FRACTIONATION OF ISOTOPOMERS OF AROMATIC HYDROCARBONS BY BIS (THIOCYANATO) TETRAKIS (4-METHYLPYRIDINE) NICKEL (II)
Abstract
Bis(thiocyanato) tetrakis (4-methylpyridine) nickel (II) in the formation of inclusion compounds with a variety of guest molecules, such as p-xylene and benzene, shows a preference for one compound over the other; benzene is preferred over toluene in a competitive inclusion study.('46) This phenomenon has been employed in the separation of isomers of nitrotoluenes, p-, o, m-xylenes and a host of aromatic hydrocarbons.('52,53) In the experiments described here inclusion in the complex, Ni (4-mepy)(,4) (SCN)(,2), is used for the fractionation of isotopomers, benzene and hexadeuterated benzene, protiated and deuterated p-xylene. Two methods were employed, a static equilibration and chromatography. In the static experiment, the complex was shaken with an excess of a mixture of p- (CH(,3))(,2) C(,6)H(,4) ('(h(,10))) and p- (CD(,3))(,2) C(,6)H(,4) ('(d(,6)h(,4))) and (CD(,3))(,2) C(,6)D(,4) (d(,10)) dissolved in n-pentane. The inclusion compounds which form were shown to have a higher D/H ratio and the mother liquor a lower D/H ratio than that of the original mixture. The values of the ratios (D/H) were measured with the mass spectrometer for the original mixture, R(,O), the solid phase, R(,S), and the equilibrium liquid, R(,L). Plots of 1n R(,s) vs 1n R(,L) gave a straight line of slope 0.970 for the d(,6)h(,4) - h(,10) and 0.943 for the d(,10) - h(,10) system. These slopes were taken to be unity leading to the conclusion that the solid phase is an ideal solid solution of one inclusion compound in the other. Since by Gibbs-Duhem equation the activity coefficients must be one and the separation factor is proportional to the ratio of the activity coefficients it follows that (alpha) is a constant. Plots of R(,S) vs R(,L) also gave a straight line of slope 1.090 and 1.067 for d(,6)h(,4) - h(,10) and d(,10) - h(,10) systems respectively. The slope of these lines thus represent a kind of average value of the separation factor, (alpha). In preliminary chromatographic experiments benzene and toluene were separated on a 1.5 cm column of Ni (4-mepy)(,4) (SCN)(,2) (.) y (4-mepy) in 2 ml total volume of elution with n-pentane solution .43M in 4-methylpyridine. Fractionation of protio- and deutero-pxylene was accomplished by elution with MeOH/H(,2)O solution, 0.4 M in 4-methylpyridine and 0.4 M in NH(,4)SCN, on a column prepared by equilibrating the complex in this base solution for a week. The chromatographic method accentuated the single stage separation factor; an average (alpha) value of 1.405 was obtained with 2 cm columns and a constant flow rate of 8 - 12 ml/hr. A disadvantage in the process was the fact that the p-xylenes were not completely eluted by the base solution. The preference of the deuterated p-xylene over the protiated compound was due to the difference in their polarisability. Their shape is the same and the difference in the molecular dimensions is not very significant in the static equilibration experiments. In chromatographic separation, this is important as diffusion through the channels is sensitive to size and steric effects. The higher (alpha) value in chromatography may be due to the contribution of this factor.
Subject Area
Organic chemistry
Recommended Citation
OFODILE, EDOKOBI SAMUEL, "FRACTIONATION OF ISOTOPOMERS OF AROMATIC HYDROCARBONS BY BIS (THIOCYANATO) TETRAKIS (4-METHYLPYRIDINE) NICKEL (II)" (1980). ETD Collection for Fordham University. AAI8020078.
https://research.library.fordham.edu/dissertations/AAI8020078
