Consider the structures of cis-1,2-dimethylcyclopropane and trans-1,2-dimethylcyclopropane:
(a) Which compound would you expect to be more stable? Explain your choice.
Hey guys. So, here are two structures for our cyclopropanes and it wants to know which one's going to be more stable, we have the one that's trans here and then the other one that's cis. So, as far as stability goes, which do you think going to be more stable? what do you guys think? Well, we have different ways that we can solve this problem, one of them is to be thinking of the interactions that will be involved between these two methyls being on the same side as far as energy goes and here notice that we have a methyl and then a hydrogen and then over here a methyl and a hydrogen, so these are CH threes and those same interactions will be going on for our cis isomer, but notice that the energy involved with these two methyls be on the same side will actually have not only sterics but also some strain involved with it and we have different values that we can use to determine which one's going to be more stable, and notice that this interaction between these two methyls on the same side of our plane, there's going to be energy involved that's 11 kilojoules per mole, okay? Because that is the interaction between two eclipsed CH3's, okay? Now, notice that the other one only has CH3 with an H, okay? Like this. Now, this value, you guys remember what this value will be? this one's only going to be six kilojoules per mole. So, essentially both of these molecules will have very similar interactions but the cis one will have that eleven kilojoules per mole value for the CH3 and a CH3 overlapping each other, alright? So, which isomer do you think is going to be more stable then? Well guys, the one that's going to be more stable as the one it doesn't have that sterics and strain involved with those two methyls, which is our trans molecule, so we can go ahead and say that trans will be more stable than our cis isomer. Alright guys. So, does that make sense? Now, let's take a look at your next question.