202 Mr. J. F. Brown on a General Method of Substituting 



me to speculate as to its cause, and to contrive a method of 

 producing the required substitution which has proved entirely 

 successful, and has the further advantage of being perfectly 

 general, so that its application will enable chemists to obtain 

 iodine substitution compounds in cases in which they have 

 hitherto failed. 



A few preliminary observations on the cause of substitution 

 will render intelligible the nature of the method in question. 

 Selecting the production of bromopyromecouic acid and of 

 trichloracetic acid, as characteristic examples of substitution, we 

 have the following formulae representing the changes which 

 occur : — 



CIO H4 06 + Br2 = C^o H3 Br O^ + H Br 



Pyroraeconic acid. Bromopyromeconic acid. 



C H'* O" + CF = C* H CP C+a H CI 



^_^_. ^_ — ^_ 



Acetic acid. Trichloracetic acid. 



In these, as in every other case of substitution, the chlorine 

 and bromine obviously perform a twofold function, one portion 

 entering into the complex atom in the place of an equivalent 

 quantity of hydrogen, which is eliminated in combination with 

 another quantity either of chlorine or bromine ; and the new 

 product contains the same number of atoms, and is commonly 

 said to belong to the same type. In talking of such substitu- 

 tion, it is not unfrequently said that the atom of chlorine 

 simply displaces or pushes out the atom of hydrogen, and so 

 comes to occupy its place, but a very slight consideration 

 enables us to see that it is really dependent, not so much on 

 the quantity of chlorine which remains in the compound as on 

 that which escapes in combination with hydrogen. The ori- 

 ginal substance in all cases of substitution, forms a perfect 

 molecular group in which the individual affinities of the dif- 

 ferent elementary atoms are properly balanced, and the whole 

 remains in a quiescent state. But when another element, such 

 as chlorine, comes in contact with this complex group of atoms, 

 it immediately exerts its affinity for the hydrogen, with which 

 it easily combines, and withdraws the whole or part of it from 

 the compound. In this way a gap is produced in what was 

 before a perfect group, the balance of the affinities of its ele- 

 ments is destroyed, and there must either be a complete read- 

 justment of its molecular arrangements, or some other element 

 must stop the gap and produce another perfect molecule, not 

 differing from the original substance in the arrangement of 

 its parts, but only in the presence of one or more atoms of a 



