Hydrogen bonding Modified phenolics

THF and methanol employed as organic modifiers of mobile phase provided a considerable difference in selectivity based on the polar interactions between solutes and the organic solvent molecules in the stationary phase. Acidic compounds, phenols and nitroaromatics, were preferentially retained in the THF-based mobile phase, whereas esters and ketones were preferentially retained in the methanol (a hydrogen-bond donor) containing mobile phase. The system presented here seems to be very practical because any laboratory possessing two sets of HPLC equipment and two C j g columns can attempt similar 2D HPLC by simply changing the mobile phase for the two dimensions. 

This hydrogen bonding capacity expressed by the polar selectivity is not limited to an increase in retention for phenols. Packings with embedded polar groups that excel in this feature also exhibit increased retention for analytes with other functional groups. Specifically, carboxylic acids at acidic pH and with acetonitrile as the organic modifier and compounds with sulfonamide functions at acidic and neutral pH exhibit significant... 

However, the reactivity of phenolic hydroxy groups can be modified by a fused heterocyclic ring. Thus, hydroxy groups peri to a carbonyl group, e.g. (600), are hydrogen bonded they do not react with diazomethane, and are difficult to acylate. This allows selective reactions in polyhydroxychromones. 

Copigmentation involves complexation phenomena, generally at low energy (hydrogen bonds and hydrophobic interactions), either between the various forms of anthocyanins or between antho-cyanins and other, mostly colorless, phenolic compounds (coumarins, phenolic acids, flavonols, fla-vanols, etc.) These bulky complexes modify the cation resonance and prevent the substitution of carbons 2 and 4 (Mazza and Brouillard, 1987). 

On occasion, as the organic phase becomes loaded with the metal-extractant complex, the solubility of the latter in the diluent is exceeded and the metal complex comes out of the solution as a third phase. This is obviously a severe disadvantage in processing, so other compounds are added to the organic phase to remove any such tendency for third-phase formation. These compounds are called modifiers and usually are compounds such as nonyl phenol, decanol, or similar compounds capable of forming hydrogen bonds with the metal complex. Again, there may be a problem of differential solubility of the modifier and extractant. 

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