Nomenclature: Crash Course Chemistry #44

Nomenclature: Crash Course Chemistry #44

October 16, 2019 100 By Stanley Isaacs


There are some of you out there, taking chemistry, and feeling a little bit like there’s an international body whose job is simply to make your life more difficult, to force you to cram tons of information into
your head, to suck all the wonder and excitement out
of learning about how the world works at this beautiful, fundamental level of chemicals. You are not actually imagining this.
That organization actually exists. It’s called the International Union of Pure
and Applied Chemists or IUPAC, and really, they’re looking out for your
best interests, in just the same way that my mom was looking
out for my best interests when she said that I couldn’t go to the
“Violent Femmes” concert with Megan Cross. And yeah, I hated my mom for, like, a full year after that but she was right, that girl was trouble. So yes, you are allowed to hate IUPAC for
changing the name of ‘cinnamaldehyde’, the aldehyde that makes cinnamon smell like
cinnamon to ‘trans-3-phenylprop-2-enal’. Yes, they’re sucking all the fun out of the world, but trust me when I say, they’re doing it for your own good. [Theme Music] ‘Cinnamaldehyde’ is a lovely name for
a lovely aldehyde, just like ‘cadaverine’ is a creative name
for a rancid-smelling amine. ‘Citronellol’, ‘angelic acid’, ‘vanillin’,
they’re beautiful names, but there are tens of thousands of regularly
used organic compounds, and we can’t come up with pretty names for
all of them. More important than that, though, there’s
no way to translate those names into chemical formulas
and IUPAC hates that. They’re like, “Cinnamaldehyde, yeah, sure,
gives cinnamon its lovely smell, but we think ‘trans-3-phenylprop-2-enal’
is way better of a name!” And it actually is, because if I say ’trans-3-phenylprop-2-enal’
to somebody properly educated, they can just write down the chemical structure
without even thinking about it. And that’s, like, almost a kind of magic, where words go from relating to reality as we experience it (this chemical comes from cinnamon so name
it after cinnamon) to relating to the actual physical reality of the universe, and that’s a magic worth knowing. IUPAC created a language that could be understood
by everyone with minimal, even zero misinterpretation and that’s no
easy feat. That’s why, as we’ve discussed organic
chemistry, when we start counting carbons, we make sure
that the first thing named on the molecule is the lowest possible carbon, not because there’s something special about lower numbers but because they had to make a rule, otherwise one chemical could be named two
different ways. Let’s go through the process of naming a
chemical, it’s a multi-step process. Step 1: how long is the carbon chain? Five carbons will have ‘pent-’, seven
‘sept-’, three ‘prop-’, one ‘meth-’ Don’t feel weird if you have to look these
up. I, for example, had no idea that a twenty-three
carbon atom chain had the prefix ‘tricosa-’, until I looked it up just now. The only tricky thing here is to make sure
you find the longest carbon chain. Sometimes, a hexane can look like a pentane
with an ethane sticking off of it. So now that you know your longest carbon chain, you’ve got your prefix, time to figure out the suffix. If there’s nothing but hydrogen and carbon, your suffix just depends on whether you have
any double bonds or triple bonds; you know these, propane, propene, propyne. Then there are a ton of other groups that
can change the suffix. Like last week we talked about alcohols and
aldehydes and ketones, but we didn’t talk about how they affect
nomenclature and boy do they. When working with functional groups, each one adds a particular suffix to the end of the name of a compound. Usually, these are pretty easy to remember because they come straight off the name of the functional group. Alcohols add an ‘-ol’, so for methane
you drop the ‘e’ and you get methanol; ethane, you get ethanol. Aldehydes, instead of ‘-ol’ you add ‘-al’,
so from ethane you get ethanal. Ketones, add ‘-one’, so a ketone of a hexane
would be hexanone, and amines, you just add ‘-amine’ to the end
so methenamine or propanamine. Super easy. For caboxylic acids, you have to identify
it’s an acid, so you end in acid, that’s obvious, but instead of adding ‘-ylic’ you add ‘-oic’. As far as I can tell, this is just because ethanoic acid is much more fun to say than ethanylic acid. Going over that again: alcohol, on an ethane,
is ethanol, aldehydes add ‘-al’ giving ethanal, ketones add ‘-one’ pronounced ‘eth/an/own’,
amines, add amine for ethanamine, and carboxylic acid, on an ethane, is ethanoic
acid. And of course, there are literally dozens of other named functional groups that will change the suffix. It’s terrible, that’s why they’re on Wikipedia. So if you got a functional group, there’s
a name that defines the suffix of the molecule, and that is called the parent functional group. Now it is perfectly possible that a molecule
will have two different functional groups, so how do we know which one is the parent?
Which one defines the suffix? Well, different groups have different precedence. The most weighted of all is the carboxylic
acid functional group, so no matter what else is on a six carbon chain, if there’s a carboxylic acid, then it’s a hexanoic acid. That precedence is decided by a list that
was, you guessed it, created by IUPAC to minimize confusion by
creating yet more rules. The list is linked in the description. OK, so now we have created our base name for
the molecule; it’s time to start looking at what else is
attached to it. Any side chains or functional groups sticking
off the side have to be identified and named. If it’s a six carbon chain with a carboxylic acid on it and an alcohol on one of the other carbons, our name has to do two things: identify that the hydroxyl group is there and identify where it is. If it’s got a double bond, we likewise have to identify that it is there and where it is, all with our words. Use your words! Also, use your numbers. Counting out carbons can be a little confusing
but try it both ways, in both directions, to make sure that you get the lowest possible
number on the group that defines your suffix. All right, enough of the talking, let’s try
to do some actual examples here. I mentioned earlier angelic acid. It’s not actually particularly angelic, it was just named after a flower it was derived from. But it’s actually found all over the place
in nature, so ‘angelic acid’ tells us pretty much nothing
about the molecule, except that it’s acidic. So in order to figure out the structure I
have to look it up. That’s no fun, but here it is.
Let’s figure out a better name for it. Count up the carbons, 1-2-3 if you count that chain, 1-2-3-4 if you count this one so it’s a butane derivative. Actually it’s a butene derivative because
it has that double bond. It’s also got the carboxylic acid though,
so let’s go ahead and call it butenoic acid. Except that there’s a methyl group sticking
off so it’s methylbutanoic acid. Of course, with that name, without any numbers, no one has any idea where anything is on the chain, so, time to number. From this direction, 1-2-3-4
we get 3-methylbut-2-en-4-oic acid,
which is obviously wrong. Mostly because that parent functional group, the one that defines the suffix, is supposed to be as low as possible and in fact it is as high as possible, so
that can’t be right. Also it just sounds terrible. Numbering the other way, 1-2-3-4, the carboxylic
acid is on carbon 1 (that’s pretty ideal). The double bond and the methyl group are both
on the second carbon, much better, and thus it is named 2-methylbut-2-enoic acid. Two important notes here, if the parent functional group is on carbon one we just leave the number off. And two, there’s a double bond in this molecule and we haven’t identified whether it’s cis or trans, and we can’t do that so we haven’t actually
completely named it. We should stick a cis in front of the name because the carbon chain continues on the same side, not the opposite side as it would be in the
trans form. Of course, it works both ways, that’s the
beauty of it, you can build a name from a molecule and a
molecule from a name. Earlier, I told you that cinnamaldehyde was
trans 3-phenylprop-2-enal. Can you build it? Well, start with the three carbon chain, because of that prop, and count out the carbons, 1-2-3. We know, because it’s prop-2-enal with that
‘e’ there’s a double bond from the second to third
carbon and we know it’s trans. We also know that there’s a phenyl group on
the third carbon. The word ends in a non-numbered ‘-al’, so finally we can stick an aldehyde on carbon number one, and yeah, that was actually pretty easy! Now look, IUPAC does it’s best but past a
certain point of complexity, no one expects anyone to know all this by heart unless you work for them and name compounds for a living. That’s why we have the internet and textbooks
and libraries to go look things up. Because I’m a dork, I actually find these things to be kinda delightful puzzles and they’re often super fun for me, but taken one or two steps beyond the level
of complexity we have here, my personal viewpoint is that you’re better off looking it up than memorizing the novel-sized list of functional groups, names, precedence lists,
prefixes and suffixes. As long as you can recognize why trans 3-phenylprop-2-enal
is in fact a superior name to cinnamaldehyde, and no, I am not going to do the cinnamon
challenge. Ever. Thank you for watching this episode of Crash
Course Chemistry, if you were paying attention you learned that
IUPAC is the international organization responsible
for your unhappiness, and that their goal is to create a system
where every organic compound has one and only one
name to prevent confusion. They do this by giving prefixes to compounds
for the number of carbons in a chain, suffixes based on parent functional groups, and ranking functional groups somewhat arbitrarily for precedence. You also learned that you have to number your
carbon chains, so that your parent functional group has the
lowest possible number, and that double bonds require you to label
the molecule as either cis or trans, and hopefully, you learned that this is a
kind of word magic worth learning. This episode was written by me, Hank Green,
and edited by Blake de Pastino. Our Chemistry consultant is Dr Heiko Langner, it was filmed, edited, and directed by Nicholas Jenkins. Our script supervisor was Caitlin Hofmeister,
our sound designer is Michael Aranda, and our graphics team was Thought Cafe.