FUNDAMENTAL CONCEPTIONS 207 



i 



acid, HON = O, sulphur trioxide, O = S^ etc.; (2), those in which 



X o, 



an atom itself is unsaturated, i.e., does not exert its maximum valence 

 capacity, as, for instance, amines, R 3 N, thioethers, R, =S], methylene 

 derivatives, etc. We must assume that the remaining affinity units are 

 latent, or, what is far more probable, especially where two or four 

 affinity units are available, that they mutually polarize each other in 

 a manner entirely similar to unsaturated compounds containing 

 doubly or triply linked atoms. 



Finally we have a third class of unsaturated compounds, (3) those 

 containing closed atomic chains such as trimethylene, 

 CH 2 O 



/ \ / \ 



CH 2 CH 2 ,propyleneoxide, CH 3 CH CH 2 etc., which show 



apparently a saturated molecular system like the paraffines, and yet 

 react in a manner perfectly analogous to defines and methylene de- 

 rivatives. Fundamentally considered, these three classes of unsatur- 

 ated compounds manifest their chemical activity in the same way; 

 they absorb a great variety of other molecules and thus form com- 

 binations, called addition products. How does this union take place? 

 An unsaturated compound with its affinities polarized represents in 

 reality a saturated system; it cannot per se show chemical activity. 

 This is also true of molecular systems in which the atoms are bound 

 to one another by single affinity units. The sole basis for reactivity 

 in either case is the presence of a relatively greater or smaller number 

 of dissociated particles. The reactivity of any unsaturated, as well as 

 of a saturated compound, must in fact be directly proportional to the 

 ratio of such active particles present. If that ratio is very small, the 

 substance may be entirely inert; if it is greater, absorption of reagents 

 proceeds with regularly increasing speed. 



Experience has shown, furthermore, that many unsaturated 

 compounds cannot be isolated, but polymerize spontaneously. It is 

 clear that when the per cent of active particles present in an unsatur- 

 ated compound becomes relatively great, the possibility of their 

 uniting with each other to form condensed molecules increases 

 in fact, we may imagine a condition in which the active molecules 

 simply cannot be prevented from combining with each other. This 

 shows us why we cannot isolate and keep substances like formalde- 

 hyde, H 2 C = 0, or alkylcyanates, R O C = N, in the monomo- 

 lecular form. Similarly in many cases where attempts were made 

 to isolate methylene derivatives like mono- and diphenyl methyl- 

 ene, benzoyl and acetyl methylene, cyanmethylene-carboxylate, 



