Studies on the chromatographic behaviour of substituted phenols.

The effects of variations in extra-molecular, and molecular factors on chromatographic behaviour is reviewed. The relation between molecular structure and the chromatographic behaviour of substituted phenols is considered for the following systems. (a) Adsorption chromatography on alumina surfaces. (b) Reversed-phase thin-layer chromatography on cellulose impregnated with ethyl oleate. In adsorption chromatography, the mechanism responsible for the chromatographic process is thought to be hydrogen bonding between the phenolic group and the hydroxylated alumina surface. In the systems studied, the gross effect of this inter- molecular hydrogen bonding is so strong that, in the simple alkyl phenols, with no ortho-groups, the effects of substituents on chromatographic behaviour is negligible, and hence the Martin relation is not valid for these compounds. Where alkyl substituents are present in one or both ortho-positions, the values increase with an increase in the chain length of the substituent. This is attributed to a steric effect rather than to electronic ones. The bulkier is the ortho-substituent the greater is the steric effect. For the halogenated phenols, intermolecular hydrogen bonding between halogen atoms and the hydroxylated surfaces is important. In mixed halogeno-; alkyl-substituted phenols, the effect of the addition of a group to the molecule is dependent upon the natures of the groups already in the molecule. This is also so for the substituted nitrophenols. This represents a real deviation from the Martin relation. Internal hydrogen bonding, and external hydrogen bonding are both important in governing the chromatographic behaviour of the nitrophenols. The effects of other substituents in the nitrophenol molecules in modifying the strengths of these hydrogen bonds are also of importance. In the reversed-phase thin-layer chromatography, the behaviour of the phenols is dependent upon the removal of the phenol from the stationary phase into the mobile phase by the solvation of the phenolic group by the mobile phase. The Ivlartin relation is shown to be approximately valid in these systems, but is modified by the presence of other structural features in the molecule. Steric effects are considered to be more important than electronic effects in modifying chromatographic behaviour. The effect of internal hydrogen bonding is significant in the chromatography of the nitrophenols. Solvation of the nitro-group by the eluent is also important. For the polyamide/cellulose system, the chromatographic behaviour of the phenols is dependent upon the nature of the eluents. Por non-aqueous systems some real failures of the llartin relation are seen. In other cases, e.g. where ortho-groups are present in the molecule, the chromatographic behaviour is dependent upon the steric hindrance of the approach of the phenolic group to the polyamide surface. Here the Martin relation is approximately valid. In aqueous eluents, solvation of the pheiiolic group by the eluent is thought to govern the chromatographic process. The Ivlartin relation is approximately valid, but is modified by other constitutive effects, e.g. internal hydrogen bonding, the presence of double bonds and steric effects. In all systems it appears that, though steric effects cannot be entirely separated from electronic effects, the former are more important than the latter.