Chromatographic reviews最新文献

The separation of m- and p-disubstituted benzene derivatives has been studied in four systems. It is shown that the R_M values of these compounds cannot be correlated with the existence of charge separations or with the vectorial dipole moment. The p-isomers generally obey the group additivity principle, whereas m-isomers, when they differ, depart from this principle. The separation of m- and p-isomers is explained in terms of a competition between intra- and intermolecular hydrogen bonding. It is shown how in m-isomers, the presence of an ortho/para-directing group can affect hydrogen bonding by inducing a fractional negative charge on the para-carbon atom. Benzene derivatives containing two meta-directing groups appear to be inseparable by partition chromatography, and this is discussed. Certain compounds, such as substituted anilines, have anomalous R_M values and may contain, in hydroxylic solvents, small amounts of compounds of the type, RNH₃⁺OH^-. The chromatography of halophenols shows no influence of the inductive effect of the halogens; they separate according to their molar volume. There was no ortho-effect in halophenols in the reversed phase system studied, confirming that this effect is not spatial but mainly polar in origin.

The effect of tautomerism on R_M has been studied. The general nature of the effect on chromatography is discussed and equations obtained by means of which the existence of tautomerism can be verified chromatographically. If sufficient data are available from the study of archetypal compounds in which each form of the tautomeric group occurs free from other constitutive effects, it is possible, by these equations, to calculate the position of tautomeric equilibrium in a given compound. The procedure is illustrated with reference to the tautomerism of thienol (2-hydroxythiophen), p-nitrosophenol and 4-nitroso-I-naphthol. The procedure followed rests on the adherence of many group and structural ΔR_M parameters to the additivity principle of $\text{Martin}$'s equation.

The separations of steroid oestrogens achieved by paper chromatography have been reviewed with special reference to studied involving epimeric oestrogens and to oestrogens which have been isolated recently. Methods of detection of these compounds on paper chromatograms have also been discussed. The mobilities of thirty-four different oestrogens and their derivatives in many solvent systems have been tabulated.

It is hoped that this review will be found useful in providing a collection of data from which separations of mixtures of oestrogens can be planned.

Attempts to correlate the chromatographic behaviour of substances with their chemical structure must be based on accurate knowledge of R_F values and a survey of published work indicates that these may often be in error because of the practical difficulties of carrying out chromatography under ideal conditions. The theoretical limitations of chromatography in tanks are also discussed. In order to overcome certain of these difficulties, a new method for the accurate determination of R_F values, tankless or flat-bed chromatography, has been studied and its advantages examined. Chief among these are its extreme reproducibility, the possibility of running very large numbers of compounds together under near-ideal conditions and the precision of the experimental R_F values. A series of alkyl dinitrobenzoates was shown, by this method, to obey $\text{Martin}$'s equation with respect to ΔR_M(CH₂).

The chromatographic behaviour of seventy-seven phenols and closely related substances has been studied in a reversed phase system (ethyl oleate against 25 % aqueous ethanol) and the relationships between their R_M values and their structures elucidated and discussed. Constitutive effects in chromatography were studied by means of a new method, the use of atomic ΔR_M parameters. The methods of calculating these parameters for carbon, hydrogen and oxygen are described and illustrated. By this method it is possible to calculate the R_M value of any of these compounds from the R_M value of phenol itself. It is shown that the ΔR_M parameters for CH groups (arbitrarily expressed for convenience as atomic ΔR_M(H) parameters) vary depending on their proximity to the aromatic ring. Similarly, the atomic ΔR_M(O) parameters in ethers are profoundly influenced by the nature of the substituent vicinal to the oxygen atom. These effects are considered to be produced by permanent polarizations due to the resonance effects of alkyl groups in the molecules under consideration.