88 MOLECULAR STRUCTURE 



molecule H,CNCS, whilst in formation from H 3 CSCN only 

 a single one, as shown by the symbols 



H 3 C I NO I S and H 3 CS I ON. 



O I O 



Intramolecular atomic displacements. The question 

 thus becomes very important, whether this assumption as 

 to the mechanism of reaction always corresponds to the 

 facts. With regard to this, a distinction must first be 

 made between inorganic and organic, i. e. carbon compounds. 

 In the former, which show the phenomenon of isomerism 

 exceptionally, or not at all, the product is independent 

 of the original condition of the components ; e.g. barium 

 sulphate (BaSO 4 ) from oxide (BaO) and sulphur trioxide 

 (SO. } ) is identical with that from the superoxide (Ba0 9 ) 

 and sulphur dioxide (SO 2 ) ; here the peculiar rigidity which 

 gives the compound the characteristic of its origin is 

 absent. In organic compounds, however, with the appear- 

 ance of isomerism, this is not the case. And that the 

 rigidity is such that only the smallest possible change 

 occurs in reactions, as in the case of mustard oil discussed 

 above, is rendered probable by the fact that different 

 determinations of constitution, all based on this principle, 

 yield the same result : thus the formation and decomposition 

 of methyl mustard oil lead to the same conclusion, and 

 several other methods of formation and decomposition not 

 mentioned here. There are cases, however, in which such 

 agreement is wanting. 



We may first consider such a case, which was at first 

 difficult to understand, but was afterwards fully explained, 

 so affording a prospect of removing other uncertainties 

 previously existing. The body in question is allyl thio- 

 cyanate, NCS(C 3 H 5 ). The generally applicable method of 

 preparation of thiocyanates, in which potassium thiocyanate 

 (NCSK) is treated with the iodide of the radicle it is 

 desired to introduce in this case allyl (C 3 H 5 ) leads to a 

 body of the required composition, according to the equation 



NCSK + IC 3 H 5 = NCS (C 3 H 5 ) + KL 



