Conformations
Chair and Boat Shapes for Cyclohexane Chair and Boat Shapes for Cyclohexane
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- For all the cyclic molecules we've dealt with so far, we've
- just drawn them as rings.
- For example, for cyclohexane we've literally just drawn it
- as a hexagon.
- So we've drawn cyclohexane like that.
- Now, we know from the last several videos that all the
- bonds for carbon don't sit in the same plane.
- If we take the example of methane, that's
- the simplest example.
- You have your carbon sitting in the middle.
- You'll have kind of a hydrogen popping out like that, another
- hydrogen that's in the plane of the screen, another one
- that's behind the screen, and another one
- that is straight up.
- So you kind of have this tetrahedral structure, and in
- the case of methane you have that 109.5 degree bond angles.
- Carbon likes to form bonds of this shape.
- It won't always be 109.5 degrees.
- It'll be something close to it, depending on what the
- different atoms or molecules are that it is bonded to.
- So given that, what would a cyclohexane molecule actually
- look like if we try to visualize it in three
- dimensions?
- So to think about that, let's think about these two bonds
- first. I'll try my best to draw it in one of its
- three-dimensional shapes.
- So those bonds right there, I will draw like that.
- And then this down here, in orange, I will draw like this.
- And then this up here, in magenta, I
- will draw like that.
- And then, let me see, in, in purple, I'll do these two
- right over here, and I'll draw them like this.
- So you have that and like that.
- This hopefully makes clear that over there is that end
- over there, this end over here is this end over here, and
- this way that I've drawn the cyclohexane is called a chair
- configuration.
- Chair shape.
- And it might be obvious.
- It looks like a chair.
- That's the back of the chair, this is where you would sit
- down on the chair, and I guess the back of your calves would
- go against here.
- Your knees would sit on it someplace like that.
- That's called the chair configuration.
- Now another configuration that it could be in is called the
- boat configuration.
- And so if I were to put this exact one in the boat
- configuration, if I take it from a slightly different
- perspective, if I'm looking at it, kind of, head on, it would
- look something like this in the boat configuration.
- It would look like this.
- Now I want to use the purple.
- It would look like that.
- Now, the first thing you're probably saying is, Sal, you
- said that the reason why it looks like this is because
- carbon likes to form these kind of tetrahedral, or this
- tripod shaped bonds.
- I don't see the tripod shaped bonds either here or here.
- Let me draw that boat a little bit, at least this end of the
- boat a little bit better.
- There you go.
- And you say, well I don't see that tripod shape over there.
- And to see the tripod shape, you just
- have to draw the hydrogens.
- So let me draw some hydrogens here.
- So let me draw a hydrogen here that will go
- straight down like this.
- A hydrogen that goes straight down over here, a hydrogen
- that goes straight up over here, straight up over here,
- straight down over here, straight up over there.
- I've now drawn one hydrogen on every carbon.
- And now let me draw some hydrogens.
- Let me draw a hydrogen here that goes straight up, not up
- really-- to the side over here.
- So a hydrogen there, let me draw a hydrogen over here that
- does the same thing.
- So those guys have their hydrogens.
- A hydrogen right over here.
- And then, let's see, this guy needs his hydrogens still, so
- he'll have a hydrogen that goes down like that, and a
- hydrogen and it goes like that.
- And this guy will have a hydrogen that goes like that.
- And when you see it like this, if you look at any one carbon
- on this molecule, if you look at any one carbon, you can see
- that's forming the same tetrahedral shape that has a
- tripod at every one.
- Over here, you have that close to, roughly 109 degree, 110
- degree angle between each of the constituents that are
- bonding to the carbon.
- Now, I've drawn the different hydrogens that are coming off
- of these carbons in different colors, and I've
- done it for a purpose.
- The ones that are going straight up or straight down,
- we call those axial hydrogens.
- And the ones I drew in orange that are kind of going to the
- side in some level, we call these equatorial.
- These are equatorial hydrogens.
- And the reason why it's useful to know that name is when we
- talk about the different configurations, the different
- chair and boats, whether something is equatorial or
- axial can change if this were to flip up, or vice versa, and
- things like that.
- And we'll talk more about that in the next video.
- And the reason why they're called equatorial is if you
- think about it, and it's sometimes hard to visualize,
- this bond right here is parallel to this bond right
- over there.
- And this bond right over here is parallel to that.
- It's parallel, the equatorial bonds are parallel, to some
- part of the ring.
- So that one is parallel to that right over there.
- Actually I should even, I could even color-code that.
- This, well, I don't want to use that same color.
- This is parallel to this and this is parallel to that.
- And we could do it for all the equatorial bonds.
- So for example, I don't want to-- I'm running
- out of colors here.
- So this right here is parallel to this, and this, and that
- over there.
- So we could keep doing it for all of them.
- I could do it for the other set right here.
- This guy right here is parallel to
- that guy over there.
- I didn't quite draw it like that, but hopefully it makes
- the idea clear.
- And I'll do one more of these just to show
- what's parallel to what.
- This bond is parallel to that.
- So the ones that are parallel to some part of the ring we're
- calling equatorial.
- And the ones that kind of jump out of the ring, that aren't
- parallel to any other part of the ring, we're
- calling those axial.
- And the way I've drawn it here, the axials are the ones
- that point up and point straight up and
- point straight down.
- We can do the same thing on a boat configuration.
- Now, one question you might ask is, well, there's these
- two configurations.
- Both of these would result in tetrahedral type shapes at
- each of the carbons.
- In fact, let me draw it for you.
- So this axial hydrogen is pointing straight down, this
- one is pointing straight down.
- Here, this hydrogen is actually going to point
- straight down because we flipped it up.
- And then over here you would have a hydrogen point straight
- up, and then one that's kind of pointing down.
- This gives a tripod there.
- To have the tripod over here, you'll have to have a hydrogen
- that points a little bit like that, one that's pointing a
- little bit like that, along, well, you can kind of view it
- along the same plane as this guy would be parallel.
- It's hard to see it in this, but he would
- actually parallel to that.
- This guy would be out like this, and then this guy would
- have an axial hydrogen, and then he would have one
- equatorial one just like that.
- So you could draw the tripod shapes in either the chair or
- boat configuration.
- But one question is, well, what's more stable?
- That's actually one of the main points of being able to
- visually think about the three dimensional structure of any
- of these hydrocarbons, or in this case cyclohexane.
- So in this situation, we know from past videos, that all of
- these carbons with their hydrogens around them, these
- bonds, these have electron clouds around them.
- The electron clouds are negative, and so they want to
- get as far away from each other as possible.
- In this chair configuration, you have this carbon up here,
- the ch2 we could consider it, has two hydrogens and is
- connected to the rest of the ring.
- It's as far as possible from this ch2 as possible.
- So in that situation, we have a lower potential energy, or
- it is a more stable shape.
- Or more stable configuration.
- In the boat configuration, this ch2 up here is much
- closer to this ch2, I mean, that's really the main
- difference between the two.
- And they want to get away from each other.
- They want to repel ech other.
- So this one will have higher potential energy, or it will
- be less stable.
- So this is just a starting point of how to visualize
- cyclic hydrocarbons and we'll use this information in the
- next video to think a little bit more about, maybe, the
- different chair configurations that a molecule could have,
- and what could be more stable.
- In this situation, in the case of just cyclohexane, the two
- chair configurations are equally stable.
- And let me just touch on that a second.
- So you have, well, I don't have to--
- actually, let me see.
- I won't copy and paste.
- I'll just redraw the other chair
- configuration for this guy.
- Actually let me just do it separately over here because
- I've made the colors here so confusing.
- Let me draw two, the same cyclohexane, but in two
- different chair configurations that it could
- be equilibrium in.
- So you could have this one, you could have this one, so
- this could be one chair configuration, and I'll draw
- it like this.
- And then the same hydrocarbon could be in-- or the same
- cyclohexane could be in equilibrium with the this
- other chair configuration that looks like this.
- Let me have a little more space here.
- So it looks like this.
- Let me do the pink.
- It goes up like that, like that.
- Let me make sure I'm-- no, I want to do it actually.
- This pink guy goes like this.
- And then the blue guy is going to be just like this.
- So notice, in this situation this carbon appears kind of at
- the top of the chair, and this carbon is at the bottom, and
- then they've flipped.
- But these are equally stable configurations.
- But one way to think about is all of the axial guys on this
- carbon here turned into equatorial on this carbon and
- vice versa on the two.
- Let me show it to you.
- Let me just draw the hydrogens on this carbon.
- This carbon's hydrogens has an axial hydrogen, and has an
- equatorial hydrogen, whose bond would be parallel to that
- just like that.
- And this guy would have an equatorial hydrogen whose bond
- is parallel to actually both of these guys.
- And an axial hydrogen.
- But when it flips, and I'm just drawing those guys'
- hydrogens, but when this structure flips like that,
- what happens?
- Well, this hydrogen over here goes into this position, and
- this yellow hydrogen over here goes into this position.
- So over here, it was equatorial, and
- now it becomes axial.
- The same argument can be made over here.
- This equatorial hydrogen, when it flips-- when this whole
- blue part flips down-- now becomes axial.
- And this axial hydrogen, when you flip it down, becomes
- equatorial.
- And you can actually do that for all of the hydrogens.
- Over here you have an axial hydrogen.
- Once you flip it, you have an axial hydrogen, and then you
- have an equatorial hydrogen.
- When you flip it, these two equatorial
- hydrogens become axial.
- So they become axial and then both of these guys become
- equatorial.
- So let me do that in yellow.
- Both this guy and this guy become equatorial So this and
- that become equatorial.
- They become parallel to the other end.
- And you could do it for these two hydrogens, as well.
- So that's another interesting to think about.
- And this is really just practice on visualizing what's
- going on when we when we visualize--
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