150 



K. S. PITZER 



Fig. 4 



The balance, even in the four-membered ring which is of much less interest, 

 seems to be in favor of a little distortion from the plane, although there it is 

 very small. As I said before, if you put atoms other than carbon into the ring, 

 the torsional forces are usually reduced and the balance will therefore move from 

 this back toward a planar ring. 



Likewise, if you put double bonds in a five membered ring the torsional forces 

 favor a flatter ring because the portion of the molecule aroung the double bond 

 tends to be planar. Cyclopentadiene is no doubt fiat. Cyclopentane is just on 

 the verge. The energy is substantially the same for a flat or slightly distorted 

 ring. 



In the six-membered ring (Fig. 4) the books in the older literature always 

 showed both a boat and chair conformation. If you look into those structures 

 in the same manner that we just did for cyclopentane, you find that the chair 

 structure is just ideal; it could not have been better. It staggers all the orienta- 

 tions of the single bonds and it gives tetrehedral bond angles at the same time. 

 So chair cyclohexane is a strain free structure for the molecule. 



The boat conformation has two of the C — C bonds in opposed orientation 

 and therefore presumably has a strain energy in the vicinity of 5 to 6 kilo- 

 calories. No one has found any evidence whatsoever for cyclohexane itself in 

 the boat conformation. Some highly substituted derivatives have been postu- 

 lated as having the boat form, but that arises from a compromise of other forces. 

 There is no evidence in the spectrum for any boat cyclohexane at room tempera- 

 ture or at the temperature in which the spectra were measured. I have no doubt 

 that it does appear at high temperatures and in fact included such heat capacity 



