INTRODUCTION. 21 



This kind of action is called substitution, and the 

 newly -formed body a substitution-product of the ori- 

 ginal body. Iodine, in other respects so similar to 

 chlorine and bromine, when alone acts never, or at 

 least only exceptionally, in the manner described, as 

 hydriodic acid is formed at the same time, and this 

 has the tendency to cause a reverse substitution, i.e. a 

 displacement of the iodine in organic compounds, con- 

 taining iodine, by hydrogen. If, however, a body be 

 added with the iodine which has the property of 

 removing the hydriodic acid as soon as formed, for 

 instance iodic acid, substitution-products containing 

 iodine can in many cases be obtained. An addition 

 of small quantities of iodine aids materially the substi- 

 tuting action of chlorine upon organic compounds. In 

 the presence of water, chlorine sometimes acts as an 

 oxidizing agent. Organic compounds containing chlo- 

 rine likewise result, as a rule, by the action of hydro- 

 chloric acid or the chlorine compounds of phosphorus. 



Concentrated nitric acid acts in most cases in a simi- 

 lar manner to chlorine. A certain number of hydro- 

 gen atoms is eliminated, and for each of them the 

 monovalent group NO* (hyponitric acid) enters the 

 compound ; for example : 



C 6 H 6 + N0 2 .OH = C 6 H 5 .N0 2 -1- H 2 0. 



Benzol. Nitrobenzol. 



Compounds resulting in this way are called nitro- 

 compounds, or nitro-substitution-products. The forma- 

 tion of these bodies is very much aided by mixing the 

 concentrated nitric acid with twice its volume of con- 

 centrated sulphuric acid. Nitric acid acts frequently, 

 especially by continued boiling, only as an oxidizing 

 agent. 



Concentrated sulphuric acid acts upon a great many 

 organic bodies similarly to nitric acid. One or more 

 hydrogen atoms of the compound are displaced by the 

 monovalent group SO 2 . OH; for example: 



C 6 H 6 + SO 2 = C 6 H 5 .S0 2 .OH .+ H 2 0. 



Benzol. SulpUobenzolic acid. 



