Representing Structures of Organic Molecules

Representing Structures of Organic Molecules

The one thing that probably causes the most pain in chemistry –

and organic chemistry in particular –

is just the notation and the nomenclature,

and the naming that we use.

What I want to do here, in this video –

and really the next few videos –

is to just make sure that 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 just a review of regular chemistry.

If I just have a chain of carbons –

(And organic chemistry is dealing with chains of carbons.)

So let me just draw a one carbon chain.

(So it's really kind of ridiculous to call it a chain.)

But say we have one carbon over here,

and it has 4 valence electrons.

It wants to get to eight.

That's just the magic number,

as we learned in regular chemistry,

for all molecules, that's the stable valence structure,

I guess you could say.

So a good partner to bond with is hydrogen,

because It (carbon) has four valence electrons,

and then hydrogen has one valence electron.

So then they can each share an electron with each other,

and then they both look pretty happy.

I said eight is the magic number

for everybody except for hydrogen and helium,

both of them are happy

because they are only trying to fill their 1s orbital.

So, the magic number for those two guys is 2.

So, all of the hydrogens now feel like they have 2 electrons,

and the carbon feels like it has 8.

Now, there are several ways to write this,

you could write it just like this,

where 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 about why it is 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 can imagine

that each of these bonds consists of 2 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 -

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 1, 2, 3, 4 electrons

and let me do the other carbon in the first yellow.

So now we have a 3-carbon chain

that has one, two, three, four valence electrons.

Now these other guys are unpaired and if we 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,

hydrogen there and then a hydrogen there.

Now notice, in this molecular structure that I've drawn

I have 3 carbons they were each able to form 4 bonds.

This guy has bonds with 3 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.

So what we're typically going to do is - at least this structure

we're going to see later in this video,

there's a simpler ways to write it.

So if we want to 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 almost never see it written like this

because this is kind of crazy.

Hydrogen, hydrogen, hydrogen..

At least crazy to write, it takes forever.

It might be messy,

it might not be clear where these electrons belong.

I didn't write it clearly as I could.

So they have 2 electrons there, they share with these 2 guys.

Hopefully that was reasonably clear.

But if we were to draw with the lines, it looks just like that.

So it is 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 3 carbon chain, where everything is a single bond,

is propane.

Let me write these words down.

This is methane.

And you've going to see the rhyme...

the reason to this naming soon enough.

This is methane, this is propane.

And there is 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 this is CH3: a CH3 connected to a CH2.

Which is then connected to another CH3.

And the important thing is, no matter the notation,

as long as you can figure out the exact molecular structure.

(this last CH3)

Whether you have this, this or this.

You know what the molecular structure is.

You can draw any one of these given any of the others.

Now there is 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 3 carbons, so 1, 2, 3.

Now this seems ridiculously simple.

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,

this is called a line diagram or a line-angle diagram.

This is the simplest way, and 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 is a pain to draw all of the H's.

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 is a carbon at that end point

and a carbon at that end point.

And then you know that carbon makes 4 bonds.

There are no, kind of, charges here.

All the carbons are going to make 4 bonds.

And each of the carbons here - this carbon has 2 bonds

so the other 2 bonds are implicitly going to be with hydrogens.

If they don't draw them, you assume

that they are going to be with hydrogens.

This guy has 1 bond, so the other 3 must be with hydrogen.

This guy has 1 bond, so the other 3 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 are 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 is 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 in the inside.

You assume that this end point right here is a C

and it is bonded to 2 hydrogens.

So these are all completely valid ways of... of...

drawing the molecular structures

of these carbon chains or of these organic compounds.