1 6 ORGANIC AGRICULTURAL CHEMISTRY 



H H H H H H 



III III 



H- C- C- C- I and H - C-C- C-H Propyl iodides 



H H H H I H 



or CH3 - CH2 - CH2I and CH3 - CHI - CH3 



These two iodine atoms are evidently in different positions in 

 the compound, i.e. have different relations to the rest of the 

 compound. The fact that two propyl iodide compounds exist 

 isomeric with each other indicates that our supposition of the 

 difference of the two substituted hydrogen atoms is correct. 



The two isomeric propyl iodides are known as propyl iodide 

 and as iso-propyl iodide (isomeric propyl iodide). 



If now by the reaction with sodium a new methyl group is 

 placed in the position of the iodine in these two propyl iodides, 

 we shall have two hydrocarbons formed each of the composition 

 C4H10, and in which a difference in structure exists like that in 

 the isomeric propyl iodides, i.e. : 



CH3 - CH2 - CH2I -> CH3 - CH2 - CH2 - CH3 



Propyl iodide Butane 



CH3-CH-CH3 ->CH3-CH-CH3 



I CH3 



Iso-propyl iodide Iso-butane 



Here also fact and theory agree, for there are known two 

 butane hydrocarbons isomeric with each other. Without de- 

 veloping the idea further we can gain some conception of the 

 meaning of isomerism and the possibility of the existence of 

 isomeric compounds both among the hydrocarbons themselves 

 and among all the different classes of substitution products. 

 Such isomerism, when the difference in the isomeric compounds 

 is explained by difference in structure or by difference in the 

 position which a substituting element or group takes, is known 

 as structural or position isomerism. We shall later have to do 

 with a further development of the idea of isomerism when we 

 consider the space relationship of the molecule in connection 



