PRESENT PROBLEMS 293 



Billmann * have recently described a similar case, but very few in- 

 stances of this kind are known. 



Pfeiffer 2 has recently suggested a new interpretation of van 't 

 Hoff's hypothesis as applied to unsaturated compounds. Pfeiffer 

 assumes that unsaturated compounds retain essentially the same 

 configuration as the saturated compounds, from which they are 

 derived. On this side his interpretation is closely related to the old 

 theory of free valences, which, if I understand him correctly, is favored 

 by Professor Michael. Pfeiffer also brings his interpretation into a 

 close relationship to Werner's theory of inorganic metallic compounds. 

 The most serious objection to the theory is that it supposes the 

 existence either of trivalent carbon atoms or of free valences, in ethyl- 

 ene and its derivatives, an objection which has appeared to most 

 chemists very strong in the past. Pfeiffer points out, it is true, that 

 since the discovery of triphenylmethyl we can no longer deny the 

 possible existence of a trivalent carbon atom. 3 It would seem, how- 

 ever, that the great difference between the intense chemical activity 

 of triphenylmethyl and the comparative inactivity of ethylene 

 demonstrates that, if the latter does in reality have free valences, 

 the fact that there are two such valences reduces the activity of each 

 enormously. The inactivity of carbon monoxide may be significant 

 in this connection. 



A more serious objection to Pfeiffer's hypothesis lies in the fact 

 that he supposes so slight a difference in the configuration of fumaric 

 and of racemic acids that it is difficult to see why the former as well 

 as the latter might not be split into a pair of optically active bodies. 



We must admit, then, that we have, at present, no satisfactory 

 theory of double and triple unions, and that we have here a problem 

 which demands a large amount of further work before it is solved. 

 When the solution is reached we shall probably gain a new insight into 

 the perennial question of the structure of benzene, and our knowledge 

 of tautomerism will cease to be, as it is at present, almost purely 

 empirical. It is possible, perhaps probable, that Thiele's "conjugated 

 double unions" will contribute toward the solution. 



While I have no comprehensive theory with regard to double unions 

 to advance, I will, with a good deal of hesitation, venture to express 

 some thoughts with regard to the combination of atoms in general 



1 Ann. d. Chem. (Liebig), 332, 25. 



2 Ztschr. Phys. Chem. 43, 40. 



The fact that triphenylmethyl exists as a doubled molecule in solution should 

 not, I think, lead us to discard the monomolecular formula for it any more than 

 we consider that acetic acid has, in the ordinary sense of structure, a doubled 

 molecule because it exists as a doubled molecule in solution in benzene or in the 

 state of vapor just above its boiling-point, nor because it forms acid salts. In these 

 cases the chemical evidence appears to be more important and more conclusive 

 than the physical. It is probable that the doubled physical molecule is the result 

 of forces which do not produce a stable structure in the ordinary sense. 



