206 CHEMISTRY 



Up to 1897 the presence of bivalent carbon had been established 

 in the following compounds, (1) carbon monoxide, O = C; (2) the 

 alkyl and aryl isocyanides, RN =-C; (3) prussic acid and the cyanides, 

 HN = C, MN = C; (4) fulminic acid and the fulminates, HO - N = C, 

 MO N = C. (2), (3) and (4) are all compounds containing the iso- 

 cyanogen radical. 



In 1897 the presence of bivalent carbon was established in a series 

 of nitrogen free carbon compounds obtained from acetylene. They 



H 



are the mono-and dihalogen substituted acetylidenes, ^ C = C and 



X 

 X 



^ C = C [X = Cl Br or I]. The corresponding members of the acetyl- 



X 



ene series, XC=CH and XC = CX, do not exist, although we have 

 substances like CH 3 C = CI, C a H 5 C = C X, whose properties are in 

 marked contrast to those of the acetylidene derivatives. 



Diiodacetylidene, which possesses an odor deceptively like that of 

 the isonitriles, dissociates at 100 with violence into iodine and dia- 

 tomic carbon, I 2 = C = C I 2 + C = C ; the latter cannot be isolated as 

 such, but polymerizes explosively to graphite and amorphous carbon. 

 The mono- and dihalogen substituted acetylidenes are all poisonous 

 and spontaneously combustible compounds, possessing, therefore, like 

 methylisocyanide a marked affinity for oxygen Up to the present 

 time it has not been possible to isolate compounds containing 

 bivalent carbon other than those mentioned above. We are, how- 

 ever, now in a position to explain clearly why we cannot hope by 

 methods now known to isolate methylene and its homologues as 

 such, although these substances play a great role in many of the 

 fundamental reactions of organic chemistry. In order to approach 

 this point more intelligently, let us consider briefly the properties of 

 unsaturated compounds in general, their possibility of existence, etc. 



II. On the Unsaturated Compounds 



The unsaturated compounds may, first of all, be divided into three 

 categories, namely; (1) those in which two atoms, which may be the 

 same or different, are bound doubly or triply to each other by two 

 or three affinity units, such as defines, acetylenes, chlorine, C1 = C1, 



R 



oxygen, O = O, aldelhydes, ^C = 0, alkylcyanides, RC = N, nitric 



H 



