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Solution: Sketch an approximate potential energy diagram for rotation ...

Question

Sketch an approximate potential energy diagram for rotation about the carbon–carbon bond in 2,2-dimethylpropane similar to that shown in Figures 3.4 and 3.7. Does the form of the potential energy curve of 2,2-dimethylpropane more closely resemble that of ethane or that of butane? 

Video Transcript

So we're going to go ahead and figure out our Newman projection for 2,2-dimethyl propane and actually track the rotational the potential energy as a function of its rotation, okay? So guys we know that dimethyl propane, right? We're actually told that dimethyl propane and given the 2,2 but it's not so necessary to have that 2,2 in this particular case because if we didn't add our methyl groups to our middle carbon or two carbon we would end up with something else like a butane or pentane, right? So we know because we have propane and we're told 2,2-dimethyl as an extra that we've got our methyl groups there, so let's say we're looking down this bond, right? We can go ahead and say that we've got this Newman projection template, on our front carbon we just have hydrogens, right? That's all, and then on our back carbon we just have methyl groups, right? this would be our back carbon here and we've got CH3's coming off and no hydrogens, right? So we would just be CH3 over here, right? So now, let's go ahead and track our potential energy. Now, let's say that we just have something looks like this, okay? where these two are approximately the same height if not the exact same height, so we're going to go ahead and fill in what would go there, the higher potential energy tells me that we've got some groups overlapping and here since we drawn it this way, all we've got is hydrogens in the front and carbons in the back, right? So, methyl groups in the back, hydrogens in the front, so the only things that can overlap are going to be these guys right here, right? The hydrogen and methyl groups. So, our Newman projection that looks like that is going to be a little bit, it is going to be eclipsed, okay? I'm going to go ahead and draw our back carbon here in blue with our methyl groups up and then we have here eclipsed, right? It's just going to be our hydrogens in black, right? And this is our front carbon. Now, we can't really rotate anymore in any new way, right? we can just go ahead and make this happen again for this down here, so I'm going to go ahead and do this in all black so you guys get used to that because that's probably what you're going to see on your exam, so we should see is just hydrogen, hydrogen, hydrogen CH3, CH3, CH3, okay? And then up here, well, actually it's just this again, right? We're going back to higher energy, which means that we're having to these guys eclipsed and the only that we can have eclipse is methyl groups eclipsed with hydrogens, so it's just going to be the same thing again for these next two and then our CH3, okay? So these are all the clips, I know I'm a bad artist but they're all eclipsed and then the exact same thing for this last one and then this would be just this that we've made already, right? So this is our rotational energy diagram for our Newman projection, as you rotate we expect that we're going to have different groups overlapping in different ways, so we're going to have peaks and valleys here. Alright, so guys, let me know give any questions, if not, let's move on.