Iodine for Hydrogen in Organic Compounds. 203 



different sort from those which it previously contained. Con- 

 sidered in this point of view^ it becomes at once obvious that we 

 fail to produce the substitution of iodine for hydrogen, not from 

 any inability of the former to occupy the place of the latter, but 

 simply because it has not a sufficiently powerful affinity for 

 hydrogen to withdraw it from the compound, so as to leave an 

 empty space into which another portion may enter. It occurred 

 to me that as chlorine and bromine in causing a substitution 

 exercise the two different functions already alluded to, one 

 quantity withdrawing hydrogen, and the other simply slipping 

 into the vacant space, it might be possible to produce an iodine , 

 substitution by associating that element with some substance 

 having a sufficiently powerful affinity for hydrogen, to open the 

 door as it were, and leave nothing for it to do but to step into 

 the place prepared for it. 



For this purpose I selected the bromide of iodine, as being 

 most likely to fulfill, the required conditions. It was prepared by 

 agitating bromine-water with a considerable excess of iodine, 

 and decanting the reddish-brown solution from the undissolved 

 residue. When this bromide was added to pyromeconic acid, a 

 change rapidly occurred, the solution became colourless, and 

 iodopyromeconic acid was produced. Having succeeded in this 

 way, I then tried the chloride of iodine, and having found it to' 

 act equally well, I made use of it in all my subsequent experi- 

 ments. The chloride of iodine was prepared by passing a rapid 

 current of chlorine through finely-powdered iodine suspended 

 in a small quantity of water and kept in continual agitation, 

 care being taken to stop the process before the iodine was 

 entirely dissolved. A moderate heat is produced during the 

 combination, and the fluid should be kept as cool as possible. 



In order, therefore, to obtain iodopyromeconic acid, a freshly- 

 prepared solution of the chloride of iodine is mixed with a cold 

 saturated solution of pyromeconic acid. The solution is in- 

 stantly decolorized, and the new acid is deposited in the form 

 of fine delicate plates, which are so abundant as to render the 

 fluid almost semisolid. The only precautions necessary are, to 

 avoid the use of a hot solution of the acid, and an excess of the 

 chloride, as the product, under such circumstances, becomes 

 coloured, owing to the occurrence of a further decomposition, to 

 be afterwards referred to. The change which first takes place 

 is represented by the equation — 



CIO H4 0^ -f I CI = CIO H3 I 0^ -f- H CI. 



The crystals of iodopyromeconic thus precipitated are in a 

 short time filtered off and washed with cold water ; they are 

 then finally dissolved in boiling alcohol, from which they again 



