Hydrocarbon Power!: Crash Course Chemistry #40


In which Hank introduces us to the world of Organic Chemistry and, more specifically, the power of hydrocarbon. He talks about the classifications of organic compounds, the structures & properties of alkanes, isomers, and naming an alkane all by observing its structure.

Transcript Provided by YouTube:

00:00
You’ve heard this before, but it bears repeating. Carbon is the element of life.
00:03
So much so that when we explore other planets the first thing we look for is compounds that contain carbon.
00:08
In fact, there was a time when we thought carbon compounds could only be produced by living things.
00:13
So early chemists called them, as we still do today organic compounds.
00:18
Scientists back then considered biological molecules to be almost mystical in origin.
00:22
Until, 1828 when German chemist Friedrich Wöhler discovered that urea, a component of urine,
00:28
could be synthesized simply by heating ammonium cyanate, an inorganic compound.
00:33
That proved biological molecules were just chemicals that could be created and manipulated in the lab.
00:38
Suddenly a new branch of chemistry was born, organic chemistry. It’s like my favorite chemistry.
00:44
So what is it about carbon though, that makes it so special? Well, a lot of things.
00:49
Like silicon, which we talked about a few weeks ago, carbon is in group 14 on the periodic table,
00:54
and like all of the elements in that group, it has 4 valence electrons.
00:59
In carbon those 4 electrons can bond to other atoms in a really promiscuous number of configurations
01:04
to form all kinds of structures.
01:07
Which is why carbon is to biology, which silicon is to geology.
01:10
Just as silicon forms the basis, not only for sand,
01:13
but also most of the rocks on earth, carbon is the foundation of most biological molecules.
01:19
Really all biological molecules…right? Yup.
01:23
The simplest organic molecules are pure hydrocarbons containing only carbon and hydrogen. Hydro-carbon.
01:29
They are where we’re going to start our six week exploration of organic chemistry.
01:33
And they’re a good place to start, partly because they play by the most straight forward rules.
01:38
When all carbons in a pure hydrocarbon are bound to the maximum number of atoms, 4 atoms each,
01:42
so that there are no double or triple bonds anywhere; these compounds are considered to be full or saturated.
01:48
That means that all the carbons have 4 bonds, either with other carbon atoms or with hydrogen atoms,
01:52
in which case the hydrogens are bound to one carbon.
01:55
No questions, no exceptions.
01:57
These are the simple rules that govern some of the world’s most useful, or at least, used compounds.
02:01
The hydrocarbons that we use as diesel fuel, gasoline, methane, propane.
02:05
You’re gonna learn what these and other compounds look like, what they’re names mean,
02:09
and how they take part in the reactions that fuel our lives.
02:12
Welcome to organic chemistry!
02:14
[Theme Music]
02:24
The fully saturated hydrocarbons I just described are usually called by the much simpler name, alkanes.
02:29
The simplest of the alkanes is one you’ve heard of before, methane, or CH4, the main compound in natural gas.
02:35
The next simplest alkane contains 2 carbons side by side, each one of them in bonded to 3 hydrogen atoms.
02:41
This is ethane, C2H6. Another gas, and it’s mostly used in the production of plastics.
02:47
If we add another carbon and enough hydrogens to fill all those spaces we get our next alkane:
02:52
propane, C3H8.
02:54
Also a gas at room temperature and normal atmospheric pressure,
02:57
propane is a common fuel for cooking, heating, and vehicles,
03:01
as well as a propellant for everything from aerosol cans to paintball guns.
03:05
And we could do this all day, adding carbons to the chain and giving each compound a name,
03:10
but that would be pretty boring.
03:12
Things get more interesting, though, with the next alkane, butane, C4H10,
03:16
because there are two different forms of it.
03:19
The first is what you’d expect:
03:20
just a chain of carbons with hydrogens stuck wherever they’re needed to make each carbon have 4 bonds.
03:26
This is called normal butane or n-butane.
03:29
But you can also arrange the 4 carbons differently by making a chain of 3
03:33
and then branching the fourth one off the center of the chain.
03:36
This is called isobutane or i-butane.
03:38
And even though it has the same chemical formula as n-butane, its structure gives it different properties.
03:43
For example, n-butane boils at -0.5 degrees Celsius while isobutane boils at -11.7 degrees Celsius.
03:49
These different structures for compounds that have the same molecular formula are called isomers.
03:53
As you add more and more carbon atoms to the molecule, there are more and more ways that you can arrange them.
03:58
So the number of atoms is butane only allows for 2 isomers, n-butane and isobutane.
04:03
But pentane, C5H12, has 3 possible isomers and C6H14, known as hexane, has 5.
04:09
Again, I could do this all day.
04:11
But looking at this table of the number of possible isomers you could see that that escalated quickly.
04:16
The take away here is that molecules that have the same mass and number of atoms can form different structures.
04:22
And as their structure changes, their properties also change.
04:25
As a general rule, the larger and more complex alkanes are, the more densely their molecules can pack together,
04:30
which means that they tend to be liquid or solid instead of gaseous at room temperature.
04:34
So alkanes with 5 to 18 chains of carbon atoms like octane and gasoline are liquids at room temperature
04:40
and those with more than 18 carbon atoms like paraffin or other waxes are solids.
04:44
Now you’re probably picking up on a lot of words that you’ve heard before, even outside of chemistry class:
04:49
octane, propane, methane, paraffin, and so on.
04:52
You can chalk that up to the enormous popularity of these compounds in our daily lives.
04:56
Like I said, hydrocarbons are super useful because of the types of reactions they can take part in,
05:01
which I will explain more in a bit.
05:03
But first, I think it’s high time you know what these names actually mean.
05:06
Much like the general language of chemistry that we talked about months ago,
05:10
organic nomenclature has its own system of prefixes, suffixes, and numbers
05:14
that tell you what’s in the compound being named.
05:17
Now you gotta know the prefixes because they indicate how many carbon atoms are present.
05:21
Here’s one that I know you’ve heard before: meth.
05:23
Meth- in a name always indicates a molecule or branch containing one carbon atom.
05:28
So the difference between amphetamines doctors prescribe
05:31
and methamphetamines that are sold on the streets is that
05:34
methamphetamine has a methyl group, CH3 with one carbon,
05:37
where amphetamine just has a single hydrogen atom.
05:40
Hopefully, that’s helpful to you. Don’t do drugs.
05:42
Eth- in a name means 2carbon atoms.
05:45
Prop- means 3. But- means 4.
05:48
From there, most of the prefixes will be familiar from geometry class
05:52
and you can review them in tables and learn them.
05:55
I’m not gonna go through them all. There are a few naming rules that are specific to alkanes.
05:58
First, alkanes are always named based on the longest possible continuous chain in their structure.
06:03
For example, even though this looks like a 5 carbon chain intersecting with a 6 carbon chain,
06:08
it actually contains an 8 carbon chain if you look at it close enough.
06:12
So this is considered an octane with two carbon chain attached to one of its carbon atoms.
06:16
When shorter carbon chains are attached to longer ones like this, they’re still named using the same prefixes,
06:21
but we stuff a little -yl onto the end to show that they’re just attachments.
06:25
Since this attachment has two carbons, we call it an ethyl group.
06:29
And the attachment with just one carbon that turns amphetamine into methamphetamine, that’s the methyl group.
06:33
Attachments are also given a number to show you where along the chain they’re attached.
06:38
The long chain is always numbered carbon by carbon
06:41
in the direction that gives the attachments the lowest numbers possible.
06:44
So, if we number the chain the right way, the ethyl group will end up at position 4.
06:48
But if you do it the wrong way, it’s in position 5.
06:51
Low numbers win so it’s numbered from left to right in this case.
06:55
So when we put it all together, this compound is called 4 ethyl octane.
06:59
Congratulations! You just named an organic compound.
07:01
Now particularly astute and studious students would have noticed something here.
07:05
Earlier, I introduced you to isobutane, a compound with four carbons that are not all in a chain.
07:11
They call that isobutane and it is an isomer of butane,
07:14
but according to these all important rules of nomenclature, it’s not actually any sort of butane at all.
07:19
The longest carbon chain is just 3 carbons long, so it’s propane with one methyl group sticking off of it.
07:24
If we wanted to give a technical name for it, isobutane would be 2-methylpropane.
07:28
Though, since the second carbon is the only place where the methyl group can go without the molecule,
07:33
once again becoming butane, properly proper chemists just drop the two and call it methylpropane.
07:39
Now suppose you have more than one of the same size group attached to the same chain,
07:43
like two methyl groups on the same alkane.
07:46
In this case, you put a number for both of them
07:47
and then prefixes like di- and tri- are used to indicate multiple attachments.
07:52
So for instance, if an octane chain has methyl groups attached with second and fifth carbons,
07:57
it’s called 2,5-dimethyloctane.
07:59
On the other hand, if you have attachments of different lengths,
08:02
you just name and number each one separately, being sure to list them in alphabetical order.
08:06
The structure we just used had a methyl group on it and an ethyl group on its fifth,
08:11
it would be 5-ethyl-2-methyloctane.
08:13
This is super useful for several reasons.
08:16
One, because there are trillions of ways that organic compounds can come together.
08:20
But also because you can work backwards from a name and build a structural formula from it.
08:25
Let’s try that out. We’re gonna build 2-ethyl-3,5-dimethylnonane.
08:30
Start with the main chain, nonane. The prefix non- indicates 9 carbons.
08:35
Then, add an ethyl group, a 2 carbon chain on number 4 and then methyl groups, just 1 carbon on carbons 3 & 5.
08:43
Our final step is to add enough hydrogen atoms to give every carbon atom 4 bonds.
08:48
And now, the molecule is complete. It’s like a puzzle that we got to make.
08:52
Of course these compounds don’t exist in isolation.
08:55
Like any other compound, they can undergo a whole variety of reactions.
09:00
But there are 3 types of alkane reactions that are important enough for us to cover right now right here.
09:04
The first is the kind that made alkane the most common fuel for combustion or burning.
09:09
You’ll note here that I’m saying burning, a common misperception, even among chemistry students,
09:13
is that combustion somehow equals explosion.
09:16
While that would definitely make things more interesting, also more dangerous,
09:20
those two things are not synonymous.
09:22
Combustion is the type of reaction that powers your car and your propane grill,
09:27
even candles among many other alkane fuels.
09:29
The general reaction for combustion requires a hydrocarbon, oxygen, and a source of heat energy.
09:35
In this example, we’re using methane, but it works the same for any pure hydrocarbon.
09:39
The only thing that changes is the coefficients.
09:41
The products of a complete combustion of a pure hydrocarbon are always
09:45
carbon dioxide and water vapor, just those two things.
09:48
The next major reaction that alkanes experience is halogenation,
09:52
when halogen atoms like fluorine or chlorine are substituted for one or more hydrogen atoms in the alkane.
09:57
For example, the rather well-known compound chloroform is more correctly called trichloromethane.
10:03
It’s a molecule of methane that is reacted with a chlorine gas,
10:06
resulting in three of the hydrogen atoms being replaced with chlorine atoms.
10:10
The final reaction type is dehydrogenation, and it, somewhat obviously,
10:14
is the removal of hydrogen atoms from alkanes.
10:17
For example, ethane can be dehydrogenated by this reaction,
10:21
and as you can see, the result is that the carbon atoms are no longer saturated with hydrogen, thus,
10:26
requiring the formation of double or triple bonds to give the atoms the 4 bonds that they need.
10:31
Hydrocarbons that contain double or triple bonds have
10:33
their own specific groups with different rules, reactions, and properties than alkanes.
10:38
And those are the topic of next week’s episode.
10:41
For now though, thank you for watching this episode of Crash Course Chemistry.
10:44
If you were listening, you learned about some of the different classifications of organic compounds,
10:48
the structure and properties of the simplest alkanes.
10:51
You also learned about isomers and why they’re important, how to name an alkane based on its structure,
10:56
and how to build an alkane structure from its name.
10:59
And finally, you learned a few important types of chemical reactions that alkanes experience:
11:03
combustion, halogenation, and dehyrdogenation.
11:06
This episode was written by Edi Gonzalez, it was edited by Blake de Pastino,
11:09
and our chemistry consultant is Dr. Heiko Langner.
11:12
It was filmed, edited, and directed by Nicholas Jenkins. The script supervisor was Caitlin Hofmeister.
11:17
And Michael Aranda is our sound designer. Our graphics team, as always, is Thought Cafe.


This post was previously published on YouTube.

Photo credit: Screenshot from video