“2 
Cas) 
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 formule representing the changes which 
occur :— 
C,, H, 0, + Br, =C,, H, Br O, + H Br 
sulreal css Bis ant pg ee alae, 
Pyromeconic Acid. Bromopyromeconic Acid. 
C,H, 0, + Cl, =C, HC, 0, + 3HO 
—— ——— 
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 eithér 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. ve) 
