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Solution: Sketch an energy diagram showing a conformational analysis o...

Question

Sketch an energy diagram showing a conformational analysis of 2,2,3,3-tetramethylbutane. Use Table 4.6 to determine the energy difference between staggered and eclipsed conformations of this compound.

Video Transcript

Hey guys. So, for this question, we're looking at our compound which is 2, 2, 3, 3 tetra methyl butane, okay? So, here. Notice I've drawn in an energy diagram and then I've drawn in the compound or the structure for a regular butane. So, in order to make this 2, 2, 3, 3 tetra methyl butane, we need to add in methyl groups at positions 2 and 3. So, say we number this thing starting over here, give this r1 r2 r3 and r4, right? this is a regular butane and we're going to add methyl groups, we're going to add a methyl right here we're going to add a methyl right here and then two more methyls over here at two, okay? So now we have the correct structure for this compound. Now, what we need to do is convert this into a Newman projection, right? So, just remember, let's draw any other methyls on our butane. Notice that we're going to have another methyl right here and another methyl right here, so now say we wanted to draw the Newman projection, we know that a regular Newman projection is going to look something like this, where we have three groups coming off the front carbon, three groups coming off the back carbon. So, say that carbon in the front is going to be carbon two and that carbon in the back is going to be our carbon three, so what three groups do you see coming off of carbon 2? Well, you see we have a methyl another methyl and a third methyl. Now, what about the back guys? Notice that in the back, we're going to have three methyls as well, so we should expect that our Newman projection for the front carbon is going to have a methyl here, here and here and for the carbon in the back we're going to expect a methyl here and here, the two coming off the top, right? Of our carbon 3 and another one at the bottom, so notice that we just drew a regular confirmation for our Newman projection and everything coming off of it is a methyl group, right? Okay, so now say, we try to rotate this thing and now made this eclipsed, okay? So, in an eclipsed conformation, we would get something that looks like this, where now in that carbon in the front is going to stay the same but that carbon in the back of going to be rotated, it's going to be rotated so that now we have something that looks like this and again the carbon in the front is going to have methyl, methyl and methyl and then the carbon in the back is again going to have methyl, methyl and methyl and what I'm getting at is that when we draw a Newman projection you know that it's going to have different Peaks and different valleys, so remember that 180 is going to represent anti, 60 is going to represent gauche and then 120 is going to represent our eclipsed conformation, okay? Or zero degrees both of them, right? one is going to be partially eclipsed the other one totally eclipsed but notice that can we draw any other kind of Newman projection than these two? no, right? Because we had all the same groups coming off our carbon 2 and our carbon 3.

Remember that anti is going to be a lower energy at 180, gauche on 160 at 60 degrees though is going to be at the, same energy level same thing with our 0 and 120 cuz those are both eclipsed, these are the only two confirmations, so we should expect that our Newman projection is going to look something like this, we start out with high energy, low energy, high energy, low energy, high again low again and then high, we're going to end up at the same, where notice that this conformation on the left is going to be represented at 60 degrees, 180 degrees and 300 degrees, where the one on the right is going to be represented at 0 degrees, 120 degrees to 40 degrees and then again at 360, okay? So, those are going to be where our Newman projections lie on our graph. Now, we need to figure out the energy associated with them, so now let's take a look, for this one on the Left notice that we're going to have the only interaction is going to be between two different methyls and that's going to be a gauche interaction. So, for that gauche interaction we're going to say that the energy between those, do you remember what it is? Well guys, it's going to be around 3.8 kilojoules per mole, okay? And what about the one on the left, excuse me, on the right where we have now this eclipsed interaction between two methyls? Well, guys this one's going to be around 11 kilojoules per mole, okay? So let's write it right here, 11 kilojoules per mole, can you guys see that? so now we need to figure out the energy difference between these two Newman projections, for on one on the Left, we're going to say that we have 3.8 times 6 and that's going to give us 22.8 kilojoules per mole, okay? So what about the one on the right one on the right? well, on the right, we're going to say, we only have three interactions, right? So, for that one we're going to say we have 11 kilojoules per mole times 3, notice what that is going to give us guys? 33 kilojoules per mole, so now what is the difference between these two Newman projections? Well, we take our 33 minus by our 22.8 and what are we going to get? we're going to get a difference of 10.2 kilojoules per mole, okay? And that is our final answer and this is an energy diagram that relates to our two different newman projections for our 2, 2, 3, 3 tetramethyl butane. Alright guys, so I hope that makes sense and let me know if you have any questions.