MOLECULAR SHAPES 



149 



puckered 



2 6 kcal 

 66 



92 



CYCLOPENTANE 



bond an^le 

 rotation 



Total strain 



planar 



I kcal 



140 



Fig. 3 



force the molecule into some other form. This is probably not a completely gen- 

 eral conclusion but we have enough information to make it a pretty safe gen- 

 eralization. One should be, I think, very cautious about postulating a change in 

 structure of the molecule in order to accommodate the lattice arrangement. 



I want to talk a little about rings. Fig. 3 is cyclopentane, and you will recall 

 that all the older books indicate that the five carbon atoms are in a single plane. 

 So far as the bond angle strain is concerned, that is the optimum orientation. 

 On the other hand, cyclopentane does not exist in this geometry and the reason 

 is simply the torsional forces about the single bond. In this case all five single 

 bonds are in the worst possible orientation and you have about 14 or 15 kilo- 

 calories strain, if we may take the ethane value for the potential. This is a large 

 strain. 



What happens, of course, is that nature finds the compromise between the 

 various forces which yields the minimum net energy. By puckering the rings 

 somewhat at least some of the bonds can twist to near their potential mini- 

 mum. By accepting as much as 2^2 kilocalories bond angle strain you can lower 

 the torsional strain much more and gain an appreciably lower energy. 



