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Course: Organic chemistry > Unit 3
Lesson 3: Conformations of alkanesConformational analysis of propane
How to analyze the staggered and eclipsed conformations of propane.
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Doesn't propane have two CH3 groups? This looks like ethane.(5 votes)
That's because the carbon of one of the methyl groups is hidden behind the central carbon atom of the propane model, but you can see its hydrogen atoms.(4 votes)
Why does CH3 eclipsing H cost more than H eclipsing H? Thank you.(2 votes)- Aren't the conformations shown in this video Gauche conformations? What is the difference then between Gauche and staggered conformations?(1 vote)
- Gauche is a special case of staggered in which the groups on two adjacent C atoms are at 60° to each other.
In propane there is a CH₃ on one carbon but only H on the other, so it is not called a gauche arrangement.(4 votes)
Wouldn't the energy cost of an eclipsed methyl group change as that methyl group spins around its bond?(2 votes)- The methyl group is always spinning, and we can't stop it.
It is behaving like a blob of charge density, so that is the basis on which the energy is measured.(2 votes)
When they measure the potential energy, for measuring the difference in energy between the staggered and eclipsed conformations, how do they know when a molecule is in staggered or eclipsed formation? And do they need all measured molecules to be in one conformation to calculate the difference in potential energy?(2 votes)- The molecule with lower potential energy is the one with a staggered conformation and the molecule with higher potential energy is the one with an eclipsed conformation.(1 vote)
Can gauche-staggered conformations be formed with two adjacent carbons with only Hydrogens attached to it? The only example of gauche I saw in the videos was with a methyl group. That's how I could differentiate between gauche and anti. But is that the case(1 vote)
So conformational analysis can be applied to any molecule with single bonds capable of internal rotations. So we can have gauche staggered and anti staggered conformers which plenty of molecules, even those with groups that don't include hydrogens.
Being gauche staggered doesn't require the presence of hydrogen. A conformation being staggered just means that the groups of the two rotating carbons have a dihedral angle of 60°, and that they're not eclipsing each other and creating steric strain. And within that staggered group we can have gauche and anti conformations. There's a single anti conformation which states that the most bulky groups have a dihedral angle of 180° and are as far apart as possible to reduce steric strain. While every other staggered conformation is gauche when the most bulky groups are not 180° apart.
These bulky groups could be as simple as methyl groups, but they could also be something like ethyl groups or tert-butyl groups. And the smaller less bulky groups could be hydrogen, but they could also be methyl groups too. So it's not necessary to have hydrogens and methyl groups for the conformation definitions to apply since they're generalizations that work for a wide array of molecules.
Hope that helps.(2 votes)
- What is energy cost? Can't find it in the previous video as mentioned(1 vote)
- The cumulative energy value of a conformation results from the individual groups of the adjacent carbons interacting with each other and encountering steric strain. So at least in this video they're using these eclipsing interactions as synonyms for energy cost. It seems to be a term that Jay uses for his own purposes because I've not seen their use of energy cost used elsewhere in chemistry classes or literature. I guess you could use it in the sense that it costs 14 kJ of energy to get the molecule to rotate from a staggered conformation to an eclipsed formation.
Hope that helps.(2 votes)
Do you know where do you get those molecule models?(1 vote)- how do you know/calculate what the torsional strain is?(1 vote)
- You calculate torsional strain by subtracting the lower potential energy from the higher potential energy as you go from staggered to eclipsed conformation. The amount of energy from each rotation is degenerate. For propane, there is a difference of 14 kJ/mole. The values for torsional strain will change as you add more carbons (as seen in the next video about the conformational analysis of butane).(1 vote)
Video transcript
- [Voiceover] Here we have a
model of the propane molecule. And if we stare down this
carbon carbon bond here we will see a Newman projection and this is a staggered conformation. If I rotate about that carbon carbon bond that we're sighting down we
get another conformation, this is the eclipsed conformation. And I'm leaving it a little bit off so you can actually see the
bonds in the back there. So that's an approximately
an eclipsed conformation. I can rotate again and get
a staggered conformation. And if I keep rotating here the next one would be an
eclipsed conformation, so again, I'm leaving the
bonds slightly off to the side so you can actually see
the ones in the back. I rotate again to get a
staggered conformation, and the next one would
of course be eclipsed, so let's look at that. So the eclipsed conformation. And then finally, one
more time to get back to our staggered conformation of propanes. So there's your staggered conformation. Here we have the energy diagram for the conformations we saw in the video. And make sure you've seen the conformational
analysis of ethane video before you watch this one. On the y-axis we have potential energy, so as you increase in the y-axis you're increasing in potential energy. And we started with the staggered
conformation of propane. And we rotated it 60 degrees, we held the back carbon stationary and I rotated the front carbon 60 degrees to give us this conformation, which is the eclipsed
conformation of propane. Notice the difference
in potential energies between these two conformations. The staggered conformation
has a lower potential energy and the eclipsed conformation
has a higher potential energy. Remember, the lower the potential energy, the more stable the conformation, so the staggered
conformation is more stable than the eclipsed conformation. So it takes energy to go from
the staggered conformation to the eclipsed conformation. And the analogy that I
used in the earlier video was a boulder. So if you have a boulder
at the bottom of the hill and you're trying to push
the boulder up the hill to the top here, it
takes energy to do that. And at the top of the hill
the boulder is less stable. So higher the potential
energy, less stable. Lower the potential energy, more stable. So our staggered
conformation is more stable than our eclipsed. As we rotate and we go from
this eclipsed conformation to this staggered conformation that would be a decrease
in potential energy. Going from this staggered conformation to this eclipsed would be an
increase in potential energy, going from the eclipsed to this staggered would be a decrease,
and you see the pattern. Going from staggered up to this eclipsed would take energy and then
going from the eclipsed down to this staggered is a decrease
in the potential energy. All of our eclipsed conformations have the same value for
the potential energy. They are degenerate in terms of energy. Same thing for the
staggered conformations, these all have the same
potential energy value. So there's a difference
in potential energy between the eclipsed conformations and the staggered conformations. And that difference in energy turns out to be 14 kilojoules per mole. So we're talking about
the energy difference between the eclipsed and
the staggered conformation. We know there's an energy difference of 14 kilojoules per mole between the staggered
conformation of propane and the eclipsed conformation and that's called the torsional strain. Let's go ahead and draw
a Newman projection for each one of these conformations, so just as practice. Let's start with the
staggered conformation. And we'll start with this
carbon in front here, which is represented by a point. So I'll draw in a point here. What is bonded to that carbon? Well, there is a CH3 group,
a methyl group up here, so let's draw a line straight up and draw in a CH3. And there's a hydrogen going to the right, and a hydrogen going to the left, so there's my hydrogen going to the right, and there's my hydrogen going to the left. We know there's a carbon
behind this carbon that I marked with a point here, we just can't see it
because the front carbon is eclipsing the back carbon, but we know that these
hydrogens in the back here are attached to that back carbon. So we represent the back
carbon with a circle when we're doing Newman projections, so that's supposed to
represent the back carbon. And then we would have
a hydrogen coming out to the right, like that,
so that's this hydrogen, a hydrogen coming out to the left, that's this hydrogen, and a
hydrogen coming straight down, so that would be this hydrogen. So there's your staggered
conformation for propane. Next let's draw the eclipsed conformation as a Newman projection. So a little bit harder. But let's start with this carbon again, so this is the one's at the front carbon, so this is represented
with a point right here. And then we would have a CH3, a methyl group going off to the right, so let's draw that in. So we have a CH3 going off to the right. We have a hydrogen going down. And I'm gonna draw this
a little bit off center, so instead of drawing it straight down, I'm gonna draw it a little
bit off to the left, just as I did in the picture here, to make it easier to see
the bonds in the back. So there's a hydrogen going
down a little bit to the left, and then we have a hydrogen
going in this direction, so let me go ahead and draw that in here, so here's a hydrogen. Next let's think about the back carbon. We can't see it, but because
this front carbon here is eclipsing the back carbon, but we know that the back carbon has three hydrogens attached to it. This one, this one, and this one, which we can just barely see. So let's add those in on
our Newman projection. So the back carbon is
represented by a circle here, and let's start with this
hydrogen right back here. That would be going in this direction, so it's being eclipsed
by the methyl group, but we draw it a little
bit off to the side, so we can still see it's there. Next let's do this hydrogen. So it's going down,
pretty much straight down, so we'll draw that in there. And then finally, this hydrogen over here. So this hydrogen we could
represent it like that. So now we have Newman projections for the staggered conformation and for the eclipsed conformation. Let's go back to that
14 kilojoules per mole, that torsional strain. So let me write that in here, so 14 kilojoules per mole. In the video on conformations of ethane we already know that each
pair of eclipsed hydrogens has an energy cost of
four kilojoules per mole. So this pair of eclipsed hydrogens, alright, that's four kilojoules per mole as an energy cost right here. Same with this one, so this
one's four kilojoules per mole. So now we can figure out the energy cost associated with a methyl
group eclipsing a hydrogen. Because we know the total
should add up to equal 14, so four plus four plus
what is equal to 14. Obviously the answer is six. Alright, so this must be six kilojoules, six kilojoules per mole. So six plus four plus four gives us our total torsional strain of 14 kilojoules per mole. So now we know that the energy cost of a methyl group eclipsing a hydrogen must be six kilojoules per mole.