Ethyl groups, analytical determination

Channaine was first isolated by Bodendorf and Krieger from the ethyl acetate fraction of the Channa alkaloids. Analytical data provided the empirical formula CieHigNOs (6) but there were some indications that channaine is represented by a dimeric formula (11). Channaine formed crystals from methanol or ethyl acetate and was found to be optically inactive 11). Spectral and analytical properties showed that channaine contains two methoxyls, probably in a veratryl group. NMR- as well as IR-spectra indicated that the alkaloid has no A -methyl nor carbonyl functions. From Zerewitinoff determinations and NMR-spectra values... 

Table 2 gives the thermal and analytical properties of the series of p-alkylphenyl esters of 1,4-cyclohexanedlcarboxyllc acid. Four members of this series were prepared In which the alkyl group was varied from ethyl through n-pentyl. The first two members, the ethyl and propyl substituted compounds, have straightforward thermal transitions, both showing only enantlotropic nematic mesophases. A smectic phase first appears In the n-butyl member and the butyl and pentyl compounds are both smectic and nematic. The solids In this series are less complex than In the alkoxy series, and there Is no Indication of solid polymorphism. The n-pentyl does become more complex In Its mesophase and three smectics are observed, two of which are monotropic. The transition temperatures of these are too close to allow resolution of the peaks on the DSC, and no determination of the separate heats was made. 

The hydrophobicity of a stationary phase is responsible for the retention of the analytes. This is mainly determined by the specific surface area and the carbon content, discussed earlier. But as far as the selectivity is concerned, in reversed-phase (RP)-chromatography hydrophobic interactions usually play a much less important role than is generally believed, ionic/polar interactions, and steric aspects - if they come into play - are more dominant factors. Only by means of these influencing factors can analytes be differentiated. This is also the reason why a correlation between retention and selectivity factors is rather seldom observed, discussed later. Such a correlation may at the best be expected from a uniform RP mechanism in which van der Waals interactions predominate, which is seldom the case (homologous series or simple pairs of analytes such as toluene/ethyl benzene). This is because the molecules to be analyzed mostly have groups that are capable of ionic/polar interactions. Even with hydrophobic aromatic compounds, we are dealing with induced dipoles in addition, the polarity may increase through resonance. 

Following the information obtained from CE separations, Chung et al applied I8C6H4 to CCC, using a CCC analytical device with a toroidal coil column (internal volume 7.4 mL). The baseline resolution of small amounts of gemifloxacin was achieved in a biphasic solvent system consisting of 1-butanol - ethyl acetate - bis(2-hydroxyethyl)amino-tris(hydroxymethyl)methane (Bis-Tris) acetate buffer. The presence of multiple ionisable groups in the derivatized crown ether CS and in the racemate makes pH an important factor in this separation. A pH value around 6.0 was determined to be optimal. The influence of CS concentration on enantioselectivity was also studied. 

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