Experiment 5: Column Chromatography

Transcription

1 Experiment 5: Column Chromatography Separation of Ferrocene & Acetylferrocene by Column Chromatography Reading: Mohrig, Hammond & Schatz Ch. 18 pgs watch the technique video on the course website!

2 Types of Chromatography 1. Thin Layer Chromatography (TLC) stationary phase: spread over glass or plastic sheet mobile phase: liquid; drawn up plate by capillary action 2. Column Chromatography stationary phase: contained in a column mobile phase: liquid; passes through column (gravity or pressure) 3. Gas Chromatography (GC) stationary phase: contained in a column mobile phase: gas; passes through column (pressure)

3 Column Chromatography One of the most important methods for purification of organic compounds Based on same principles that govern TLC, except now stationary phase packed into a glass column rather than spread over a thin plate. - separation based on the distribution of individual components of the mixture between stationary and mobile phases - effectively establish an equilibrium for each component Differences in equilibrium allow separation Recall: components adsorbed on the stationary phase do not move components dissolved in the mobile phase move with the flow

4 Column Chromatography Like with TLC: the more strongly a compound adsorbs to stationary phase, the slower it will travel - adsorption is based on interactions of functional groups in the molecule with the stationary phase (silica or alumina) hydrogen bonding electrostatic interactions coordination Van der Waals interactions Also true: the more polar the stationary phase, the faster a compound will travel - any compound will spend more time in the mobile phase when the solvent used is more polar these two principles are NOT mutually exclusive (be sure you understand why)

5 TLC vs. Column Chromatography Two compounds: A ( ) & B ( ) TLC: Column Chromatography: develop before mobile phase moves up the plate after mobile phase moves down the column A is the less polar compound A moves furthest up the plate (R f A > R f B) A moves fastest down the column (first compound to come off)

6 Column Chromatography Sample is loaded carefully to the top of the column add Column is run (eluted) by adding solvent carefully to the top Eluent is collected in fractions at the bottom Fractions are analyzed by TLC Pure compounds are isolated collect TLC of column chromatography fractions Chromatography Column

7 Next Week (October 10 - October 14) Experiment 6: Column Chromatography A. Chromatography of a Mixture of Ferrocene & Acetylferrocene separate two compounds by column chromatography DUE: Extraction Lab Report (exp 4) Lab Reports are due at the beginning of your regular lab session

8 Experimental Details Column Assembly - plug tip with cotton don't go overboard, solvent must flow through - clamp column securely column should be vertical - add a layer of sand forms a level bed for the silica - add dry silica gel use the small funnel in your micro kit tap column carefully with pencil as you pour to remove air bubbles - solvate silica, by carefully pushing ~10mL hexanes through the column do not disturb the sand (use a pipet to add solvent) apply air pressure Not too much! do not let the column run dry

9 Experimental Details Prepare & Load Sample - weigh out ferrocene:acetylferrocene mix (get an exact mass) - mix sample with dry silica & add resulting powder to top of column. mix well; don't forget to use the funnel - top sample with a small layer of sand helps protect sample as you add solvent - add enough hexanes to just wet the added silica use a pipet; do not disturb the surface of the silica works best to run the solvent down the side of the column

10 Experimental Details Elute the Column - first with hexanes, then with an ether/hexane mix - fit thermometer adapter to top of column to help control air flow forms a nice seal, but still easy to adjust if pressure gets too high - apply air pressure rate of elution should be rapid, but should still see individual drops do not let the column run dry! - collect two individual fractions (one yellow, one orange) tip of the column should sit just below rim of the collection beaker avoid spatter (messy, lowers recovery) rinse tip of column fequently with additional solvent

11 Experimental Details Isolate fractions - concentrate using the rotary evaporator don't forget to weigh (tare) the flask first - alternatively, blow a gentle stream of air over the sample (not ideal) Sample cools as solvent evaporated, concentrates water from air if you choose this method, do it in the hood! Analyze fractions - TLC, melting point, and IR TLC most easily done before you concentrate your fractions IRs will be distributed - analysis can be done in any order spread out!

12 Some Pointers: Minimize air bubbles when packing column air pockets cause channelling - can ruin separation Do not disturb top layer of sand when adding solvent may cause dilution of sample separation will be less efficient Do not let the column run dry stationary phase may crack; air channels may ruin separation Be sure to label your fractions!

13 Writing the Lab Report: Exp #4 Extraction Purpose - technique experiment: what will you learn? - what conclusions will you reach? - a general discussion of theory/expected results is not a purpose! Results & Discussion - Identify contents of flasks #1, #2, and #3 clearly explain how your data supports your identification mp: compare to known values TLC: what should the relative Rf values be? consider the functional groups that are present IR: what absorbances should you see in each case? tabulate IR data (one table for each compound) include position (cm -1 ), vibration, & functional group example shown in "Interpretation of Experimental Data"

14 Writing the Lab Report: Exp # 4 Extraction Results & Discussion - IR example: ethyl levinulate O O O 2950 cm cm cm -1 position (cm -1 ) vibration functional group 1720 cm -1 C=O ketone 1740 cm -1 C=O ester 2950 cm -1 C-H alkane

15 Writing the Lab Report: Exp #4 Extraction Results & Discussion - Discuss the success of your separation basically: how well did the compounds separate? in other words, how pure are they? 1. show & discuss reactions that allowed separation of compounds be sure the equations are balanced don't discuss the procedure in gory detail! 2. discuss the purity of your compounds consider both TLC and mp data (be specific! ) what should you see if each compound is pure? if each is impure? how does your data compare? 3. Was the separation successful or not?

16 Writing the Lab Report: Exp #4 Extraction Results & Discussion - Discuss the efficiency of the extraction process how much of each compound did you get back (%recovery) how much should you get back (max)? Remember! You started with a 1:1:1 mixture by weight account for any discrepancies in your % recoveries did recover too much of a given component? too little? explain why! - As always: do not include a drawing of your TLC plate, measurements or calculations in this section!

17 Writing the Lab Report: Exp # 4 Extraction Conclusion - a brief recap of your findings - make a general statement about extraction (what did you learn?) - should be brief (2-3 sentences) Appendix A: Calculations - Percent Recovery started with a 1:1:1 mixture each compound comprizes 1/3 of this mixture - Rf value % recovery = amount of compound recovered (g) 1/3 original material (g) x 100 Appendix B: Spectra - Page of IR spectra (provided)