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Hydrocarbon Power!: Crash Course Chemistry #40

Hydrocarbon Power!: Crash Course Chemistry #40


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

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