Hydrogen bonding. Part 14. The characterisation of some N-substituted amides as solvents: comparison with gas–liquid chromatography stationary phases

Abstract

Equations previously used for the characterisation of GLC stationary phases have been found to be equally suitable for the characterisation of common solvents. Thus equation (a) has been applied to solubility data for series of solutes on N-formylmorpholine (NFM), N-methylpyrrolidinone (NMP), N,N-dimethylformamide (DMF), and N,N-dimethylace amide (DMA).

SP =c+r·R₂+s·π₂*+a·α₂^H+b·β₂^H+I· log L¹⁶(a)

In equation (a), SP can be log V°_G or log L for a series of solutes on a given solvent where V°_G is the specific retention volume and L is the Ostwald solubility coefficient. The solte parameters are R₂, a polarisability parameter; π₂*, the solute dipolarity; α₂^H, the solute hydrogen-bond acidity; β₂^H, the solute hydrogen-bond basicity; and log L¹⁶ where L¹⁶ is the solute Ostwald solubility coefficient on n-hexadecane at 298 K.

It is shown that at 298 K all four amides have about the same dipolarity, as judged by the s-constant, and have nearly the same hydrogen-bond basicity, as judged by the a·α₂^H term: all have zero hydrogen-bond acidity so that b= 0 in equation (a). Comparison can be made between results for NFM and NMP at 393 K and results for some GLC stationary phases. The two amides are less dipolar than tricyano(ethoxy) propane and diethyleneglycol succinate, about the same as Zonyl E-7®and Carbowax®, and more dipolar than poly(phenyl ether). The amides, however, have rather more hydrogen-bond basicity than any of the above five GLC phases. It is suggeted that equation (a) can be used as the basis of method for characterising condensed phases, such that common solvents as well as GLC stationary phases can be included within the scope of the method.