Chromatographic Purification

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Chapter: Organic Chemistry : Structure Determination of Organic Compounds

The first step in the identification of any compound is to obtain that material in pure form. The most common way to achieve this goal today is to use chromatography.


CHROMATOGRAPHIC PURIFICATION

The first step in the identification of any compound is to obtain that material in pure form. The most common way to achieve this goal today is to use chromatography. While a discussion of the many separation and purification techniques which utilize some form of chromatography are outside the focus of this book, all rely in one way or another on the interaction of molecules with a surface. Such interactions depend much more on the chemical properties of a molecule (functional groups, polarity, unsaturation, etc.) than on physical properties of the bulk substance (boiling point, vapor pressure, etc.). Furthermore the interactions of a compound with a surface allow it to be resolved (separated) from other molecules by placing it in a flowing system (mobile phase). When the molecule is not adsorbed to the surface, it moves over the surface at the same velocity as the mobile phase (Figure 11.1).


When it is adsorbed to the surface, it does not advance with the mobile phase. Since adsorption is an equilibrium process, those compounds which are only weakly adsorbed (M in Figure 11.1) to the surface spend a greater portion of time in the mobile phase and thus move over the surface faster than compounds which are more strongly adsorbed (M in the figure) and thus spend more time immobilized on the surface. 

Because different compounds are adsorbed differently on the surface, each can travel at a different rate over the surface. By collecting the effluent from the surface as a series of fractions, individual com-pounds can be separated cleanly from other components in the original mixture because each component exits the surface at a different time.

Many different mobile phases have been utilized to provide the forward velocity for nonadsorbed molecules. If the mobile phase is a gas, then the technique used is gas chromatography (GC). In GC, the surface to which the molecules adsorb can be a wide variety of materials which are often prepared by coating an inert surface with a polymer whose properties are related to its structure. In this way the surface properties and hence adsorption of the solid surface can be varied to give the best chromatographic resolution.

If a liquid is used as the mobile phase, the technique used is liquid chromatography (LC). The solid adsorbent is constrained in a tube or column through which the liquid mobile phase flows. Any number of solvents, buffer solutions, or supercritical fluids can be used as liquid mobile phases. High-pressure liquid chromatography (HPLC) is used if pressure is needed to force the liquid phase through the tube. If the liquid phase moves over a thin adsorbent surface propelled by capillary action, the technique used is thin-layer chromatography (TLC). In general, two types of surfaces are used as the solid phase.

In “normal”-phase LC systems, the solid phase is a polar solid such as silica gel (most common) or alumina and the liquid is generally an organic solvent of low polarity. In such a case, polar compounds bind more strongly to the polar silica gel surface and thus travel more slowly along the surface, whereas nonpolar components have a lower affinity for the polar surface and a greater affinity for the nonpolar eluting solvent. They consequently elute from the column more rapidly. In reversed-phase systems, the surface of silica gel is modified to produce a nonpolar hydrocarbon-derivatized surface, and the mobile phase often is a polar, aqueous solvent mixture. In this case polar compounds have a low affinity for the nonpolar surface; they remain dissolved in the polar mobile phase and elute more rapidly. Nonpolar components have a higher affinity for the nonpolar surface than the polar mobile phase and elute more slowly. Using various chromatographic techniques, it is possible to separate most mixtures into the individual components efficiently and very rapidly.

In addition to chromatographic techniques, traditional purification methods such as recrystallization, distillation, or sublimation are also employed. Such methods often require much more material than chromatographic techniques.

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