DETERMINATION OF CONSTITUTION 87 



All these reactions lead to a common characteristic of 

 the constitution of the isomers, viz. the presence of a 

 methyl group (CH 3 ), which is regularly contained both 

 in the materials required for formation and the products 

 of decomposition. The constitution of each is therefore 

 definite to the extent of being 



(H 3 C)CNS. 



The difference is now simply shown in the above reactions, 

 both by the materials required for formation and by the 

 products of combustion, to lie in the fact that the methyl 

 group of the mustard oil is connected to nitrogen ; but that 

 of the thiocyanate to sulphur. This may be expressed by 

 the constitutional formulae 



H ; ,CNCS(methylmustardoil) ) H 3 CSCN (methyl thiocyanate). 



The following schemes will then serve to express the 

 reactions : 



For methyl mustard oil : 



H 3 CNH 2 + C1 2 |CS and 



For methyl thiocyanate : 



H 3 CS|CH3_4 1 Br 1 CN and 



It should be noticed that in this choice, made on the 

 ground that the formation and decomposition can easily 

 be expressed, something has been assumed as to the course 

 of the reactions, viz. that in reacting, a molecule changes 

 as little as may be. To illustrate this by an example, 

 we may note that hydrocyanic acid might conceivably be 

 formed from a body H 3 CNCS in this manner : 



H 3 C 



But then a double break would occur in the original 



