Representing structures of organic molecules | Biology | Khan Academy


The one thing that probably
causes some of the most pain in chemistry, and in organic
chemistry, in particular, is just the notation and the
nomenclature or the naming that we use. And what I want to do here in
this video and really the next few videos is to just make sure
we have a firm grounding in the notation and in the
nomenclature or how we name things, and then everything
else will hopefully not be too difficult. So just to start off, and this
is really a little bit of review of regular chemistry,
if I just have a chain of carbons, and organic chemistry
is dealing with chains of carbons. Let me just draw a one-carbon
chain, so it’s really kind of ridiculous to call it a chain,
but if we have one carbon over here and it has four valence
electrons, it wants to get to eight. That’s the magic number
we learned in just regular chemistry. For all molecules, that’s the
stable valence structure, I guess you could say it. A good partner to bond
with is hydrogen. So it has four valence electrons
and then hydrogen has one valence electron, so
they can each share an electron with each other
and then they both look pretty happy. I said eight’s the magic number
for everybody except for hydrogen and helium. Both of them are happy because
they’re only trying to fill their 1s orbital, so the
magic number for those two guys is two. So all of the hydrogens
now feel like they have two electrons. The carbon feels like
it has eight. Now, there’s several
ways to write this. You could write it just like
this and you can see the electrons explicitly, or you
can draw little lines here. So I could also write this
exact molecule, which is methane, and we’ll talk a little
bit more about why it’s called methane later
in this video. I can write this exact structure
like this: a carbon bonded to four hydrogens. And the way that I’ve written
these bonds right here you could imagine that each of these
bonds consists of two electrons, one from the carbon
and one from the hydrogen. Now let’s explore slightly
larger chains. So let’s say I have a
two-carbon chain. Well, let me do a three-carbon
chain so it really looks like a chain. So if I were to draw everything
explicitly it might look like this. So I have a carbon. It has one, two, three,
four electrons. Maybe I have another carbon here
that has– let me do the carbons in slightly different
shades of yellow. I have another carbon
here that has one, two, three, four electrons. And then let me do the other
carbon in that first yellow. And then I have another carbon
so we’re going to have a three-carbon chain. It has one, two, three, four
valence electrons. Now, these other guys are
unpaired, and if you don’t specify it, it’s normally going
to be hydrogen, so let me draw some hydrogens
over here. So you’re going to have a
hydrogen there, a hydrogen over there, a hydrogen over
here, a hydrogen over here, a hydrogen over there, a hydrogen
over here, almost done, a hydrogen there, and
then a hydrogen there. Now notice, in this molecular
structure that I’ve drawn, I have three carbons. They were each able to
form four bonds. This guy has bonds with three
hydrogens and another carbon. This guy has a bond with two
hydrogens and two carbons. This guy has a bond with three
hydrogens and then this carbon right here. And so this is a completely
valid molecular structure, but it was kind of a pain
to draw all of these valence electrons here. So what we typically would want
to do is, at least in this structure, and we’re going
to see later in this video there’s even simpler ways
to write it, so if we want at least do it with
these lines, we can draw it like this. So you have a carbon, carbon,
carbon, and then they are bonded to the hydrogens. So you’ll almost never see it
written like this because this is just kind of crazy. Hyrdrogen, hydrogen– at
least crazy to write. It takes forever. And it might be messy, like it
might not be clear where these electrons belong. I didn’t write it as
clearly as I could. So they have two electrons
there. They share with these
two guys. Hopefully, that was
reasonably clear. But if we were to draw
it with the lines, it looks just like that. So it’s a little bit neater,
faster to draw, same exact idea here and here. And in general, and we’ll go
in more detail on it, this three-carbon chain, where
everything is a single bond, is propane. Let me write these words down
because it’s helpful to get. This is methane. And you’re going to see the
rhyme– you’re going to see the reason to this naming
soon enough. This is methane; this
is propane. And there’s an even simpler
way to write propane. You could write it like this. Instead of explicitly drawing
these bonds, you could say that this part right here, you
could write that that part right there, that is CH3, so you
have a CH3, connected to a– this is a CH2, that is CH2
which is then connected to another CH3. And the important thing is, no
matter what the notation, as long as you can figure out the
exact molecular structure, as long as you can– so there’s
this last CH3. Whether you have this, this,
or this, you know what the molecular structure is. You could draw any one of these
given any of the others. Now, there’s an even simpler
way to write this. You could write it
just like this. Let me do it in a
different color. You literally could write it
so we have three carbons. So one, two, three. Now, this seems ridiculously
simple and you’re like, how can this thing right here give
you the same information as all of these more complicated
ways to draw it? Well, in chemistry, and in
organic chemistry in particular, any of these– let
me call it a line diagram or a line angle diagram. It’s the simplest way and it’s
actually probably the most useful way to show chains
of carbons or to show organic molecules. Once they start to get really,
really complicated, because then it’s a pain to draw all of
the H’s, but when you see something like this, you assume
that the end points of any lines have a carbon on it. So if you see something like
that, you assume that there’s a carbon at that end point, a
carbon at that end point, and a carbon at that end point. And then you know that carbon
makes four bonds. There are no charges here. All the carbons are going to
make four bonds, and each of the carbons here, this carbon
has two bonds, so the other two bonds are implicitly going
to be with hydrogens. If they don’t draw them, you
assume that they’re going to be with hydrogens. This guy has one bond,
so the other three must be with hydrogen. This guy has one bond, so the
other three must be hydrogens. So just drawing that little line
angle thing right there, I actually did convey the exact
same information as this depiction, this depiction,
or this depiction. So you’re going to see
a lot of this. This really simplifies things. And sometimes you see things
that are in between. You might see someone draw it
like this, where they’ll write CH3, and then they’ll
draw it like that. So that’s kind of combining
this way of writing the molecule where you write the
CH3’s for the end points, but then you implicitly have
the CH2 on the inside. You assume that this end point
right here is a C and it’s bonded to two hydrogens. So these are all completely
valid ways of drawing the molecular structures of these
carbon chains or of these organic compounds.