Let's go ahead and look at an example. We're just going to do this as a worked example together. Here it goes. It says the barrier to rotation for the following molecule is 22 kilojoules per mole. Okay? So, that means that is the total amount of energy cost that it's going to take me to make these molecules or make these bonds go eclipse to each other. Okay? So what we want to do notice that right now it's staggered, 22 kilojoules per mole is what it's going to cost to make it go eclipse.
So what it's asking is to determine the energy cost associated with the eclipsing interaction between a bromine and a hydrogen atom. Okay? So, this is the way we think about it. First of all, did I make you memorize the value between H and bromine that are eclipsed? No. I could literally fill up a table of all the different atoms that could overlap, and I could make you memorize all those energies. But that's not going to be a really good use of your time. Okay? Especially because these questions will always give you some easy values that you can then back calculate the unknown value from. So all we do is we just look at each interaction, and we see how much each interaction is going to cost me. So overall, this rotation is going to cost me 22 kilojoules per mole. But we know that hey, if this H is going to overlap with this Br, that means that I'm also going to have an H and an H overlapping, and I'm also going to have a methyl and a methyl overlapping. So then I go ahead, and I get those formulas from my memory because I memorized them and I write them down. So H2 is 4 kilojoules per mole. Methyl-methyl is 11 kilojoules per mole. Okay?
Now we only have one more interaction that could be making my 22. The 22 is basically made out of 3 interactions summed together. Okay? So what that means is I basically have 4 + 11 + some unknown number x, and that's going to give me 22.
4 + 11 + x = 22
Does that make sense? So all I have to do is I just have to solve for x, and this is super easy. You can just use really easy math. So this is 15, so I'm going to subtract 15. So it's x equals 22 - fifteen. So that means that x equals 7.
x = 7
So that means that the energy cost for this interaction here is that HBr eclipsed is equal to 7 kilojoules per mole. Does that make sense? So all we did was we used the values that we're supposed to memorize, and we were able to figure out the unknown bond through that value. Cool. Right?
So now what I want you guys to do is do the next practice completely on your own. It's the same concept except it's a little bit harder because now you're going to have to draw out the compound yourself. So you're going to have to draw the Newman projection yourself and then you're going to have to work with it. You're going to have to play with it and figure out use the values that I told you to memorize to figure out the unknown bond. Alright? So go ahead and try to do