smallest possible numbers.

  • In this case, if one of the methyl is in the 1 position, the other is in position 2.

  • Note that the numbering can also be done like this,

  • but it doesn't matter as long as the branches are at the lowest possible numbers.

  • That makes this molecule 1, 2-dimethylbenzene.

  • Now if you swap the methyls out for a couple of other slightly more complicated groups,

  • then you get acetylsalicylic acid, or aspirin.

  • Aromatic rings can be used the other way around too, as substituents on dihydrocarbons.

  • Consider this molecule, a hexane with a benzene ring attached to carbon number 2.

  • When benzene is used as an attachment, we call it a phenyl group, so this is 2-phenylhexane.

  • Or 2-phenylhexane. [Different pronunciation] It doesn't matter.

  • What's important is that it is not a benzyl group. Why?

  • Well, it was English genius Michael Faraday who first isolated benzene in 1825 from the gas used in lamps.

  • In honor of this, his French contemporary chemist Auguste Laurent started calling the derivatives of benzene "phene,"

  • from the Greek "I illiminate." It's a lovely gesture, very sweet, but, like, seriously?

  • But just to make sure you're, like, good and solidly confused, benzyl groups do exist.

  • They're a phenyl group joined by a methyl group instead of just directly.

  • So now that we know where the word "phenyl" comes from,

  • I bet you're just salivating with curiosity, wanting to know where "benzene" comes from, because,

  • like, your desire to know things is stronger than any of your other desires, right?

  • Yeah, I thought so. So here's how it happened.

  • Back in the 1400s when the spice trade drove the majority of the global economy,

  • Arabic traders sold European traders a resin used in perfumes and medicine that they called

  • "luban jawi" or "incense of Java."

  • Of course, it didn't come from Java, it came from Sumatra,

  • but the world was a big and complicated place so we can't really blame them for being a little bit confused.

  • The "lu" was mistaken by the Italians for the definite article "la", and so it was dropped, making it "banjawa,"

  • and then the French called it "banjoin," and eventually the German and English made it "benzoin."

  • This is also, by the way, the same root as the name Bon Jovi (it's not...).

  • When we started doing some chemistry, we found that most of benzoin was composed of an acid,

  • which chemists named benzoic acid, or acid of benzoin.

  • Then, a little chemistry knocked the acid off, creating what was clearly an alkene, so they named it benzene,

  • because it was an alkene that was from the benzoin resin.

  • History is hiding everywhere! And so is chemistry.

  • Now benzoic acid is one of literally infinite possible benzene-based compounds.

  • There's naphthalene, the main ingredient in mothballs.

  • It's basically two benzene rings stuck together.

  • And there's anthracene, with three benzenes, which is probably the dye that's making your blue jeans blue.

  • And it can get crazy complicated all the way up to humic acid, a component of soil.

  • Now you might be wondering how something so complicated as that could ever even come into being.

  • Like any organic compound, aromatics can undergo tons of reactions that produce all sorts of molecules.

  • By far the simplest and most common are substitution reactions in which one substituent is substituted for another.

  • Like one of the hydrogen atoms on a benzene might be changed to a propyl group,

  • or that a halogen might come in and replace that propyl group with a bromine.

  • These changes alter the compound, of course, but it may not be that major a change, like changing your shoes.

  • (Unless you put on some really weird shoes.)

  • Another fairly common reaction of aromatic hydrocarbons is coupling, the joining of two aromatic structures,

  • and it basically works the same as a substitution reaction.

  • The only difference is that the new substituent is a another aromatic structure

  • and it typically requires some kind of catalyst.

  • A final common reaction of aromatics is hydrogenation, which we talked about a little last week,

  • the addition of hydrogen atoms to remove double bonds.

  • Now of course getting rid of even a single double bond in a phenyl group always destroys the resonance,

  • and thus it is no longer an aromatic compound after hydrogenation.

  • Now obviously we haven't covered all the possibilities here,

  • but you can already see that organic molecules have the potential to become extremely large and complex,

  • integrating straight chains, cyclic structures, and aromatic groups in the same molecule.

  • This literally infinite variety is what makes organic compounds the basis for the infinite variety of life on Earth.

  • So we'll leave you to jeans and basil pesto and aspirin now to appreciate them on a whole new level

  • that simply was not possible just ten minutes ago.

  • Meanwhile, thank you for watching this episode of Crash Course Chemistry.

  • If you paid attention, you learned about the structure of cyclic organic compounds

  • and how to name them and their substituents.

  • You also learned what an aromatic compound is, what resonance is,

  • and how to name aromatics and their substituents.

  • And finally, you learned a few common reactions and uses for aromatic compounds.

  • This episode was written by Edi Gonzalez and myself.

  • It was edited by Blake de Pastino and the chemistry consultant was Dr. Heiko Langner.

  • It was filmed and edited then directed by Nicholas Jenkins.

  • The script supervisor was Michael Aranda, who is also our sound designer.

  • And the graphics team is Thought Cafe.

We've talked up carbon quite a bit here on Crash Course Chemistry,

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芳香族と環状化合物.クラッシュコース化学#42 (Aromatics & Cyclic Compounds: Crash Course Chemistry #42)

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    Vivian Wong に公開 2021 年 01 月 14 日
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