STUDIES O S T H E PHOTOSEPU'SITIZIKG ACTIOX B'I- I O D I S E ; ISOMERIC TRAPU'SFORMATIOK OF ALLO-CISKAblYLIDESE ACETIC ACID INTO THE NORMAL FORM I K METHYL ALCOHOL, ETHYL ALCOHOL, CHLOROFORM, A i N D I N M I X T U R E S O F ALCOHOL, CHLOROFORLI A S D W'hTER
BY J. C. GHObH, D. S . DAS GLPT.4, X. C. ROY, AND H. K. CHATTERJEE
Liebermann' discovered that allo-cinnamylidene acetic acid is very slowly transformed into the normal variety under the influence of light but in presence of iodine, this photochemical transformation is very rapid. Ghosh, Gupta and Mitra have published certain preliminary results on the above transformation in solutions of chloroform and methyl alcohol.* I n the present paper the results of a thorough study of this reaction, as photosensitized by iodine, will be given. The method of estimating the quantity of allo-acid which has been changed into the normal form, depends on the fact that the normal acid is sparingly soluble in benzene. The following experimental procedure was adopted. 2 C.C. of the solution containing the allo-acid and normal acid were taken out and poured into a I O O C.C. stoppered bottle containing anhydrous sodium thiosulphate. The iodine immediately reacted with sodium thiosulphate and the solution became colourless. The bottle was then connected by a rubber cork, through a tower filled with fresh soda-lime with a n oil-pump of large capacity. The solution was evaporated a t 2ooC gradually by working the pump; the allo-acid during the process of evaporation did not undergo any further transformation into the normal form. 3 C.C.of dry benzene were then added and the bottle shaken vigorously to dissolve the acids in the solid residue. The benzene solution was then poured into a long stoppered tube and kept overnight in a thermostat. I C.C. of the clear liquid was sucked up by a pipette through a plug of glass wool fitted a t its lower end and titrated with baryta solution. The allo-acid is freely soluble in benzene but the normal acid is only slightly soluble. Table I gives the solubility of the normal acid in benzene at different temperatures in gram molecules per C . C .
TABLE I Temperature in "C.
*
Solubility
x
I06
15
24.5
22
30,4
Temperature in "C.
Solubility
30 35
44.4
Ber., 28, 1446 (1895). J. Indian Chem. SOC., 2, 241 (1925);3, 273 (1926).
x
I06
57.5
2 i j 2
GHOSH, DAS GUPT.4, ROY A S D CHATTERJEE
From the solubility curve of normal acid in benzene, the quantity of normal acid at the tempm-ature of the thermostat was determined and the difference between the total quantity of acid in I C . C . of benzene and the solubility of normal acid gave the amount of allo-acid in the solid residue of the bottle which has dissolved in I C.C.of benzene. The total 3.110-acid in the residue is three times this quantity and was present in 2 C.C.of the solution. Experimental arrangement :-.I 1000 c.p. point-0-lite lamp TWS used as the source of light. The lamp was run by a constant current of 6 amperes. Convex lenses of different f o r d lengths were used to obtain parallel beams of light, of varying intensil y. The light was passed through filters (description of which will be given afterwards) and entrred the reaction vessel (4 X 4 X I cm.) enclosed in a double-jacketed copper box with a glass window to admit light. This copper box was maintained at a constant temperature by circulating water from a thermostat. The intensity of incident radiation was measured by a A1011 thermopile and a Moll galvanometer calibrated by means of a Hefncr lamp. \\.ith the :lid of coloured filter solutions, recommended by Plotnikow, parallel beams of light covering a very narrow strip of the speetrum, were isolated, and their photochemical action at different intensities was studied separately. Preparation of t h e R e - a g r n t s : - Allo-cinnnmylidene acetic acid CGHiCH:CH.CH :C".C'OOH was prepared according to Liebermann's method. It is essential that the acid should always be obtained in an extra pure condition by repeated crystallisations. Otherwise, the values of the velocity co-efficients of transformation, under identical conditions, become erratic and irreproducible. Extra pure iodine was further purified by sublimation. IIerck's extra pure chloroform was further distilled over fused calcium chloride and only the fraction having the constant n. P. 61OC was collected anti preserved in a green stoppered bottle in darkness. Pure methyl alcohol and ethyl alcohol were further distilled over lime and the fractions at, boiling points 6j0c and ;8"C' respectively, were used for preparaing the solutions. IIerck's pure benzene was distilled over fused calciuni chloride. Freshly distilled mater was once more distilled over potassium-permanganate and strong sulphuric acid and preserved in a resistance glass bottle. This was mixed up with the pure ethyl alcohol and used as 3 mixed solvent. It has been observed by Ley and Engelhnrdt and by Stobbe and Schmidt that solutions of iodine in alcohol on keeping, more particularly in light, show a variation in the absorption spectra in the ultraviolet due probably to the formation of hydriodic acid and hydrogen tri-iodide. Ghosh and Mitra (loc. cit.) also observed that the ageing of the solutions of iodine and of the allo-acid in methyl alcohol vas responsible for considerable diminution in the value of the velocity co-efficients of photo-catalysis; hence in our investigation Polutions were freshly prepared before each experiment and exposed to light immediatdy after mixing them. End-point of the Reaction:-Ghosh and Gupta found that the percentage conversion is loo in chloroform 3s solvent. I n pure methyl alcohol, ethyl Ber., 28, I441 (189j).
PHOTOSENSITIZING ACTION O F IODISE
2773
alcohol and in the solvent mixtures used in these investigations, it was found that the percentage conversion of allo-acid to normal variety a t equilibrium was 90. This percentage conversion was found to be independent of ( I ) the concentration of iodine, ( 2 ) the intensity of incident light, (3) the wavelength of light used, (4) the concentration of allo-acid, and ( 5 ) the temperature of the system. Such behaviour is to be expected if both iodine and light act simply as catalysts. The negligible effect of temperature in the percentage conversion at equilibrium is to be ascribed to the fact that the energy change in this transformation is very small. The simple equation
K
= I
an
t log -for the velocity of a monomolecaa - x
ular reaction cannot be applied to this case. We have rather to use the more complicated equation for homogeneous opposing unimolecular reaction:-
_- -2 . 3 t,
log;, Z-X,
Where Kl is thr velocity of transformation of allo-:icid to normal form, a is the initial conc. of allo-acid. z is the conc. of norninl acid at rquilibrium and x1is the quantity of allo-acid convert4 into normal form in time t:. In the following tables thp value? of I
