TLC (Thin Layer Chromatography)

TLC (Thin Layer Chromatography)

Abstract

TLC (Thin Layer Chromatography) is a technique that is highly utilized in monitoring reactions. Also, it helps to determine the appropriate solvent system that helps in carrying out separations while utilizing column chromatography. In the conducted TLC experiment, Silica was used as the stationary phase because of its high polarity. Furthermore, silica plates were used. The solvent mixture consisted of ethyl acetate and hexane. The mixture was put in solution on the plate, and then the plate was run through, allowing the solvent combination to move to the top of the plate via capillary action. Based on the polarity of those components that were within the mixture, different travelled different distances towards the top of the plate. Those compounds that travelled short distances were considered to be more polar and thus stuck to the silica gel that was polar. Those who travelled for long distances were considered to have the ability to diffuse into the solvent and hence they were non-polar. The distance travelled by the compound was measured, and this is what was termed as Rf (retardation fraction). The value of Rf is between one and zero and is determined through measuring the distance in which compounds move from the line at the base, and the value is divided by the length of the solvent from baseline.

Results

From the experiment, the fractions were identified. Fraction three was identified as acetylferrocene, while fraction one was recognized as Ferrocene. This was done by identifying the colour of compounds because it was easily noted from the TLC plates. The Refer of these compounds was determined by measuring the distance in which compounds move from the line at the base, and the value is divided by the length of the solvent from baseline. In this case, Ferrocene Rf values were 0.89; after travelling a distance of 8.9, and it was divided by 10. On the other hand, the acetylferrocene Rf value was 0.31, and the mixture Rf was 0.29, respectively, as demonstrated in the table.

As the experiment was carried out, the need to use a cap that was tightly sealed was shown because it ensured that the sample was not out from the beaker sides.

Discussion

The TLC technique utilized in the experiment helped to show that column chromatography had been used to obtain acetylferrocene and Ferrocene. TLC was able to separate the components because of their functional group polarity. When the separating technique is used, there is a stationary and a mobile phase (Pavia et al., 2010). As noted, from the experiment that was carried out, ethyl acetate at 10% was the solvent together with hexane at 90%. Silica formed the stationary phase. Because a higher percentage of pure ethyl acetate would be too polar, only 10% of it was used (Schrenk & Gheorghiu, 2003). It was clear that, when U.V. light was used, fraction 2 was invisible, and this meant that acetylferrocene and ferrocene concentration was too low.

In the experiment, the movement of more polar compounds was quite slow because they were bound more tightly. While less polar compounds attach themselves less securely, something that causes their progress to be rapid, thus, it is bright that the solvent polarity has a crucial role in ensuring separation is good while various solvents that are different or solvent mixtures are used (Schrenk & Gheorghiu, 2003). TLC helps to achieve analysis objectives. Through using a known compound or a standard, it is easier for another unknown compound to be identified. Also, it is used to follow reaction progress over some time through monitoring product formation and the initial material disappearance (Pavia et al., 2010). Analysis of TLC is easy and very fast to carry out and requires the material to be used to be in a small amount.

Conclusion

It was evident that TLC could have analyzed the effectiveness of column chromatography. After running TLC, only a spot appeared, and this showed that the purification was proper. Furthermore, conceptual confirmation of acetylferrocene and Ferrocene was possible. This was because compared to acetylferrocene, ferrocene molecules were less polar. Thus, the solvent front carried Ferrocene with it, but acetylferrocene was not moved. The reason behind this was that acetylferrocene had an acetyl group that came from one of its rings, and this made it polar, while Ferrocene had two that was aromatic, and this made it a stable molecule because it was uncharged.

Questions

Question 1

1. The results obtained from TLC indicated one Rf value and one spot, and this showed that one compound was present.
2. If the compounds present are more than one, then it would be better for the composition of the solvent system to be changed, and the TLC be rechecked.

Question 2

R.f = The spot distance traveled/solvent front distance traveled

= 5.7cm/13 cm

= 0.43

Question 3

The solid phase provided by Silica is polar, while the mobile phase of cyclohexane is non-polar. Thus, within the column, there are polar compounds that will highly interact with the solid phase and travel less distance, making them stay for a longer period at the point of origin, and this will give a lower Rf value. The compounds that give an Rf value, which is high, had a lower polarity. Thus, the expected order is that the non-polar compounds with the highest Rf and polar compounds will be having the lowest Rf. The compound with the lowest Rf will be benzoic acid due to their polar carboxyl group, and then benzyl alcohols will have the next lowest because they are slightly polar. At the same time, biphenyl will have the highest Rf value because they are non-polar.

References

Pavia, D.L.; Lampman, G.M.; Kris, G.S.; Engel, R.G. Small Scale Approach Organic Laboratory Techniques, 3rd ed.; Cengage Learning: Ohio, 2010; pp 262-282.

Schrenk, J., & Gheorghiu, M. (2003). 5.310 Laboratory Chemistry, Spring 2003.

The University of Virginia. Fall 2013 Organic Chemistry Laboratory Manual; P.S. Publishing, 2013; pp 57-62.