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Hi! I’m Deboki Chakravarti and welcome to Crash Course Organic Chemistry!
Even when we speak the same language, we have different ways to say the same thing.
Do you call a carbonated beverage a “pop,” “soda,” or just “Coke”?
Or maybe you’re looking for a “drinking fountain” and everyone keeps pointing you to a “bubbler.”
Other times, we’re not saying what we think we mean.
For example, an American asking for chips in the UK isn’t going to get a bag of crispy potato circles.
If they shout out for their friend “Ken” in Japan, people might wonder why they’re yelling about a sword.
Or if they’re trying to say they’re embarrassed in Peru with a fragmented “I’m embarazada” -- they’re actually saying they’re pregnant.
Language is complicated, and it can be especially confusing in the chemistry lab.
If we ask our lab partner to pass the dichloromethane and they hand us a bottle labeled methylene chloride… is that the same chemical or not?
To unite the scientific community, the International Union of Pure and Applied Chemistry—otherwise known as IUPAC—was established in 1919.
This group wanted to make sure that chemists around the world could communicate accurately.
A hundred years later, IUPAC is still the guiding organization for all things nomenclature.
Thanks to them, we have some standard rules for naming organic chemicals!
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By the time IUPAC came along with standard rules, organic chemistry as a field was already over 100 years old.
Many chemicals had common names given to them by the people who discovered them based on their source or smell or color.
For instance, vanillin is isolated from vanilla beans and cinnamaldehyde is isolated from cinnamon.
Alcohol, ether, acetone, and acetic acid are common names too.
While some common names might sound fun, you have to memorize them.
And they don’t tell you much besides, like, there’s a chemical responsible for the flavor in vanilla ice cream called vanillin.
Not super useful for learning chemical structures.
Sometimes chemists even disagreed about what each common name meant, sort of like regional differences about what to call a carbonated beverage.
Despite IUPAC’s best attempts to make organic chemistry nomenclature less confusing, many common names are still used today…
probably because “vanillin” is easier to say and put on a bottle than 4-hydroxy-3-methoxybenzaldehyde.
But that longer name is a systematic name that follows a set of rules, which means anyone who knows those rules can draw the structure.
There are three basic steps to IUPAC systematic naming.
We’ll add more as we discover more about organic molecules throughout this series.
But for now, these will get us started.
Number 1: Find the longest carbon chain and give it a root name.
Number 2: Identify the highest priority functional group and add its suffix to the root name.
And Number 3: Identify the types of substituents and their positions on the carbon chain, then add a numbered prefix to the root name.
So we start building any systematic name with the root name, which comes from the longest carbon chain in the molecule.
The word longest is key, because some molecules will have long chains sticking out that can throw you off!
So make sure to do a little extra counting to make sure you’ve actually found the longest one.
Carbon chains containing up to four atoms have arbitrary-sounding root names.
One carbon is meth-, two is eth-, three is prop-, and four but-.
This can be a little tricky to remember, but you can use this mnemonic to help: Monkeys Eat Purple Bananas.
For carbon chains with five to twelve atoms, the root names are similar to geometric shapes.
For example, a pentagon in geometry is a five-sided ring.
And if a carbon compound has the word pent- as its root, then it’s a five-carbon chain.
A six carbon chain is hex-, seven is hept-, eight is oct-, nine is non-, ten is dec-, eleven is undec-, and twelve is dodec-.
If you need a root name for a carbon chain with more than twelve atoms… I don’t have it memorized.
There are plenty of resources out there to look them up.
Chemists don’t usually deal with longer chains and there are plenty of other things to remember besides what a 56-carbon chain is called.
Back to our IUPAC rules.
Once we’ve counted our carbons and have a root name, we need to add a suffix to indicate what kind of organic molecule we’re dealing with.
The simplest organic molecules are hydrocarbons, which only have hydrogen and carbon atoms.
There are four kinds of hydrocarbons: alkanes, alkenes, alkynes, and aromatics.
But we’ll only talk about the first three for now, because smelly aromatic compounds are a little more complicated.
Alkanes only have single bonds between carbons.
For these compounds, we take the root name and tack on the suffix –ane.
For example, this skeletal structure is a six-carbon chain, so the root is hex-.
And because there are only single bonds, it’s an alkane, and we add the suffix -ane. It’s hexane!
Alkanes are kind of boring compared to their hydrocarbon cousins.
They’re low-energy couch potatoes and don’t interact with many other compounds.
On the other hand, alkenes and alkynes have functional groups.
Remember, last episode we said that functional groups are where all the cool chemistry happens!
In alkenes, the functional groups are double bonds between carbons, and in alkynes, the functional groups are triple bonds between carbons.
Alkenes and alkynes can do fun reactions to make things like light-emitting polymers,
which are big molecules with regions where electrons kind of move around on a kind of race track that absorbs energy and emits light…
But I’m getting ahead of myself!
To name an alkene, we find the longest chain of carbon atoms and it must include a double bond.
If the longest carbon chain doesn’t include a double bond, it’s not named as an alkene.
Sorry, I don’t make the rules, IUPAC does.
Since IUPAC rules are all about communication, we want to let people know where that double bond is on the chain.
So we’ll number the carbons, starting on the end closest to the double bond.
For the sake of consistency, we want to keep the numbers as low as possible.
Then, we’ll take the lowest number touching the double bond, combine it with the suffix -ene, and add that to the root name of our chain.
For example, the longest chain in this molecule is a five-carbon chain, so the root is pent-.
Because there’s a double bond, we know the suffix is -ene but we also have to number the carbons.
Starting from the end closest to the double bond, that’s 1-2-3-4-5.
And because the double bond is between carbons 1 and 2, this compound is pent-1-ene.
Notice the punctuation here: we stick dashes around the number.
To name alkynes, the hydrocarbons with a triple bond, we do the same thing.
First, we find the longest chain of carbon atoms and pick a root name.
Then, we number the chain from the end closest to the triple bond to let people know where it is, and tack on the suffix -yne.
For example, an eight-carbon chain with a triple bond in the center would be called oct-4-yne.
To finish the IUPAC rules, once we have a root name and suffix, we might be done if the molecule doesn’t have anything fancy going on!
But frequently, organic molecules have substituents, where plain old hydrogens on a carbon chain can be replaced by other carbon chains or halogen atoms like chlorine, bromine, and iodine, for example.
We communicate where substituents are by adding prefixes before the root name.
Plus, just like we had to count carbons for alkenes and alkynes, we need to communicate where every substituent has done its substituting.
If we’re dealing with an alkane, where the longest carbon chain only has single bonds, we’ll number the chain so that the carbon atoms with the substituents get the lowest numbers.
For example, let’s take our hexane from before and replace one hydrogen on a carbon with a -CH3 group.
So in this case, we have an alkane, and we want to number the longest 6-carbon chain so that the carbon with the substituent is 2 not 5.
Remember, we want to keep the numbers as low as possible!
The -CH3 group takes the same name as a 1-carbon-chain root, meth-.
And because this group is a substituent it gets a new ending spelled -y-l.
So the prefix we add is 2-methyl, to communicate where and what it is.
And this compound is 2-methylhexane.
If the molecule is flipped over, we could start from the left and it would still be 2-methylhexane!
They’re the same compound, no matter if the methyl is drawn on the right or the left.
If we have two or more of the same substituents, we add di-, tri-, tetra- and so on to the prefix.
So if we took 2-methylhexane and added another -CH3 group to a different carbon, we get 2,4-dimethylhexane.
Notice the punctuation here, too: the numbers are separated from each other by a comma, and are separated from the words by a dash.
But there are other substituents besides carbon chains.
Let’s say we take our 2,4-dimethylhexane and replace a hydrogen atom on carbon-3 with a halogen like a chlorine atom.
So the molecule name gets another prefix!
Halogens get the -ine in their name replaced with the letter -o.
So chlorine becomes chloro-, bromine becomes bromo-, and iodine becomes iodo-.
That means we need to tack on the prefix 3-chloro here.
Lastly, when we add substituent prefixes, IUPAC rules say we need to put them in alphabetical order.
And the multipliers di, tri, and tetra do not count when we’re alphabetizing.
So this molecule is named 3-chloro-2,4-dimethylhexane.
These rules about substituent prefixes are mostly the same for alkenes and alkynes.
Except, when it comes to numbering the chains, the double and triple bonds between carbon atoms have priority over substituents sticking out.
So we start numbering the chain closest to the bonds instead of the substituents.
If we take our alkene friend pent-1-ene and add a ethyl group (that’s two carbons and six hydrogens) at carbon 3, we’d name this 3-ethylpent-1-ene.
Or if we put two methyl groups on the 2nd and 7th carbons of the alkyne oct-4-yne, meet the spectacular 2,7-dimethyloct-4-yne.
Now that we have the basics, we can start naming any compound!
Even if they look kinda complicated with more than one functional group and substituent.
Like this molecule:
But we got this! Don’t worry!
We have our trusty steps.
Number 1: Find the longest carbon chain and give it a root name.
In this case, we see there’s a seven-carbon chain here, so the root is hept-.
Number 2: Identify the highest priority functional group and add its suffix to the root name.
Alkenes and alkynes have equal priority when it comes to numbering.
But when an alkene and alkyne tie for lowest number, the alkene wins.
We’ll count from both ends to see what’s up.
And look: because the alkene and alkyne tie for lowest number, the alkene wins.
So we’ll use the numbering where the double bond is lower.
Now we have to name our double- and triple-bond functional groups, and the alphabetical order rule still applies here.
So with the root and the suffix combined, we have hept-1-ene-6-yne.
Finally, number 3: Identify the types of substituents and add a numbered prefix to the root name.
In this case, there’s a bromine on carbon 4 and a methyl group on carbon 3.
Taking alphabetical order into account, we add 4-bromo-3-methyl before the root and suffix.
And we’ve conquered this puzzle!
It’s 4-bromo-3-methylhept-1-ene-6-yne.
It’s kind of a mouthful to say, but so much more useful to chemists than if I named it Debokiyne or something.
Now, remember that conundrum from the beginning, when we asked our lab partner for dichloromethane and got a bottle of methylene chloride?
Let’s build those molecules.
Looking at the name dichloromethane, we know from meth- that this is a single carbon and -ane that it’s an alkane with single bonds.
The dichloro- tells us that we have two chlorine atoms connected to the only carbon, no substituent number required.
That’s the skeletal structure.
And if we want to get all fancy with a Lewis structure, we know that carbon often makes four bonds, so we can add two hydrogen atoms to finish it off.
Methylene chloride is trickier because we can’t use our IUPAC rules.
We just need to know that methylene is a common name for a CH2- group.
Then, because chloride is in the name, we can get to the final structure by filling in the two open spots with chlorine.
The end structure is the same, so our lab partner did know what they were doing!
But calling this compound dichloromethane makes it easier for scientists to know they’re talking about the same thing.
In this episode, we learned that:
There are three steps to naming molecules, which include the root name, a suffix based on functional groups, and a prefix based on substituents.
And we number the longest carbon chain by giving the highest priority functional group the lowest possible number, or whatever gives the substituents the lowest possible numbers.
Next time, we’ll learn how to give molecules systematic names when they have functional groups with heteroatoms!
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