Representing Structures of Organic Molecules Representing Structures of Organic Molecules
Representing Structures of Organic Molecules
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- 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.
- Let me just draw a one carbon chain,
- it is really kind of ridiculous to call it a chain.
- But if we have one carbon over here
- and it has 4 valence electrons.
- It wants to get to eight.
- That's just the magic number
- we'd 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.
- It 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 two.
- So, all of the hydrogens now feel like they have 2 electrons,
- the carbon feels like it has eight.
- 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.
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