00:01
Okay, so we've got four substituted butanes, right? we've got two methylbutane.
00:11
We've got 2 -2 -dymethyl butane.
00:17
We've got 2 -3 -dymethyl butane, and we've got 2 -2 -3 dimethylbutane.
00:33
And we need to draw confirmations along the newman projections taking sight along the c2 -3 bond and show the most stable and least stable confirmations for each of these.
00:45
So, right? that means we're going to be looking along this bond right there.
00:52
And i like to color code these.
00:53
All right.
00:54
So when i'm building this, i'll say, okay, this carbon two is blue and carbon three is green.
01:01
Okay.
01:01
And i'm going to be looking down that bond.
01:08
Okay.
01:09
So then i'll come here and i'll say, okay, we're in the front here with our blue carbon.
01:14
And it looks to me like we have a methyl that is up, and then i consider the different ways i can draw this.
01:28
It's not shown with stereochemistry because there's no actual stereotypical information, but we can kind of draw this as though it's a wedge and that this will be going out to the right, methyl there, and that we'd have a hydrogen to the left.
01:44
And we can just do that in either direction, because this is not a stereo center, so it really doesn't matter.
01:53
Okay, now we're going to draw our carbon that's in the back.
01:55
That's our green carbon.
01:57
There we go.
02:00
And we're going to show the substituents coming off of this.
02:05
So we've got a methyl and two hydrogens.
02:08
Once again, there's no stereocenter, so it doesn't really matter where we draw the meth or the hydrogens.
02:18
Okay.
02:20
And now we're tasked with drawing, say, the most stable confirmation of this, right? so what are we trying to avoid? we're trying to avoid methyl being next to each other, right? methyls being next to hydrogen is preferable to metals being next to methyls.
02:38
Because recall that each of these methyl actually has three hydrogen sticking off of it, and that these are repelling each other, right? the hydrogens are positively charged.
02:51
They're kind of bumping into each other.
02:53
That's not good.
02:54
That's not stable.
02:55
So we want to draw this in such a manner that the hydrogens, that the methyl groups are not next to other methyl groups.
03:04
Okay.
03:07
So how can we determine the different possible configurations we could draw this in? basically, we can just consider rotating about this bond.
03:16
And the way i like to do that is just swap everything one place.
03:20
Right.
03:21
So this methyl moves here.
03:24
This methyl moves there.
03:26
And this hydrogen moves there.
03:27
And then i leave the back as is.
03:29
Right and that's how i might you know withdrawing move through the different confirmations or even just sort of mentally do it in my head but in this case i will draw it okay so we're assuming that we're rotating everything 60 degrees everything in the front we're rotating at 60 degrees clock once okay so as i said this will now be hydrogen and the methyl's over the other ones and then in the back, we leave everything the same.
04:03
Right.
04:03
So we're still going to have hydrogen, hydrogen, and a methyl.
04:11
Okay.
04:12
So now it's like, well, is this any better? you know, did we get lower energy, higher energy? what happened? look, we had methyl, a ghost to methyl, right? methyl next to methyl and another methyl next to methyl.
04:29
We had two methyl -methyl -methyl -methal interactions, right? here, we've got one methyl -methyl interaction, these two interacting, but nothing here, right? this is next to hydrogens.
04:42
So this is going to be lower energy, right? and we can actually use this table 3 .5 to assign strain energy values.
04:50
Now, i do not have this textbook.
04:52
I don't know, but i do know that there are, that these values are pretty standard.
05:03
And, right, so basically we can say methyl, the strain energy from that and k -cals per mole is about 0 .9, right? whereas, you know, methyl hydrogen or hydrogen, we consider to be zero.
05:37
So hydrogen, hydrogen, methyl hydrogen, we call that zero or zero, right? so of the two, right, we can say that this one might have an eclipsing strain of 1 .8, right, because it's got two methyl -methyl -gosch interactions, right? whereas this molecule only has one, right? so its eclipsing strain is only about 0 .9.
06:11
Right? so obviously this is the lowest energy conformer.
06:18
And if we want to be thorough, what we would do is we would rotate it an additional 60 degrees clockwise.
06:27
In fact, this is going to give us the same thing that we currently have.
06:30
But that's not always the case.
06:34
It's just because we have two methyl in the front here that this is going to be the exact same.
06:38
But let's consider rotating this.
06:47
Right.
06:47
So now the hydrogen is down here.
06:50
And in the back, it's going to be the same.
06:54
Hydrogen up to the left, hydrogen down, and a methyl.
07:00
There and as you can see once again we have a single methyl methyl methyl goche interaction and then no other methyl metals only a methyl hydrogens and hydrogen hydrogens which we set are zero so this is also about 0 .9 so either of these structures is going to be the lowest but there's something we're missing here right we also need to show the highest right so you'll notice the way i drew this is that there's never any eclipsing atoms right like everything's as spread out as it can be show you what i mean right so we've done something like this right the rear atoms are always spaced in between the front atoms and this would correspond to a structure like say something like this right but we can actually draw this a different way in which everything is lined up with everything else which would correspond to a molecule being drawn like this.
08:20
Right.
08:21
So the corresponding newman projection would be we've got this are three substituents in the front and our substituents in the back are overlapping.
08:38
And now we've got a whole different set of values for the energy of, say, a methyl overlapping with a methyl or a methyl overlapping with the hydrogen or a hydrogen overlapping with hydrogen for example right so whereas these interactions are referred to as goche interactions these are referred to as eclipsing right so our gosh interactions only really matter if it's methyl methyl but for eclipsing and i recall this is about 0 .9 okay for eclipsing we've got methyl methyl, which is about 2 .6.
09:36
We've got methyl hydrogen, which is about 1 .4.
09:45
And we've got hydrogen, hydrogen, which is about 1 k -cals per mole.
09:52
Right.
09:53
So because of that, in fact, if we want to draw the highest energy conformer, there's some more things to consider.
10:00
So let's revisit what we said, what's the lowest energy conformer, keeping these values.
10:06
In mind right so we said lowest energy we had let's see methyl metal okay methyl's in the front methyl nitrogen whereas in the back we had one methyl here hydrogen here and hydrogen here right but for our highest energy conformer we actually don't want everything to be spaced out like they are.
10:45
We want them to be overlapping.
10:46
All right.
10:47
So we said that this is about 0 .9 k kals per mole.
10:52
But for our highest energy conformer, we can leave the front the same.
11:01
Methyl down to the right, methyl down to the left hydrogen here.
11:05
We're going to rotate the back such that our methyls are lined up together, or at least the two that we can line up together or line up together.
11:17
Right.
11:18
So now you see, this table, we can determine that, well, we've got a methyl -methyl eclipsing.
11:24
That's 2 .6.
11:25
We've got a hydrogen eclipsing.
11:27
That's one...