Chromatography and Molecular Polarity

Oxidation of sulfides results both in sulfoxides and sulfones, as well as starting material. 

These can usually be easily separated by thin layer chromatography (TLC) on silica gel. 

Electrostatic potential map for methyl cyclohexyl sulfoxide shows negatively-charged regions (in red) and positively-charged regions (in blue), either of which is capable of sticking to a polar stationary phase. 

There is actually no unique way to calculate (or measure) atomic charges, simply because there is no way to uniquely partition a molecule s electrons among the atoms. For example, it is impossible to say what fraction of the electrons contained in the electron density surface for hydrogen fluoride belongs to fluorine. None of the partitions shown below is more reasonable than any of the others. 

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Normally in chromatography (except affinity chromatography) effects of molecular shape on retention are of minor importance in comparison to the effects of molecular size and molecular polarity. In the case of planar non-polar PAH isomers, retention was linearly related to a shape descriptor (a degree of elongation of the analyte molecule) [43,541. 

The separation of the polysaccharide components utilizes their different solubUities, polar groups, extents of branching, molecular weights, and molecular flexibUities and may be accompUshed batchwise or with easUy automated column techniques such as column or high performance Uquid chromatography. These procedures have been summarized in several reviews (3,141—143). 

Figure 4.27 Flow chart for coluwi selection based on sample type (m - molecular weight). PLC precipitation-liquid chromatography SEC = size-exclusion chromatography lEC - ion-exchange chromatography HIC hydrophobic interaction chromatography LSC liquid-solid chromatography RPC - reversed-phase liquid chromatography BPC (polar) bonded-phase chromatography and IPC - ion-pair chromatography.

CO, reforming reaction was conducted at 500-750°C, reactants mole ratio of CH3 CO, He = 1 1 3, and space velocity = 20000-80000 1/kg/h. Methane oxidation was conducted at 150-550 °C using 1 % CH in air mixture (2 ml/min CH4 198 ml/min air) at space velocity = 60000 1/kg/h, and MIBK (4000 ppm in 150 ml/min air introduced by a syringe pump) combustion at 100-500°C and space velocity of 10000-30000 h 1. Catalytic reactions were conducted in a conventional flow reactor at atmospheric pressure. The catalyst sample, 0.1-0.3g was placed in the middle of a 0.5 inch I.D. quartz reactor and heated in a furnace controlled by a temperature programmer. Reaction products were analyzed by a gas chromatography (TCD/FID) equipped with Molecular Sieves 5A. Porapak Q, and 15m polar C BP 20 capillary column. 

T Rezanka, M Podojil. Preparative separation of algal polar lipids and of individual molecular species by high performance liquid chromatography and their identification by gas chromatography-mass spectrometry. J Chromatogr 463 397-408, 1989. 

The follow-up section will deal with separation methods based upon (a) molecular size and related to it hydrodynamic volume (size-exclusion chromatography and ultrafiltration), (b) molecular size and related to it molecular diffusivity (field-flow fractionation), and (c) charge/size ratio and related to it molecular polarity (electrophoresis and mass spectrometry). Also reviewed will be hyphenated techniques or those that combine separation by chromatography or electrophoresis with spectral detection. 

Liquid-Liquid Chromatography. Liquid-liquid chromatographic (LLC) separations result from partitioning of solute (analyte) molecules between two immiscible liquid phases (10). The liquid mobile and liquid stationary phases ideally have little or no mutual solubility. The stationary liquid phase is dispersed on a column of finely divided support. The use of a nonpolar mobile phase and a polar stationary phase is referred to as normal phase LLC. Under these conditions, less polar solutes are preferentially eluted from the column. Reverse phase chromatography employs a nonpolar stationary phase and a polar mobile phase. Generally, polar compounds elute more rapidly with this technique. Reverse phase chromatography, useful for the separation of less polar solutes, has found increased application in occupational health chemistry. It is optimally suited to the separation of low-to-medium molecular weight compounds of intermediate polarity. 

Wheeler, J.R. and McNally, M.E. 1987. Comparison of packed column and capillary column supercritical fluid chromatography and high-performance hquid chromatography using representative herbicides and pesticides as typical moderate polarity and molecular weight range molecules. Journal of Chromatography, 410 343-53. 

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