Organic Chemistry Lab Report #4 PDF

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Melina Rosado Rosales

Chemistry 2612 Organic Chemistry Lab

Lab Report #4

TLC and Column Chromatography

02-27-17

Introduction Chromatography is a technique used to purify individual chemical compounds from a mixture of two or more compounds, by taking advantage of the polarity of each individual compound which determines if the compound will be in the mobile phase or the stationary phase. The stationary phase is a solid or immobile substance and is known as the absorbent. During the experiment alumina was utilized as the absorbent in the column chromatography and silica was utilized as the absorbent for the thin layer chromatography (TLC). The most commonly used absorbents are silica gel and neutral alumina. During the column chromatography, a thin layer of sand is placed on top of the alumina to maintain the structural integrity of the alumina so a straight band of chemicals will form in it and when the compounds are separated it will result in a purer fraction. The mobile phase consists of a liquid called the eluent which makes the compounds separate and move down the column. The type of eluent used varies in polarity based on what compounds are going to be separated. The first solvent used tends to be less polar because the less polar compound has an affinity for the less polar solvent (like dissolves like) such as petroleum ether. The more polar compound will be more likely to stay in the stationary phase with the alumina while the less polar compound will want to be in the mobile phase and is then fractioned out. Once this fraction is completed a more polar eluent is then added such as dichloromethane or DCM. This eluent has an affinity for more polar compounds and makes the remaining, more polar compound move down the column faster because of the affinity for like properties. Once all the fractions are taken a TLC (Thin Layer Chromatography) is then performed. Using silica gel paper, the fraction is dotted on the bottom of the paper and then placed in solvent in a developing chamber which is then absorbed via capillary action. The purpose of the TLC is to identify the RF

(Retardation Factor) with that of a known compound to support the evidence that it was separated correctly. What the RF describes is the distance traveled by the individual molecules. If the distance traveled is the same, then this may indicate the molecules are the same molecule, or simply that they have similar RF values.

Chemical Properties and Safety Compound

Molecular Weight (g/mol)

Boiling Point (°C)

Melting Point (°C)

Fluorene (C13H10)

166.21

295.00

116.50

-

Irritant of skin, eye, ingestion & inhalation

9-Flourenone (C13H8O)

180.21

342

83.50

-

-

Irritant of skin, eye, ingestion & inhalation Toxic for aquatic life

-

Flammable Volatile

-

Reacts with metals Corrosive Toxic vapors

Petroleum Ether (CH)

Dichloromethane (CH2Cl2)

42 - 62

84.92

40.00

-97.00

Hazards

Experimental Procedure The chromatography column was set up vertically with stopcock closed. Funnel was placed on top of column ad 12.0 g of dry neutral alumina was added. Alumina was covered in thin layer of sand approximately 1 cm, and approximately 25 mL petroleum ether was added to the column. The petroleum ether was used to wash inner walls to get remaining alumina. The stopcock was then opened and solvent was allowed to drain through until it was at the top of the sand. Last step was repeated to allow the sand to tightly compact in the tube and to ensure the alumina was proper wet. 0.1 g of a 50:50 mixture of fluorene:9-flurenone was placed in a vial and dissolved with petroleum ether approximately 0.5mL. Using a Pasteur pipette 7-10 drops of DCM was added to vial allowed to completely dissolve the mixture. The solution was then transferred into the top of the column and the sides of the column were rinsed with additional petroleum ether approximately 2mL. The stopcock was then opened and the solution was allowed to drain to the top of the alumina. The column was then filled with fresh petroleum ether approximately 35 mL and drained into test tubes so 5 mL fractions could be taken. Once petroleum ether reached the top of the sand, 20 mL DCM was added to the column. More fractions were taken for a total of 10 fractions and tested via TLC.

Data and Observations

Distance traveled by solvent 1.2 cm

Flourene 0.5 cm

9-Flourenone

0.7 cm

Calculations

RF = Fluorene RF = (

(

Distance Traveled by molecule ) Distance Traveled by solvent

0.5 cm ) 1.2cm = 0.416

9-Flourenone RF = (

0.7 cm ) 1.2 cm = 0.583

Conclusion/Discussion Column chromatography separates compounds based on the polarity of the compounds. During this experiment, we observed the less polar compound fluorene have a higher affinity to the less polar solvent petroleum ether, and the more polar compound 9-flourenone have a higher affinity to the more polar solvent DCM. When petroleum ether was added to the column a yellow band moved down the column slowly indicating the less polar compound fluorene being in the mobile phase, and the more polar compound being in the stationary phase. During this time the less polar compound fluorene was being fractioned out of the column. When the more polar solvent DCM was added, the yellow band was observed moving down the column indicating that the more polar compound 9-flourenone was in the mobile phase being fractioned out rather than being in the stationary phase. The affinity of the two compounds to the two solvents was determined by the IMF’s of the compounds and the solvents. Like dissolves like, polar solvents will dissolve polar compounds and non-polar solvents will dissolve non-polar compounds. The same concept occurred during the TLC. A strip of silica gel chromatogram sheet was utilized as the absorbent and due to the polarity of silica it kept the polar molecule 9-flourenone in the stationary phase; whereas the petroleum ether initiated the non-polar molecule fluorene into the stationary phase. By manipulating the mobile phase, organic compounds can be separated efficiently taking advantage of the difference of the polarity. This theory is supported during the TLC when the distance traveled by the non-polar fluorene was 0.5cm yielding a RF of 0.416; whereas the distance traveled by the polar 9-flourenone was 0.7cm yielding a RF of 0.583. This is due to the 9-flourenone having a higher affinity to the silica gel chromatogram sheet and

staying in the mobile phase, but fluorene has a higher affinity to petroleum ether so it traveled upwards and further....