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Course: Organic chemistry > Unit 3
Lesson 3: Conformations of alkanesConformations of ethane
How to represent the staggered and eclipsed conformations of ethane.
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Would the eclipse conformation ever occur on its own? As the Hydrogen has a partial positive charge would any of the other carbon bonded hydrogen's also have a positive charge, and as equal charges repel the hydrogen's favored state would be spaced out as far from each other as possible as is depicted in the staggered conformation(3 votes)
It technically does. Note that in reality molecules vibrate and rotate all the time due to Brownian Motion. It is just that the staggered conformations are more energetically favored. In a system where there is ample energy to allow ethane to surpass the energy barrier necessary to get it into a eclipsed position, then at any given time you will find ethanes adopting a eclipsed conformation. However, given that staggered is more favored and is more energetically economical so to say, more ethanes in a given sample will have the staggered conformation. Hope this helps.(18 votes)
Is it necessary for a gausche conformation to have some bulky groups? Or can it form between the hydrogens too?(1 vote)
In Newman projection, gauche is the relationship between two atoms or groups whose dihedral angle is 60 °.
Two H atoms can be gauche, so staggered ethane has six gauche interactions.(8 votes)
- How many conformations can there really be?(2 votes)
The bond rotate infinitly, so in the real sense it is infinite. But in terms of staggered and ecclipse for ethane, there will be 1 ecliipsed and one staggered since all the hydrogens are identical..(4 votes)
What about molecules with double bonds, e.g ethene?
Each carbon in ethene has 2 hydrogens, so technically we could have both staggered and eclipse conformations.
But because the double bond can't rotate, does it mean that a molecule with a double bond will only have either staggered or eclipse conformation?(1 vote)
It’s good to remember that ethene has a π bond as the second carbon-carbon bond in the double bond. The π bond requires the two carbon’s unhybridized p orbitals to be parallel to each other for it to form. And since both of the carbons are sp2 hybridized, they have three sp2 hybrid orbitals in a trigonal planar geometry with the unhybridized p orbital perpendicular so that plane. They’re always in that orientation so we can’t move one carbon’s hydrogens (and the sp2 orbitals) so that they’re perpendicular to the other carbon’s hydrogens since moving the sp2 orbitals also necessitates a move of the unhybridized p orbital which would result in a breaking of the π bond.
So because the hydrogens are always locked in one position because of the π bond, along with the inability to rotate, we can’t say that they have conformations. They have no other choice but to be in these trigonal planar geometries around the carbons. So they’re neither staggered or eclipsed.
Hope that helps.(4 votes)
- This may sound like a very stupid question, but how you know where the plane of the page/computer screen would be? And how would you tell the difference between a bond coming out of the plane and bond going away from you in space?(2 votes)
Anything with just a normal line between atoms is in the plane of the paper (or screen), wedged lines (they look like black filled in triangles) means that bond is coming out towards you and dashed lines means the bond is going away from you(1 vote)
- how do you calculate for the potential energy of each conformation?(2 votes)
- What does a dihedral angle of 0 mean for the hydrogen atoms on each of the carbons in ethane?(1 vote)
- That means it’s an eclipsed configuration, the hydrogens are directly in line with one another if you look down the carbon-carbon bond.(1 vote)
- why does it always take an angle of 60 degree(1 vote)
- You have 6 H atoms in a circle, so they will be distributed 60 degrees away from each other(1 vote)
- Does it matter which carbon you take as the front one and which one you take as the back carbon atom??(1 vote)
For this particular molecule, namely ethane, it will not matter which carbon you put at the front since they are essentially symmetrical to each other. It would make no difference.(1 vote)
- Can the conformations (all) of ethane be called gausche conformations ? Since the condition is same all the time.....(1 vote)
Video transcript
- [Voiceover] On the left, we have one way to represent the ethane molecule. The only problem with this drawing is it doesn't give us much information about what ethane looks
like in 3D dimensions. On the right, is another
drawing of the ethane molecule, but this drawing gives
us more information. This is a wedge and dash drawing. Remember, a wedge has a bond
coming out at you in space or a bond coming out of
the plane of the page. A dash is a bond going
away from you in space or a bond going into
the plane of the page. And finally, if you just draw
a straight line like this, that means a bond in
the plane of the page. This wedge and dash drawing represents one conformation of ethane. Conformations are different
arrangements of atoms that result from bond rotation. We know that there's free rotation around this carbon-carbon single bond. And this wedge and dash drawing right now represents what's called
the staggered conformation of ethane. But if we rotate about
this carbon-carbon bond, we're gonna get different
arrangements of the atoms and therefore we would get
different conformations. This is easiest to see with a model set, so up next I have a video where
I have a model set of ethane and I rotate around the carbon-carbon bond so we can see different conformations. I'm gonna make these
hydrogens green in the video, so those green hydrogens
are attached to this carbon, which will be the front carbon. And then we have hydrogens attached to the back carbon that I will make white. So let's watch the video and look at the different conformations of ethane. Here we have the staggered
conformation of ethane, looking at it from a wedge
and dash perspective. If we rotate this a little bit, we'll see the staggered conformation of ethane from a sawhorse perspective. If we sight down the carbon-carbon bond, we'll see the third way of
looking at this molecule. And this is called a Newman projection. If I take one of these
hydrogens here and I rotate it, so I'm keeping the back carbon stationary and I'm rotating the front carbon, every time I rotate this, that represents a different conformation
of the ethane molecule. So there are many, many possible
conformations in theory. I rotate back to the
original position here, so this is the Newman
projection where we just were. The other conformation that we
really care about for ethane is what's called the eclipse conformation. So, I just rotated the green
hydrogens so they're in front or eclipsing the white
hydrogens in the back. So, this is the eclipse
conformation as a Newman projection. I could turn this a
little bit so we can see what an eclipsed conformation looks like from a sawhorse view and then finally, I can turn a little bit so we can see the eclipse conformation from
a wedge and dash perspective. Now that we've seen the video,
let's look at three ways to represent the staggered
conformation of ethane. And we'll start with the
wedge and dashing drawing. So you can see that this hydrogen,
this carbon, this carbon, and this hydrogen are
all in the same plane. So they're all in the same
plane here which is why all these bonds are
drawing as straight lines in our wedge and dash drawing. If we look at this carbon on the left, we know that there is a hydrogen in green coming out at us in
space, a hydrogen in green going away from us in space,
and then this hydrogen which is in the plane of the page. For the hydrogens in white, this hydrogen is in the plane of the
page, this one's coming out at us in space, and then we have one going away from us in space. And it's pretty easy to see
that in the picture here, so we're just going to match
the drawings to the pictures. Our next way to represent
the staggered conformation is what's called a sawhorse drawing. So for this sawhorse drawing,
this carbon right here is this carbon and it has
three hydrogens in green attached to it. So, this is a hydrogen in
green, in green, and in green. So, we also have this carbon-carbon bond so this bond here on our sawhorse drawing. And finally, for this carbon in the back, this is the one that
has hydrogens in white. So these are the three hydrogens in white. If we look down that carbon-carbon bond, if we look down this axis right here, so if you put your eye, imagine
putting your eye right here, so here's the eye and
you look down this axis, you will see the Newman projection just like we saw in the video. And so, this dot, this
point here represents the front carbon. That's this carbon right here. So let me go ahead and write that down. So this point represents the front carbon and the front carbon is bonded
to our hydrogens in green. So this would be a hydrogen in green, so is this one, and so is this one. The front carbon blocks
the carbon in the back. You can't see the carbon in the back in this drawing but we know it's there and we have to represent it somehow so in a Newman projection,
it's this circle back here that represents the back carbon. So this is the back carbon, this circle. Let me write that down. The circle is the back carbon
even though we can't see it when you actually have
the model in front of you. And the back carbon is the one that has the hydrogens in white. So this should be a hydrogen in white, so is this one, and so is this one. So you can see the
hydrogens in white back here and you know they're attached to a carbon but you can't see it
because the front carbon is blocking it. So let's talk a little bit
more about Newman projections. We can talk about the angle
between this hydrogen in white and this hydrogen in green. So think about the angle between these. It's 60 degrees so we have 60 degree angle between these two hydrogens. So I can write down
here too. So 60 degrees. This angle is called the dihedral angle or the torsional angle. So I'll write those down,
so you can call this the dihedral angle or you could also call it the torsional angle. Let me write that down here too. So torsional angle or you could also call it the torsion angle. The angle between the
hydrogens will be important when you're talking about conformations. Here the angle is 60 degrees which means that these hydrogens are not
right on top of each other. There's space between these hydrogens on the Newman projection. The green hydrogens are staggered compared to the white hydrogens. So if you go around you can see that there's space between all of these. And that's why we call this
the staggered conformation of ethane. And finally, let's look at the other important conformation of ethane which is the eclipsed conformation. And we'll start with the
wedge and dash drawing. So we have this hydrogen, this carbon, this carbon, and this hydrogen. These are all in the same plane, so I draw this line here
and we can see these bonds are all in the same plane up here on my wedge and dash drawing. For this carbon on the left,
we can see that we have a hydrogen in green coming out at us. So right, here's your hydrogen
in green coming out at you, a hydrogen in green going
away from you back here, and then this hydrogen
in the plane of the page. For the hydrogens in white,
this hydrogen is in the plane of the page, this hydrogen
is coming out at us in space, and this hydrogen is going
away from us in space. For the sawhorse drawings,
let's move on to here. This carbon is the front carbon, so that's this carbon right here. The one that's bonded to the
three hydrogens in green. So let me highlight those. So we have one hydrogen, this
one right here is going up. So that's our hydrogen in green going up. This hydrogen over here is going down, a little bit to the
right, and this hydrogen's going down and to the left. And then we have the carbon-carbon bond. So let me draw that in here. So here's our carbon-carbon bond, I made it much longer up here just so the atoms wouldn't
interfere with each other on our sawhorse drawing. And then we get to the carbon in the back. So here's the carbon in the back which is bonded to three hydrogens. And I made these the hydrogens in white. So here's one, two, and three. So here are the hydrogens
in white on the picture. Finally, we get to the Newman projection for the eclipsed conformation. And remember, the Newman
projection is what we would see if we stare down the carbon-carbon bond. So if we sight down
this carbon-carbon bond, if we put our eye right here
on our sawhorse projection and we stare down the carbon-carbon bond, we see this for our Newman projection. Alright, first we see this
point here repesenting the front carbon, so that
point is the front carbon on our Newman projection. And the front carbon is bonded
to the hydrogens in green. So here's one, two, and three. So up here, one, two, and three. So this would be the bonds,
this represents the bonds of that front carbon to those hydrogens. The back carbon is pretty difficult to see but we know that the circle
represents the back carbon. The front carbon is in the way. I'm not sure if you might be able to see a tiny bit of the back carbon here. But the back carbon should be
blocked by the front carbon. The back carbon we know
has our hydrogens in white so here are the three hydrogens in white. So back here you can just
barely see them poking out. This time when you think about the angle between your hydrogens,
this time your dihedral or your torsional angle is zero degrees. Think about this hydrogen
being perfectly upright, and the hydrogen in the back
being perfectly upright. So the angle between those is zero. So your dihedral angle
is zero degrees here for the eclipsed conformation. The green hydrogens are
eclipsing the white hydrogens. So that's why we call this
the eclipsed conformation.