Paper chromatography and chemical structure

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.