MOLECULAR SHAPES 151 



terms in our own calculations, but this is not of interest in the general area that 

 we are discussing. 



Six-membered rings can ordinarily be assumed to be chair-like and, if you 

 will notice cyclohexane more closely, there are two types of hydrogen locations. 

 The three atoms above the ring are equivalent and there are three below that 

 are the same and form one group of six. Then there are six around the middle 

 that are all equivalent geometrically. There has been some confusion of nomen- 

 clature but we now have a four-nation treaty that was published in Science and 

 Nature not long ago in which Hassel, Prelog, Barton and I recommended the 

 terms axial and equatorial, respectively, for the two groups. The organic chem- 

 ists seem to be happy with this system and are now putting the labels through- 

 out the literature. 



I will not go through all the cyclohexane conformational work, but it should 

 be kept in mind generally that almost any group can be put into an equatorial 

 position without getting into steric difficulties. But if you try to put substituents 

 into the axial position, you find that even one methyl group is crowded, and two 

 big groups would be impossibly strained. This on the other hand, is a strain 

 that the molecular models will show quite clearly. 



We do have some energy data on cyclohexane conformations. A methyl group 

 located equatorially is free of strain. But a methyl group located axially is 

 strained at two points and it turns out that the geometry here is just the same 

 at each point as occurred in normal butane in the gauche conformation. Thus 

 we can double that energy value. This is confirmed by experiments for the di- 

 methylcyclohexanes. 



I might just take a moment to discuss two big molecules and indicate how 

 differences in these single bond potentials have quite an effect on the general 

 properties. Again, this is an example that is probably remote from biological 

 interests, but I think it illustrates the sort of thing that might arise. Suppose 

 we take a straight paraffin chain. It would presumably have a CH3 on each end 

 of it. This is essentially an infinite chain. 



Then we compare this with the chain which occurs in the inner tube of your 

 tire, assuming you do not have a tubeless tire, namely, polyisobutylene. 



Offhand, you would think that the paraffin chain would have less interfer- 

 ence, and, therefore, that the normal paraffin ought to be the more flexible chain. 

 That is precisely wrong. The polyisobutylene is much more flexible chain which 

 makes it a rubber — that is the reason you can make an inner tube out of it if 

 you want to. The long paraffin chain is polyethylene, which, of course, is a 

 much harder material. 



Also, if you dissolve these materials in a solvent the paraffin is a much more 

 extended chain than the polyisobutylene. You can see this easily if you look 

 down one of the C — C bonds (Fig. 5). Suppose we take a methyl substituted 

 carbon atom at the far end and we have then CH3 groups in two positions and 



