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V. — On a General Method of Substituting Iodine for Hydrogen in Organic 

 Compounds, and on the Properties of Iodopyromeconic Acid. By Mr James F. 

 Brown,* Assistant to Professor Anderson, Glasgow. 



(Read 3d April 1854.) 



In a paper on pyromeconic acid read before this Society, and since published 

 in the Philosophical Magazine for September 1852, I have detailed the prepara- 

 tion and properties of a compound obtained by the substitution of an equivalent 

 of bromine for an equivalent of hydrogen in that acid. Having observed that 

 this substitution was very easily effected, I was induced to attempt the forma- 

 tion of an iodopyromeconic acid, in the hope of adding one to the very few in- 

 stances in which the direct substitution of iodine for hydrogen has been found 

 possible. For this purpose, I digested pyromeconic acid with tincture of iodine, 

 but no success attended the experiment, the acid remaining entirely unchanged. 

 The failure of this attempt led me to speculate as to its cause, and to contrive a 

 method of producing the required substitution which has proved entirely suc- 

 cessful, and has the further advantage of being perfectly general, so that its ap- 

 plication 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 in- 

 telligible the nature of the method in question. Selecting the production of 

 bromopyromeconic acid and of trichloracetic acid, as characteristic examples of 

 substitution, we have the following formulae representing the changes which 



occur : — 



C 10 H 4 O + Br 2 = C 10 H 3 Br 6 + H Br 



Pyromeconic Acid. Bromopyromeconic Acid. 



C 4 H 4 4 + Cl 6 = C 4 H Cl 3 4 + 3 H CI 



Acetic Acid. Trichloracetic Acid. 



In these, as in every other case of substitution, the chlorine and bromine ob- 

 viously perform a twofold function, one portion entering into the complex atom 

 in the place of an equivalent quantity of hydrogen, which is eliminated in com- 

 bination with another quantity either of chlorine or bromine ; and the new pro- 

 duct contains the same number of atoms, and is commonly said to belong to the 

 same type. In talking of such substitution, it is not unfrequently said, that the 



* It is with deep regret I have to record here the early death of Mr James Brown, who, in this 

 and a previous communication read before this Society, had given such high promise of future eminence. 

 He died at Glasgow on the 2d July, after an illness of only twelve hours' duration. — T. A. 



VOL. XXI. PART I. 



