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Organic chemistry
Course: Organic chemistry > Unit 3
Lesson 4: Conformations of cycloalkanesDrawing chair conformations
How to draw both chair conformations of cyclohexane.
Want to join the conversation?
- how can i flip the chair conformations(9 votes)
- In an example of drawing chair conformation with constituents besides all hydrogens, like say an ethyl group and an isopropyl group, how do you determine the most stable confirmation? ie. for cis-4-ethyl-1-isopropylcyclohexane?(4 votes)
- When thinking about these as 3D molecules, where do the axial and equatorial lines go? For example do the axial lines represent substituents that point out of the page on a wedge or behind the page on a dashed wedge?(3 votes)
- another way i can think of when drawing the hydrogens in chair conformations is that if the axial hydrogen on a carbon is going up, then the equatorial hydrogen would go down and vice versa, am i right or is it different in some cases?(3 votes)
- How do you know which carbon gets an up axial hydrogen and which one gets down? I know they alternate but how do you know where to start? Thanks(2 votes)
- You have to pick one randomly to start, it doesn't matter which(2 votes)
- How do you know what carbon to name as C1?(1 vote)
- It's arbitrary, you could choose any carbon to label as your first. It's just so you (and anyone involved) can reference which carbon you're talking about.(2 votes)
- Is it correct to conclude that chair structures can only be drawn with cyclohexanes because of the 6-carbon skeleton?(1 vote)
- How would you draw a chair conformation for a molecule that has two hexane rings connected to each other?(1 vote)
Video transcript
- [Voiceover] Let's get some practice drawing chair conformations. One way to do it is to start
by drawing two parallel lines that are offset from each other, so let me go ahead and
show you what I mean, so here's one line and
then here is another line. They're parallel to each
other but they're offset a little bit. Next, we're gonna draw two
horizontal, dotted lines, so the top horizontal dotted
line is going to be on level with that top point there, so, I'm just gonna draw a dotted line here. I'm gonna draw another
dotted line down here, which is gonna be level
with this bottom point, so that's what we have so far. Next, we're gonna draw in
another set of parallel lines. We're gonna start at
this point right here, where this intersects
with the dotted line, and we're gonna go from
the top dotted line down to the bottom dotted line like that, and then we're gonna try to
think about drawing a line parallel to that over here, so we're starting at this
dotted line and we're going up to this point. And then finally, we
need to connect the dots, so we're gonna go from this point to here and then from this point to here, and that's our carbon skeleton, so if you draw this properly, you should have three sets
of parallel lines for your carbon skeleton. Let's go back up to this drawing here, and let me show you those parallel lines. So we started with this set, so this bond, and this bond, that's
what we started with. Next, we drew in, let me
change colors so you can see, we drew in this set of parallel lines, and then, finally, we
drew in the last set, so let me highlight those
in blue, so this line, and this line are parallel, so that gives you your
carbon skeleton, and next, you need to think about
putting in your groups. In this case, we're talking
about Cyclohexane, so we're only dealing with hydrogens, so first, we're gonna put in the
hydrogens that we call axial or axial groups here, and axial
groups, you can think about the earth, so here is the
earth, you can think about this as being a globe and this would
be the axis going straight up and down, so hydrogens
that go straight up and down are called axial hydrogens, and we're gonna start at
this carbon right here, which I'll call carbon one,
so if this is carbon one, we're gonna draw in an axial hydrogen, so this is going straight up. Next, we're gonna go to carbon
two, so this one right here, and straight down would
be an axial hydrogen. We're gonna go to carbon
three and it's straight up for an axial hydrogen. Carbon four is straight
down for an axial hydrogen. Carbon five back here would be up, and then, carbon six would be,
would be down axial hydrogen. So you can see that we alternate. If we start at carbon
one, we start up axial, so let's go ahead and put
in the axial hydrogens on our Cyclohexane, on
our chair conformation. So here is carbon one. We're gonna draw a line straight
up and put in a hydrogen. We go to carbon two. We draw a line straight
down and put in a hydrogen. Carbon three, we draw a line straight up and put in a hydrogen. Carbon four, we draw a line straight down and put in a hydrogen. Carbon five would be up,
so we put in that hydrogen, and finally, carbon six, would
be down for our hydrogen. All right, next, we need to think about the equatorial hydrogens,
so that would be, starting at carbon one, that would be this hydrogen right here. Carbon two, it would be this hydrogen. And they're called equatorial
because you could think about them as being along the
equator of the ring, so if this is the earth, we
know that this is the equator. So these are kind of out to the side, and they alternate too. At carbon one, this
equatorial hydrogen is down relative to the plane of the ring. At carbon two, this
equatorial hydrogen is up relative to the plane of the ring. So we start down and then
up, and then at carbon three, this would be down. At carbon four, this would be up. At carbon five, this would be down. At carbon six, this would be up, so just imagine all of
these hydrogens are along the equator of the ring. So let's put in those
equatorial hydrogens into our chair conformation. We start at carbon one and we start down, so down equatorial, so
we draw a line down here and put in our hydrogen, and then we go up. So at carbon two, it would
be up, so we put that in. We alternate, so at carbon
three, it would be down, so we draw in that hydrogen. At carbon four, it would be
up, so we put in that hydrogen. Carbon five would be down,
so we put that one in, and carbon six would be
up, so we put that one in. These are the ones that students
have the hardest time with, so you could think about the
parallel lines that we drew earlier to help guide you. So let me go ahead and
show you what I mean. So we started with the parallel
lines in yellow over here, so if you look at those
lines, you should have, you should have these, right? These are also parallel. So this one is parallel and this one is parallel. Also you could look at
the ones in red next, so let me highlight those again, so this set of parallel
lines, these two right here, so that would be these hydrogens right? Or these groups, I should say. Those should be parallel. And finally, let's look
at the ones in blue. So let me highlight those
again really quickly. Let me use a darker blue so
we can see it a little bit better, so this one and this one, right, those were parallel lines. Well, we can also see that
these, right, these would be parallel to those as well. There's another way of thinking about this which might be helpful. If you look at the drawing on
the right, I'm highlighting some of the bonds in magenta, and when I'm done, this
should look familiar. You've seen this before in
the video on conformations of ethane. That was a sawhorse drawing
for the conformation of ethane, the staggered conformation. Let me draw it over here so you can see it a little bit better. So, here we have the staggered
conformation for ethane and that's helpful sometimes
when you're thinking about how to draw the different bonds
in the chair conformation. So pretty much, you can
use any method you want. I've seen lots of ways to
draw chair conformations, and lots of students come up
with their own way of doing it, so whatever works for
you, just be consistent and be careful. Take your time and you
have to practice a lot. Next, let's look at another
chair conformation that you'll be drawing. We need some practice drawing
the other chair conformation. We're gonna start the
same way we did before with two parallel lines that
are offset from each other. This time, though, the
lines will be going in a different direction
than what we did before. So let's go ahead and
put in one parallel line. So there's one, and then
another one, like that. So hopefully those are
pretty much parallel, and then we're gonna
draw in a dotted line, a horizontal dotted line that
just touches this top point here on this top line
that we drew, all right? And these dotted lines,
remember, are just guidelines. They're not actually a part
of our chair conformation. We draw another dotted line
down here that intersects with this bottom point, all
right, as our guideline. Next, we go from the top dotted line down to the bottom dotted line. So from the top down to the bottom, and then we go from the
bottom back up to the top, and finally we just connect the dots, so from this point we draw a line to here, and then we draw that line
again, a parallel line at this point, to this point. So make sure that everything is parallel. You have three sets of parallel lines. The first set of parallel
lines we drew was this one up here, on the drawing, so in yellow. Next, in red, we went from
this point down to this point and then we went from this
point up to this point, in red. Those should be parallel. And then finally, in
blue, this line in blue should be parallel with this line in blue, so make sure you have three
sets of parallel lines to draw your carbon skeleton. Next, this turns out to
be carbon one, right here, and we'll see why in the next video. So we're gonna start to put in our groups. In this case we're just
gonna put in all hydrogens, and at carbon one, you can
see that we're starting axial down this time. So we're gonna put a hydrogen axial down. That's the opposite of
what we did in the other chair conformation. At c-one in the other chair
conformation, we started axial up, so we're gonna start axial down, so we put a hydrogen
going down at this point, and we move over to carbon two. So this is carbon two right
here, and you can see, now we have axial up for our hydrogens, so we go to carbon two
and we have axial up. Carbon three would be this one. You can see it's axial down,
so carbon three is axial down, and again, this just alternates. So carbon four should be
axial up, so we draw in our hydrogen going up at carbon four. Carbon five, axial down, so
we draw in our hydrogen here, and finally carbon six is
axial up, so let's put in that hydrogen. Next, we put in the equatorial hydrogens, which are a little bit harder to draw, so let me use a different
color here so we can see. Actually, let me go ahead
and keep it yellow because this equatorial hydrogen,
right, is parallel with this set of lines, right, and also
this one over here actually, so we're doing equatorial
up for our first hydrogen, so this line is directed
up relative to the plane, so that's how you have to think about it. So we start at carbon one
and we go equatorial up, so we put in that line
and put in the hydrogen. Next, at carbon two, we already
had this hydrogen going up, so this must be equatorial down, so we have equatorial down, and this line should be parallel with
the ones in red, right? So this line should be
parallel with the ones in red that we've already seen, so
let's put in that hydrogen. All right, so this is equatorial down. At carbon three, we have equatorial up. Let me use a different color. We have equatorial up at carbon three. I used blue because, again,
with these lines parallel in blue, so we have
equatorial up at carbon three. Go down to this drawing, let's put it in, so here is our hydrogen going up. And again, we're alternating. We start at equatorial
up, then equatorial down, let we draw in that hydrogen,
and then equatorial up, and then equatorial down is our next one. So equatorial down at carbon four, and then carbon five should
be up, so here it is up, and this should be parallel
with the ones in red. This is one of those ones
that's difficult for students. Again, it's parallel with the ones in red, so that should make it easier. So let's draw that in, so we
have our hydrogen going up at this point, and then
finally, carbon six. You can see this is equatorial down, and I guess it should be blue, so let me be consistent here. So this is parallel with
everything else in blue. We have another hydrogen,
which is equatorial down, so let me draw that one in, so. Actually, that wasn't very
good, so let me do it again. This takes practice, right? Keep practicing your chair conformations, both kinds of chair conformations.