Spectrophotometric Study of the Complexation of Iodine and Bromine with Tetrabutylammonium Halides and Cryptand 222 in Dichloromethane Solution

A spectrophotometric study concerning the interactoin between iodine and bromine with tetrabutylammonium iodide (TBAI), tetrabutyl- ammonium bromide (TBABr) and cryptand 222 (C222) has been performed in dichloromethane solution at 25°C. The results are indicative of the formation of TBA+X3- and  C222X+X3-  through equilibrium and formation of C222X+X-  through nonequilibrium reactions. The stability constants of the equilibrium reactions were evaluated from the computer fitting of the absorbance-mole ratio data. It was found that iodine complexes were more stable than bromine ones. Comparision of the spectra of I2-TBAI with the I2-C222 mixtures indicates that the isosbestic points of the two systems  are not identical. A similar behavior is observed for the corresponding bromine spectra. Comparision of the spectra of iodine complexes with those of bromine also indicates that in the spectra of iodine complexes 1) the wavelength of the isosbestic point is less than λmax of the free iodine and 2) addition of C222 or TBAI decreases the absorption intensity at λmax of iodine. However, in each case the reverse is observed for the spectra of bromine complexes. The possible reasons for the observed differences in various spectra are explained.