H. Benesi, J. Hildebrand
Aug 1, 1949
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Journal
Journal of the American Chemical Society
Abstract
Introduction The various colors of iodine in different solvents have attracted the attention of investigators for over half a century. Beckmann’ showed that the differences are not due to any change in the molecular weight of iodine and suggested solvation as the cause of brown color. Lachman2 supported this explanation, pointing out that the solvents which give a violet color are saturated, those which give brown colors are unsaturated. He stated that small additions of alcohol to a solution of iodine in chloroform suffice to shift the color stepwise from violet to brown. Hildebrand and Glascock3 found that when iodine and an alcohol were both added to a “violet” solvent, bromoform or ethylene bromide, the molal lowering of the freezing point was considerably less than additive, indicating combination of iodine with alcohol. Furthermore, they investigated colorimetrically the equilibria of iodine with ethyl alcohol, ethyl acetate and nitrobenzene dissolved together in a “violet” solvent such as carbon tetrachloride, chloroform or carbon disulfide, and found in each case an equilibrium constant corresponding to a 1 : 1 compound. They calculated the heat of combination in one case. There followed a long series of investigations of the general problem, well summarized by Gmelin4 and in a recent review by Kleinberg and Davidson.6 But the original division of iodine solutions into two or more distinct color classes became untenable with the work of Waentig, Groh’ and \X7alkers which showed that the position of the absorption bands of iodine solutions in the visible region moved gradually from violet to brown, the shift being only small in the case of benzene in which the color of iodine is a red-violet. We have long been curious about the chemical basis for iodine solvates. It is not a question of dipole moment, as Walkers concluded, for we have recently shown rather conclusively that the iodine solubility, which would be altered by solvation effects, bears little or no relation to the dipole moment of the solvent mole~ule .~ The alcohols are extraordinary dipoles, and their effect is readily understandable, but the case of benzene seemed