Plane chromatography

In the period between 1906 and 1952 there were some developments of importance. For example, the techniques of plane chromatography were developed. Earliest was the use of paper as a plane support, but when thin layers of silica gel were introduced as an alternative in the late... 

In plane chromatography the flow of the mobile liquid cannot be controlled as it can in the column methods. It is dependent on the surface tension y and the viscosity of the mobile phase as well as on the nature... 

Figure 10.6. Developing chambers for plane chromatography (a) descending— used with PC (b) ascending—used with TLC and PC (c) sandwich—used with TLC (d) horizontal—used with paper as shown, but also adaptable for HPTLC.

The stationary phase used in this procedure is a sheet of filter paper and hence the name Paper Chromatography for the method. The mobile used is liquid. Since the stationary phase is in a plane rather than in a column, the technique is categorized under Plane Chromatography . 

Like paper chromatography, thin-layer chromatography is a form of plane chromatography in that the stationary phase is held on a plane rather than in a column. Table 12.1 lists important stationary phases used in TLC along with the respective predominant sorption process operative with each of them. The solid phase is supported on to glass, metal or a plastic substance. (Microscope slides... 

Apart from its obvious simplicity, plane chromatography techniques require only minute amounts of samples. Moreover, chromatography on plane surfaces, rather than with a column, offers the unique advantage of two-dimensional operation. This operation exploits selective properties of two different solvents in developing a single chromatogram. 

TLC is carried out on a thin layer of adsorbent on a glass or plastic support (other supports have been used). It has sometimes been referred to as planer chromatography since the separation occurs in a plane. Paper chromatography, which is carried out using a piece of paper, usually filter paper, is very similar to TLC and will not be covered here. 

Diastereomeric amides of l,12-dioxa[12]paracyclophanedicarboxylic acid (20, as the first example of diastereomers with a plane of pseudo-asymmetry) could be separated by chromatography on silicagel on the basis of their 1H-NMR spectra tentative configurations were assigned to the amides 38). 

Biopolymers in Chiral Chromatography. Biopolymers have had a tremendous impact on the separation of nonsupernnposable. mirror-image isomers known as enantiomers. Enantiomers have identical physical and chemical properties in an achiral environment except that they rotate the plane of polarized light in opposite directions. Thus separation of enantiomers by chromatographic techniques presents special problems. Direct chiral resolution by liquid chromatography (lc) involves diastereomenc interactions between the chiral solute and the chiral stationary phase. Because biopolymers are chiral molecules and can form diastereomeric... 

Enantiomers have identical physical and chemical properties to one another except the direction in which they rotate plane polarised light (clockwise or anticlockwise). They may be separated by interaction with a second chiral species. This gives two diastereoisomers (if the two chiral centres are the same we can describe the diastereoisomers as optically pure meso AA and the racemic or rac form which itself occurs as two pairs of enantiomers, AA and AA) which do differ in their physical properties e.g. have different NMR spectra, can be separated by achiral chromatography etc). For example, Scheme 3.1 shows the experimental resolution of [Co(en)3]3+ using tartrate. 

Maleic anhydride reacts with cyclopenta-1,3-diene in a Diels-Alder reaction. Since there is a plane of symmetry, the reaction can lead to two achiral compounds, which are diastereomers of each other, containing an endo- or exo-oriented dicarboxylic anhydride group. These differ in absolute and relative configuration at the bond shared by both rings. Under normal conditions the Diels-Alder reaction proceeds stereospecifically to yield preferentially the endo product. Note that in the tricyclic product no trans fusion in the ring system is possible as a consequence of the reaction mechanism. Subsequent reduction of the products therefore affords two diols, which are also diastereomers of each other. These may be separated by chromatography on an achiral stationary phase. 

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