Chromatography: Thin-Layer Chromatography (TLC) & Column Chromatography Part 1, p. 184: Separation of spinach pigments by TLC. (4 th Ed. P. 180) Part 2, p. 192: Separation of Fluorene and Fluorenone by column chromatography.(4 th ed p. 188) Important Concepts General Theory of Chromatography Definitions: R F, eluant, eluate, stationary phase, adsorbent, mobile phase TLC technique Column Chromatography Chem 341 - Chromatography, Copyright - D.J. Dyer & SIUC 1
Theory of Chromatography Chromatography - a technique to separate and purify organic compounds. Mobile Phase - consists of a liquid or gas that flows continuously. Stationary Phase - a solid support that typically remains heterogeneous; for gas chromatography the stationary phase is a solid support coated with a viscous liquid resin. Theory - The components of a mixture will interact differently with the solid support such that one component moves faster than the other. - Organic compounds will exhibit different affinity for the mobile phase vs. stationary phase. Thus, an equilibrium is established between the two phases with a partition coefficient (K). Since K will be different for each component (or eluate) of a mixture, they will separate out into migratory bands, which flow through the system. Chem 341 - Chromatography, Copyright - D.J. Dyer & SIUC 2
Theory of Chromatography - cont. Adsorbent - the solid phase (stationary phase), which is typically very polar. Silica gel (SiO 2 ) x or Alumina (Al 2 O 3 ) x are the most common. - polar molecules stick to the adsorbent more tightly than nonpolar molecules. Eluate - the compound that we are trying to separate and purify. Eluant - the mobile phase, usually an organic solvent or mixture of solvents. As the solvent flows through the adsorbent, the molecules will become detached from the stationary phase and move with the mobile phase. - polar compounds stick more tightly to the adsorbent, so they spend less time in the mobile phase and therefore move more slowly relative to nonpolar compounds. Chem 341 - Chromatography, Copyright - D.J. Dyer & SIUC 3
Chromatographic Techniques in Organic Chemistry 1. Column chromatography (flash chromatography) 2. Thin Layer Chromatography (TLC) 3. High Performance Liquid Chromatography (HPLC) - similar to column chromatography, but much more efficient and for smaller quantities. 4. Gas Chromatography (GC) - for volatile compounds. Chem 341 - Chromatography, Copyright - D.J. Dyer & SIUC 4
Thin Layer Chromatography (TLC) TLC is used for the rapid separation of small quantities < 1mg. Organic chemists use TLC to: - monitor the course of a reaction. - identify unknown compounds (i.e. two compounds will travel the same distance if they are identical). - determine an appropriate solvent system (eluant) for column chromatography (e.g. R F ~ 0.35). The Retention Factor (R F ) describes the distance an eluate travels relative to the eluant. RF a b (distance traveled by compound) (distance traveled by solvent) develop plate b a Chem 341 - Chromatography, Copyright - D.J. Dyer & SIUC 5
TLC - cont. R F depends on polarity of solvent More polar compound travels further larger R F Less polar compound travels less distance smaller R F How do we visualize organic compounds that are not colored? - Examine under UV-lamp for fluorescence. UV - Iodine vapor. - Misc. chemical treatments (e.g. phosphomolybdic acid). Chem 341 - Chromatography, Copyright - D.J. Dyer & SIUC 6
Procedural Details for TLC of Spinach Extract 1. Grind up spinach with a mortar and pestle in a mixture of ethanol and petroleum ether. 2. Transfer extract to a test tube with a disposable pipette. Avoid transferring solid. 3. Remove aqueous layer by sucking out from the bottom with a pipette. 4. Wash organic layer twice with water. 5. Filter through a plug of cotton in a pipette, into a new dry test tube. 6. Add sodium sulfate to the test tube in order to remove water from the organic phase. You should notice that the solvent goes from cloudy to clear after Na 2 SO 4 is added. There is no need to transfer this to an Erlenmeyer flask. 7. Mark a starting line on a TLC plate with pencil and spot the spinach extract on the line; be sure to spot the plate towards the middle and away from the edges. Cloudy Spinach Why add filter paper? Filter Pipette cotton Chem 341 - Chromatography, Copyright - D.J. Dyer & SIUC 7
Procedural Details for Part 2 - Column Chromatography ~ 1 cm Sand Alumina Glass Frit 1. Fill column 75% with eluant (petroleum ether). 2. Let a little solvent flow through the frit and stopcock. 3. Add alumina powder slowly while tapping the sides of the column to avoid large clumps. 4. Once the alumina has been added, then let solvent flow through column until it is packed tightly. Make sure that the solvent does not reach the level of the alumina, as air may cause bubbles and cracks in the adsorbent layer. It is OK to re-use the solvent that is in the collection beaker while packing the column. 5. After the column is packed tightly, add ~ 1 cm of sand above the alumina. 6. Allow solvent to drain until it reaches the level of the sand, then load the column with your crude sample. 7. Allow a small amount of solvent to flow through the sand until you are confident that your compound has been transferred to the alumina layer. Then fill the column with eluant and open the stopcock so that there is a moderate flow of solvent. 8. Since fluorene is not colored, use a watch glass to collect a drop and see if there is residual solid after the drop evaporates. If so, start collecting your first Residual fluorene on watch glass fraction. Continue collecting until there is no residue. Chem 341 - Chromatography, Copyright - D.J. Dyer & SIUC Fluorene Fluorenone 8 O
Column Chromatography - cont. 9. Once you have collected the two compounds, confirm the purity by performing a TLC experiment with 90/10 - Pet. Ether/Ethanol as the eluant. 10. Evaporate off solvent in the rotovap and weigh product. 11. To clean the column, let all the solvent flow out, then invert the column and tap out as much as possible into the plastic container in the hood. Then place on the vacuum to suck out the remainder. 12. After forcing the alumina out, squirt acetone through the stopcock, while it is under vacuum until all of the silica and sand is removed from the inside of the column. Wipe the outside of the column with a damp towel before returning. Normal vs. Reverse Phase Chromatography - In normal phase chromatography the adsorbent is highly polar, therefore polar compounds move more slowly than nonpolar compounds. Typically you start eluting with a nonpolar solvent. - In reverse phase chromatography the adsorbent is coated with a hydrocarbon so that it is very nonpolar, therefore polar compounds move more rapidly than nonpolar compounds. Typically you start eluting with a polar solvent. Chem 341 - Chromatography, Copyright - D.J. Dyer & SIUC 9
Problems with Chromatographic Separations Good Column Cracked Column Broad Bands in Column Eluant Sand Cracks open up channels and the dissolved compounds race down these channels without interacting with the adsorbent. Cracks occur when solvent goes below the level of the sand and air gets into the adsorbent. Migratory bands Poor loading and poor choice of eluant may cause broad migratory bands. The compounds diffuse out as they travel down the column and may never separate completely. Chem 341 - Chromatography, Copyright - D.J. Dyer & SIUC 10
Safety Issues - the organic solvents are highly flammable - disposable pipettes are sharp and can easily puncture your skin - alumina and silica gel powder are hazardous to the lungs, avoid dispersing dry powder into the air and breathing it in. Chem 341 - Chromatography, Copyright - D.J. Dyer & SIUC 11