Chromatography

Chromatography

I. Introduction

In organic chemistry mixtures of different compounds are used in experiments. In experiments naturally occurring materials are not 100 percent pure. Organic compounds interact with adsorbents by a variety of interactions. If the compound is nonpolar, it can only have weak Van der Waals attraction for the adsorbent. Molecules that are more polar may interact more strongly by dipole-dipole interactions, coordination, and hydrogen bonding. The most important rule of chromatography is that the more polar compounds will be adsorbed most strongly on adsorbents (stationary phases), while nonpolar compounds will be very weakly adsorbed. In a chromatography experiment, the non-polar compounds (poorly adsorbed) will be held least powerfully and move quickly through the plate. Polar compounds, on the other hand, will be slowed on their process through the plate by their strong interactions with the solid phase.

Chromatography is used in the separation of a mixture by passing it in solution over an adsorbent (Alumina or Silica Gel) to determine purity; also it involves the passage of a mobile phase across a stationary phase in a column (Usually compound mixture is present in the mobile phase). Soon as the mixture contacts the stationary phase some (or all) of the components of the mixture are adsorbed; as the adsorption of the solution process continues along the length of the column, the proper choice of mobile phase, stationary phase the mixture will be separated in the column and its various components will emerge at different times. In Thin Layer Chromatography (the physical separation of a mixture into its individual components by distributing the components between a stationary phase (the porous TLC plate) and a mobile phase (the solvent that moves through the stationary phase and carries the material that needs to be separated), a liquid solution is applied to a solid adsorbent drawing a developing solvent up the TLC plate. As this solvent passes through the capillary spot, the mixture will be dissolved and will begin to move with the solvent front. Since different materials will be dissolved and reabsorbed at different rates, separation will take place. The slide is removed from the chamber once the solvent front reaches a predetermined spot near the edge farthest from the point of spotting which is known as developing the plate. The degree of separation calculated by the retention factor (Rf ) will depend on the relative solubility and relative strength of adsorption of the components of the mixture. Column chromatography is another method that physically separates all of the components (usually nonvolatile) of a mixture. The driving force to separate components is gravity. Thin layer and column Chromatography methods work based on polarity differences between components in a sample.

II. Chemical Properties and Safety

Compound / Structure M.W (g/mol) Melting Point (oC) Boiling Point (oC) Density ( g/mL) Safety

Petroleum Ether (C6H14)

82.2 <-73 90-100 .65 Irritant to skin and eyes; Flammable

Fluorene(C13H10)

166.22 116 295 1.20 Irritant to skin and eyes; harmful to aquatic life

9-Fluorenone (C13H8O)

180.20 82-85 342 .90 Irritant to skin and eyes. If swallowed can be harmful to G.I Tract

Dichloromethane (CH2Cl2)

84.92 -96.7 39.6 1.32 Irritant to skin and eyes; harmful if swallowed

Alumina

Al2O3

101.96 2072 2977 1.06 Hazardous in case of skin contact (irritant), of eye contact (irritant), of ingestion, of inhalation

Silica (SiO2)

60.083 1713 2950 .45 Silica dust is hazardous when very small (respirable) particles are inhaled.

III. Experimental Procedure

The chromatography column was clamped in a vertical position with the stopcock closed. The funnel was placed on top of the column while approximately 12.0 g of dry neutral alumina was added to the column; followed by 1.0 cm of clean sand for a covering. 25.0 mL of petroleum ether was added to the column and petroleum ether was used to wash the inner walls of the column to remove alumina that were on the sides. The stopcock was opened and the solvent drained into an Erlenmeyer flask until the solvent reached the top of the sand. The solvent was reused and poured back into the column to allow it to completely wet the alumina; the solvent was drained to the top of the alumina to minimize bubbles. 0.1g of a 1:1 mixture of fluorene and 9-fluorenone was added in a small glass vial and dissolved in 0.5 mL of petroleum ether. A Pasteur pipette was used to add 7-10 drops of dichloromethane into the glass vial solution until the mixture was completely dissolved. A new Pasteur pipette was used to carefully transfer the glass vial solution directly to the top of the column. The stopcock was opened until the liquid level was at the top of the alumina. Approximately 1.0-2.0 mL of fresh petroleum ether was added to the top of the column to wash the sides of the column and drained to the top of the alumina. The column was filled with approximately 35.0 mL of fresh petroleum ether. The stopcock was opened to collect 5 test tubes the eluant in 5.0 mL fractions. Slow movement of a yellow band went down the column. Once petroleum ether had been drained to the top of the sand, the eluant was changed by adding 20.0mL of dichloromethane, and collected in 5 test tubes giving a total of 10 test tubes. A strip of silica gel chromatogram sheet was obtained and a pencil was used

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