General Theory

TLC is terrific analytical tool for separating mixtures in a sample. In this section are disputed the details that the separation, and also expand ~ above the general discussion of ar 2.1.B.

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In all forms of chromatography, samples equilibrate between stationary and also mobile phases. In almost all applications that TLC, the stationary step is a silica or alumina adsorbent and also the mobile step is an necessary solvent or solvent mixture (the "eluent") the rises up the key (equation 3).

\<\ceX_\text(silica/alumina) \rightleftharpoons \ceX_\text(solvent) \label3\>

Silica gelatin (shown in number 2.16) is created of a network that silicon-oxygen bonds, v \(\ceO-H\) bond on the surface, and also a class of water molecules. Silica gelatin \(\left( \ceSiO_2 \cdot x \ceH_2O \right)\) is used in this discussion, yet is structurally analogous to alumina \(\left( \ceAl_2O_3 \cdot x \ceH_2O \right)\). This really polar stationary phase is paired through a relatively nonpolar mobile phase (an organic solvent or solution), in what is referred to as "normal phase" TLC. Return this is the most common kind of TLC (and what will be concentrated on in this section), "reverse phase" TLC (with a nonpolar stationary phase and a polar mobile phase) is periodically used.

Figure 2.16 shows exactly how acetophenone would certainly cling to the surface of silica gel v intermolecular forces (IMF"s). In this case, acetophenone have the right to hydrogen bond (the IMF indicated in figure 2.16a) come the silica surface ar through its oxygen atom. As eluent flows end the sample (Figure 2.16b), an equilibrium is established in between the sample being adsorbed ~ above the stationary phase and dissolved in the cell phone phase. As soon as in the mobile phase, the compound moves increase the plate v the circulation of fluid (Figure 2.16c) to later on readsorb ~ above the stationary phase further up the plate. The result \(R_f\) the the compound is dependency on the lot of time spent in the stationary and also mobile phases.

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Figure 2.16: structural diagrams of compound bound come a silica coated TLC bowl (side view). In truth the silica great is lot thicker than displayed (\(0.25 \: \textmm\) the powder), and the surface is much more porous and also uneven 보다 implied. A) Acetophenone spotted ~ above the baseline that the TLC plate, b) Eluent crawling up the TLC plate, c) Acetophenone in the mobile step after break its IMF"s with the silica surface.

The equilibrium distribution between the 2 phases relies on number of factors:

It counts on the toughness of intermolecular forces in between the sample and also the stationary phase. A compound that forms solid IMF"s with the silica or alumina will frequently favor the stationary phase, and will spend much of the elution time adhered come the plate. This means it will certainly spend much less time in the mobile step (which is the only way for that to take trip up the plate), leading to it to finish up low on the TLC plate, and have a short \(R_f\). compounds that have actually oxygen or nitrogen atoms should be able to hydrogen bond with the stationary phase (have solid IMF"s with the stationary phase), and thus will have actually lower \(R_f\) values than compound of similar size that have the right to only interact through London dispersion forces (LDF"s). It relies on the toughness of interaction in between the sample and the mobile phase. as the mobile phase is always less polar than the stationary phase in typical phase TLC, polar link will often tend to have actually a lesser affinity because that the cell phone phase than nonpolar link (based top top the "like dissolves like" principle). Therefore, polar compounds have tendency to spend much less of the elution time mobile 보다 a nonpolar compound, therefore will travel "slower" up the plate, and have a short \(R_f\).

The degree of attraction by a compound to the stationary and also mobile phases lead to the same conclusion:

The more powerful IMF"s feasible with the stationary phase (often the an ext polar functional groups on a compound), the an ext time the compound will be stationary \(\rightarrow\) reduced \(R_f\). The an ext polar sensible groups existing on a compound, the less it tends to be attracted to the less polar eluent, and also the much less time the compound will be mobile \(\rightarrow\) reduced \(R_f\).

Thus, a compound v a lower \(R_f\) often tends to have much more polar functional teams than a compound with a higher \(R_f\) (summarized in number 2.17).

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api/deki/files/124923/Nichols_Screenshot_2-2-9(1).png?revision=1&size=bestfit&width=889&height=413" />Figure 2.18: TLC of benzyl alcohol (lane 1), benzaldehyde (lane 2), and also ethylbenzene (lane 3). The plate was eluted making use of 6:1 hexanes:ethyl acetate and visualized with UV light.

Benzyl alcohol and benzaldehyde have polar functional groups so thus had lower \(R_f\) values than ethylbenzene, which is totally nonpolar. Both compounds room able come hydrogen bond come the polar stationary phase (Figure 2.19a+b), therefore are much more strongly attractive to the stationary phase 보다 ethylbenzene, which interacts just through weak London dispersion pressures (Figure 2.19c). Together the least "polar" of the series, ethylbenzene is also the best dissolved by the weakly polar eluent. Because that these reasons, ethylbenzene spent the least time in the stationary phase and also the many time in the mobile phase, i m sorry is why it travel the furthest up the plate and also had the highest \(R_f\) of the series.

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This general trend is demonstrated in figure 2.21b+c, whereby the TLC of three UV-active compounds (lanes 2-4) to be run using two various mixed solvents. The an initial plate to be run making use of a 6:1 hexane:ethyl acetate mixture, which means the solvent was created by utilizing 6 volumes of hexane because that every 1 volume of ethyl acetate. This blended solvent is largely nonpolar due to the high percent of hexane, yet is more polar than straight hexane, due to the presence of some ethyl acetate (which has actually polar bonds, number 2.21a). The second plate was run utilizing a 3:2 hexane:ethyl acetate mixture, i m sorry is much more polar than the 6:1 mixture because there is a greater percentage the ethyl acetate present.