Leah here from leah4sci.com and in this video we're going to learn how to draw chair conformations
and ring flips.
If you thought organic chemistry was only about reactions, when you get to this chapter,
you're going to realize it's also a drawing class.
Chair conformations, boats, ring flips, fissure projections.
Now here's the thing, even though you're not graded on your arts skills meaning your drawings
can be very messy, it's still has to be very very clear and obvious, what do I mean by
this?
It's gotta be very obvious where your chair is, where your substituents lie and what direction
they're facing.
So let's take a look at how to draw these molecules quickly and easily even if you have
zero art skills to make sure you don't lose any points on exams.
We'll start with the cyclohexane hexagon, the hexagons can be very tricky and it's okay
if your molecule does not look perfect like this but you don't wanna be drawing something
that looks like that because in this molecule it's very difficult to figure out where your
carbons are, where the bonds are and you're gonna lose points.
Step one, you're gonna draw two parallel lines like this, they don't have to be perfect somewhat
next to each other.
Step two, place the dot between the parallel lines, one towards the upper opening and the
second towards the lower opening and third, connect the dots.
Now look at that, it's not a pretty hexagon but it's very clear and very obvious, same
thing works if smaller so let me show you how to do this quickly.
Two parallel lines, a dot above and below, connect the dots, and it's a little bit messy,
I kinda missed at the end but you know what, it's good to go.
It's a beautiful hexagon for organic chemistry.
Next we'll look at drawing chair conformations.
Your books are using computer generated graphics so they're perfect but when you try to imitate
it you wind up getting something that looks like this where it looks a little bit like
a duck and not very much like a chair, instead let's go with the very simple method that
will give you a full proof chair everytime.
Draw two parallel lines that are slightly offset from each other, in this case I have
the left over the right or you could have done the same thing right over left then,
we're going to place two dots towards the opening but like before, the dot over the
upper opening is going to go slightly above, the dot below the lower opening will be slightly
below, and then once again we connect the dots.
As you can see.
it looks nothing like the bowtie structure and it doesn't look very much like the chairs
in your book but it's very obvious looking at the structure.
What is going on, I see which is the carbon facing up which is the carbon facing down
and I can tell exactly where the other carbons are located but it doesn't stop there, once
you have cyclohexane you need to add in your substituents.
I like to start with a substituents at the tips where the tip gives us the direction.
The up tip as I call this carbon will have an axial substituent straight up, the down
tip will have and acyl substituent straight down.
You can start with either the up tip or the down tip.
Only one of them is needed because then you're going to follow the pattern of up, down, up,
down for the axial substituents.
Now if you're uncomfortable with the concept of chair conformations axial and equatorial,
pause this video and go watch my chair conformations tutorial link below or visit my website leah4sci.com/chairs
to learn about what goes into the chair conformation.
If you're already familiar with that, let’s continue.
We'll start with the up tip and then alternate the pattern.
Up, down, up, already have the down, up and down.
These are my axial substituents and identifying the tip of the chair made everything else
fall into place regardless of drawing skills.
Next we'll add our equatorial substituents, remembering that each carbon has one axial
and one equatorial and that they're facing in opposite directions.
So if the axial substituent is facing up, the equatorial substituent has to be facing
down and this allows me to start the same pattern of up, down, up, down, starting with
any one carbon where we identify the direction of the axial substituents, put the equatorial
substituent in the opposite direction and continue the pattern.
Before we do so, let me point something out, I've seen a lot of students lose points by
doing this.
It appears to be up and down, the problem is if you draw a parallel line, I can't tell
if that pink equatorial substituent is facing up or down and if your professor can't tell,
you're likely to lose points.
So make sure that it's very obvious for each equatorial position whether it's facing up
or down and the way you can verify is to draw a parallel line and make sure your substituent
is well above or well below.
We determine that this is a down equatorial, making this up equatorial, down equatorial,
up equatorial, down equatorial, up equatorial, those are all my substituents.
One more thing to point out is the two middle carbons tend to get confusing.
If we think about what we're looking at here, this is a cyclohexane.
This is a ring that we're looking at straight on.
That means this substituent here isn't really facing to the right or left, it's coming directly
at you.
But if we show it just as a dot, implying this substituent coming out, it's hard to
tell what's going on.
So instead, we offset it slightly to the right or slightly to the left, it doesn't matter
so long as we kind of see what's going on and recognize that's an equatorial substituent
facing down below the plain of the cyclohexane.
Now that you have the basic chair conformation, we want to add substituents and we want to
make sure they show up in the right place.
Anytime you're asked to turn a cyclohexane into a chair conformation don't think about
the substituents right away because trying to take this molecule and force it into a
chair conformation is time consuming, confusing and often gets messed up.
Instead here's what I want you to do, I want you to start your drawing by completely ignoring
the molecule that you're drawing, in this case, we're not told which substituent should
be up or down so we're gonna draw just one version of the chair.
To do this, we'll start by drawing one random chair so we have the parallel lines, we have
the dot, connect the dots, once you get comfortable you can just imagine that dot and connect
to an imaginary dot, we have the skeleton, next we'll add the substituents, I'll start
with the up tip, this is something that I call the turning your brain off part, you're
going to spend a lot of time in orgo thinking but certain aspects are much easier if you
just turn off the brain, go with the flow and prepare for the next step.
So find your up tip and just turn off the brain.
Go up, down, up, down.
up, down, don't really think about what else is going on here and then do the same thing
for the equatorial substituent.
We identify that this is axial up so lets start with equatorial down, up, down, up,
down, and up.
We have a skeleton, now we're ready to turn it into a chair.
In order to add the substituents we want to make sure that we know which substituents
go where by relating the two molecules and to do this we'll number the carbon chain.
It doesn't matter how you number it, this would be incorrect if i'm naming it to make
the methyl number one but i'm not naming it, I just need a reference, something that I
can follow to convert one to the other.
I decided to number this counter clockwise randomly just because, so we'll have to do
the same thing and number this one counter clockwise.
It doesn't matter where I start because it's a ring so I will randomly put the number one
here and continue counter clockwise two, three, four, five, and six.
Now that I know which carbon on the chair corresponds to the carbon on the flat drawing,
all I have to do is identify substituent, ask whether it's up or down and place it on
the same carbon.
Carbon number one has a methyl group on a wedge which is up, and so carbon number one
gets a methyl group up.
Up happens to be equatorial but I didn't know that before drawing the chair and it didn't
matter, all I had to recognize was carbon one, has a methyl on a wedge, the wedge is
up, find carbon number one, find the position that is up and add the methyl.
The position that is up happened to be equatorial and so I know the methyl group is equatorial.
The numbers tell us what's gonna happen.
Next we have an OH group up on a wedge on carbon number three, so we find carbon number
three, Identify the up position and hey, that happens to be equatorial, great.
Put an OH group in the up position and lastly on carbon number five we have a bromine on
dashes meaning it's going down into the page so we identify carbon number five, put the
bromine on the down position in this case, down happens to be axial and so we have our
final substituent.
If this is your quiz or exam and the lines are all present, do make sure to add in the
hydrogens otherwise you may lose points because that line without a hydrogen actually means
a methyl group, so we'll throw those in there.
And that's it, there you have your chair conformation.
What happens if you're asked to draw the ring flip for the chair conformation?
Once again, we don't really want to look at the chair that we have, instead we want to
use a simple approach to flip that chair and then do the same thing that we did here by
adding the substituents back.
The first step in drawing a ring flip is identifying the pattern, remember to draw a chair, we
had two parallel lines slightly offset from each other and on this chair the left is above
the right to do the ring flip, we're going to need the opposite chair so we're going
to draw the two parallel lines where the right is above the left and we're going to add a
dot or have an imaginary dot that we can connect the lines, same thing here.
Notice it's not very neat, might not pass an art class but it's very obvious for organic
chemistry.
Next we're gonna do the same thing with the substituents, I like to start with the up
tip so we have up, down, up, down, up, down, those are axial, next we have equatorial down,
up, down, up, down, up, then we want to number and this is where you want to be careful.
You should definitely practice this with a model kit just to make sure you understand
how the ring flip occurs.
I'll show you how to do this with the model kit in the video link below, we can easily
do the ring flip on paper by identifying the up tip or down tip carbon and flipping it
to the opposite direction.
If we grab the up tip carbon number two and drag it down, that gives us carbon number
two down on the bottom right, we could've done the same thing to carbon number five,
the down tip and drag it up which gives us number five here.
Now the most important part is your direction.
If my starting molecule has the numbers counter clockwise, make sure your ring flip is also
counter clockwise otherwise you potentially create the enantiomer which is not what we're
looking for.
To number this counter clockwise we have two so let’s continue, three, four, five, six,
and one, and here's the best part, we're not figuring out where the substituents are, we're
simply letting the numbers direct us.
On carbon number one, I have a methyl group in the up position, I don't care where or
what is going on with that up position, I simply care about putting a CH3 up position
on carbon number one, same thing for carbon number three OH in the up position, Identify
carbon three, add an OH in the up position and carbon five has Br in the down position
so we identify carbon five.
Find the down position and add a bromine.
Make sure to add in the hydrogens and that is it, the key thing that I want you to take
away from here is that we're not looking at this molecule and trying to recreate it, instead
we're looking at the pattern within the molecule, identifying where the numbers are, the direction
and using the numbers to determine where I add the substituent rather than trying to
visually rotate something in your head that is honestly just gonna be confusing.
How do you feel about drawing chair conformations?
Are you ready to do some practice problems?
First make sure to give this video a thumbs up, then click the subscribe button and bell
icon so that you don't miss any of the new videos, then go try the chair conformation
practice quiz which you can find on my website along with the entire chairs tutorial series
leah4sci.com/chairs.