The all-trans-1,2,3,4,5,6-hexaethylcyclohexane ( 1) prefers the all-equatorial conformation while the all-trans-1,2,3,4,5,6-hexaisopropylcyclohexane (2) possesses a severely destabilized all-equatorial conformation (J. Am. Chem. Soc. 1990, 112, 893–894):
(a) By examining a molecular model of cyclohexane with several all trans-equatorial isopropyl groups and another model with several all-trans-equatorial ethyl groups, determine why adjacent equatorially oriented isopropyl groups experience severe steric interactions which are lacking in the ethyl case. Draw a chair conformation of the former case which illustrates these severe steric interactions. Also draw a Newman projection looking down one of the C—C bonds connecting the cyclohexyl ring to an equatorial isopropyl group and illustrate a conformation with severe steric strain.
Hey guys. So, here on our screen we have two of the cyclohexane rings one where all the substituents are ethyl groups and notice they're alternating wedge and dash where the other ring their own isopropyl groups and in case you're wondering what an isopropyl looks like. Well, it's going to look like this. Remember, that here we have some part of a ring R and then this group at the top of your isopropyl group, so notice that at each end there's a CH3 and again a CH3 with an H here, okay? So, it's a very bulky group. Now, it says that these two cyclohexanes can adopt chair confirmations where all their groups are equatorial because of their orientation wedge dash wedge dash, so what I drew down here is notice that this is the straw that arrow coming from this one, this is one of the cyclohexane chair conformation is that this, the cyclohexane ring can undergo and it's not complete because I'm just going to highlight two of the groups and notice that over here, just like the other side, you can get an isopropyl group like this, right? And notice we could actually draw this a different way, we can draw it as an isopropyl like this, where we have methyl and methyl and then notice at the other one we can draw one going this direction where it has a methyl and methyl, right? Because notice that this group right here was equatorial up this next one's going to be equatorial down equatorial up and then equatorial down, okay? So it alternates and notice that this chair conformation explains part of the fact how, notice that this ethyl group could undergo a chair where all the groups of Equatorial as well and we'd be a lot more stable but for this isopropyl group it's actually not going to want to be in this position because notice all the sterics involved with these two groups being equatorial facing opposite directions, right? Now, maybe it's a little hard for you to be useless move on to the next. Now, this is just a Newman projection. Now, I run ahead and pretty draw it out for you because I don't want you to get confused with how it's drawn or anything like that but notice that it's just different Newman projections altogether in a fashion that makes this a ring, okay? So this is just a Newman representing a six membered ring where we actually drew out and highlight it again what's going on over here, okay? So I'm going to draw all this end where we have another methyl group coming off here we're just zooming in even more and over here this is part of the isopropyl group again. Now, we couldn't draw them as CH3 or we could draw them as C H, H, H, then again H, H and H. Now, look at all that steric involved there, right? So, in order to have these two groups all of our groups equatorial and this cyclohexane your b it's actually not going to be preferred for these reasons, right? So notice then the cyclohexane the chair derivative of it, you can already see some steric inverted bond, right? You can already see the steric involved there, if we go even further and look at the Newman projections assuming in on that area, you can see that we have these methyl groups that are going to be extremely overlapping and for that reason we can say that this ring a over here it's like will actually share with all the groups in the equatorial position will be a lot more stable than our chairs for B because remember that isopropyl group is a very bulky substituent, where ethyl in the other hand all that is, is just a CH2 and CH3, okay? So, bulky again but not as bulky the other isopropyl. So, here's where you can see the chair conformation with those isopropyl equatorial and see all the sterics involved and then again a Newman projection. Alright guys, so hopefully this makes sense and you guys can visualize how these isopropyl groups are very hindered and there's a lot of sterics involved with them when they're both in the equatorial positions. Alright guys, hopefully this makes sense and now let's take a look at your next question.