experiment

Experiment 2:
IDENTIFICATION OF UNKNOWNS BY TLC AND MP IN COMBINATION

Introduction: Thin layer chromatography (TLC) is one of the most valuable techniques in organic chemistry. This is a best method of separating and identifying mixtures of two or more compounds. The separation is accomplished by the distribution of the mixture between two phases: one that is stationary and one that is moving or mobile. Chromatography works on the principle that different compounds will have different solubilities and adsorption to the two phases, which will allow for their separation. Other than used to detect a given compound in a mixture,TLC also used to follow the course of a reaction, follow the purification of a compound, or evaluate the purity of a compound. TLC provide valuable information concerning the polarity and chemical nature of a compound.

Thin Layer Chromatography (TLC) is a solid-liquid technique in which the two phases are a solid, stationary phase and a liquid, mobile phase. The solid phase you will be using in today’s lab is a plastic plate covered with an adsorbent, in this case, silica gel. Aluminum is another common solid phase used. Because silica is a polar molecule, the components of the solution we will use in lab today will be separated based on their relative polarities. The more polar a molecule, the higher affinity it will have for the more polar silica plate and will therefore spend less time in the mobile phase. As a result, it will move up the plate more slowly. Conversely, a less polar molecule will spend more time in the mobile phase and will therefore move up the plate more quickly. The speed at which the molecules will move up the plate thus depends on the relative difference in polarity between the stationary and mobile phases, and will vary depending on the nature of the stationary and mobile phases used for separation.

The Rf or Retention factor, of a compound is a ratio of the distance a compound travels up the TLC plate to the distance the solvent traveled.This Retention Factor (Rf) can be used to identify molecules, as the value is molecule specific. The Rf for any given molecule will vary depending on the mobile and stationary phases used.

Rf = (distance migrated by spot) / (distancemigrated by solvent).

If the compound travels with the solvent front, the Rf is close to or equal to 1.0. If the compound does not travel at all and stays strongly adsorbed to the TLC plate the Rfis 0.0.

Objective:
-To study about the thin layer chromatography (TLC) technique
-To identify the unknown compound.
-To calculate the Rf,Retention Factor.
-To study about the melting point of compound

Procedure:

Prepare the TLC plate

Prepare the sample

Spot the TLC plate with the sample

Prepare the developing tank

Develop the TLC plate

Dry then visualize the developed TLC plate

Measure the Retardation Factor

Discussion: Chromatography is a sophisticated method of separating and identifying mixtures of two or more compounds. The separation is accomplished by the distribution of the mixture between two phases: one that is stationary and one that is moving or mobile. Chromatography works on the principle that different compounds will have different solubilities and adsorption to the two phases, which will allow for their separation. Thin Layer Chromatography (TLC) is a solid-liquid technique in which the two phases are a solid, stationary phase and a liquid, mobile phase. For this experiment I perform TLC by taking a TLC plate and using a pencil I make a mark where I going to be place the drops, I put the drop slightly above the eluent . The TLC trays that were used is a thin sheet of plastic coated in silica gel. Once the marks were place on the TLC plate a single drop of the mixture being tested was place between or on the mark, again being sure that the line and the drops are above the level of the eluent or solution. Once all of this was done to the TLC plate,I place the plate into a beaker with a small amount of eluent; just enough to cover the bottom of the beaker, and a watch glass was placed over the beaker.This whole time I makesure the TLC tray didn’t fall into the beaker that just the bottom edge was in the eluent. The eluent slowly ran up the TCL tray and when it was about a centimeter from the top the tray i pull it out of the beaker, and a mark was place to indicate how far the eluent or solution ran up the tray, and then I allow the tray to dry.After the TLC tray was dry it was place under a UV light in order to see how the mixtures separated and to also reveal their polarity. When under the UV light the spots were marked using a pencil.Finally I measure the Rf values,using measurement obtained.

The solid phase you will be using in today’s lab is a plastic plate covered with an adsorbent, in this case, silica gel. Aluminum is another common solid phase used. Because silica is a polar molecule, the components of the solution we will use in lab today will be separated based on their relative polarities. The more polar a molecule, the higher affinity it will have for the more polar silica plate and will therefore spend less time in the mobile phase. As a result, it will move up the plate more slowly. Conversely, a less polar molecule will spend more time in the mobile phase and will therefore move up the plate more quickly. The speed at which the molecules will move up the plate thus depends on the relative difference in polarity between the stationary and mobile phases, and will vary depending on the nature of the stationary and mobile phases used for separation. The difference each molecule travels along the adsorbent in relation to how far the mobile phase has traveled is called the Retention Factor (Rf) and can be used to identify molecules, as the value is molecule specific. The Rf for any given molecule will vary depending on the mobile and stationary phases used.

Question & Answer:
1.Define the term retardation factor?
In chemical chromatography, it is a measure of the relative mobility of components of a mixture through a stationary phase while experiencing the forces of a mobile eluent phase, based on relative intermolecular attractive forces and molecular size.In thin layer chromatography, is it the ratio of distance travelled by a component compared to the distance travelled by the eluent front from the point of contact with the mixture.
In column chromatography, it is the fraction of the component in the mobile phase at equilibrium.
2.At what position of a spot(top,middle,bottom) do you measure to obtain an accurate Rf value?
Middle of the spot.It is a ratio so you have to calculate the distance the spot has moved from the original position (measured from the spots centre) divided by the distance the solvent front has moved.
3.If you have chromatography of an alcohol and ether ,which compound would you expect to have the larger Rf value if the TLC plate was made with silica gel?
Ether is nonpolar substance whereas the alcohol is polar substance.Normally very polar substances on Aluminum Oxide do not migrate very far from the starting point.

4.If you run a chromatography(silica gel) of an alcohol(Rf 0.5) in ethyl acetate,which solvent would you add to make an Rf value of about 0.3?

5. If you run a chromatography(silica gel) of an alcohol(Rf 0.5) in ethyl acetate,which solvent would you add to make an Rf value of about 0.8?

Conclusion: For conclusion,I can say that from this experiment I able study about the thin layer chromatography(TLC) technique, about the method of applying this technique and use of this technique in chemical field.Other than that,I also able to identify unknown compound which is………………. Through this experiment.In addition. I also learn the method calculate the retention factor, Rf .The retention factor ,Rf is From this experiment I also study about melting point and use of melting point in identifying unknown compound. Finally as conclusion I can say that all the objective of this experiment is achieved successfully.

References :
1. Atkins R.C, Carey F.A 2002. Organic Chemistry: A Brief Course, 3rd Ed. New York, McGraw Hill.
2. Bailey S.P, Bailey C.A 2000. Organic Chemistry, 6th Ed. New Jersey, Prentice Hall.
3. Laboratory Manual, Organic Chemistry Lab, Parveen Jamal, 2010, IIUM

References: : 1. Atkins R.C, Carey F.A 2002. Organic Chemistry: A Brief Course, 3rd Ed. New York, McGraw Hill. 2. Bailey S.P, Bailey C.A 2000. Organic Chemistry, 6th Ed. New Jersey, Prentice Hall. 3. Laboratory Manual, Organic Chemistry Lab, Parveen Jamal, 2010, IIUM