Experiment 5: Column Chromatography

Experiment 5: Column

Chromatography

Separation of Ferrocene & Acetylferrocene by Column Chromatography

Reading: Mohrig, Hammond & Schatz Ch. 18 pgs 235-253

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) 

Column Chromatography

- separation based on the distribution of individual components of the mixture between stationary and mobile phases

- effectively establish an equilibrium for each component

• 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.

• Recall: components adsorbed on the stationary phase do not move components dissolved in the mobile phase move with the flow • Differences in equilibrium allow separation

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)

TLC vs. Column Chromatography

mobile phase moves up the plate

mobile phase moves down the column

 TLC:  Column Chromatography:

before after

develop

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) Two compounds: A ( ) & B ( )

Column Chromatography

• Sample is loaded carefully to the top of the column

• 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

Chromatography Column add

collect

1 2 3 4 5 6

Next Week

A. Chromatography of a Mixture of Ferrocene & Acetylferrocene separate two compounds by column chromatography

Experiment 6: Column Chromatography

DUE: Extraction Lab Report (exp 4)

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!

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

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)

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

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!

Writing the Lab Report:

Exp #4 Extraction

- 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"



Writing the Lab Report:

Exp # 4 Extraction

- IR example: ethyl levinulate  alkane C-H 2950 cm-1 ester C=O 1740 cm-1 ketone C=O 1720 cm-1 functional group vibration position (cm-1₎ 1740 cm-1 2950 cm-1 1720 cm-1 O O O

Writing the Lab Report:

Exp #4 Extraction

- 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? 

Writing the Lab Report:

Exp #4 Extraction

- 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!

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

Appendix B: Spectra

- Page of IR spectra (provided)

Writing the Lab Report:

Exp # 4 Extraction



% recovery = amount of compound recovered (g)_{1/3 original material (g)} x 100