For Chromatography, you might want to remember Polar Dissolves More, not like dissolves like.

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1 Chromatography In General Separation of compounds based on the polarity of the compounds being separated Two potential phases for a compound to eist in: mobile (liquid or gas) and stationary Partitioning of compounds between mobile phase and stationary phase occurs some move more in mobile phase, some stick more on stationary phase, resulting in compounds moving at different rates which can then be separated 1. Mobile Phase Solvent System The polarity of a solvent is defined as its ability of the solvent to dissolve increasingly polar organic compounds. The more polar a solvent is, the larger the numbers of compounds that can dissolve into the solvent, starting with non-polar, then a little polar, more polar and finally (for very polar solvents) very polar compounds! Compounds dissolved into the solvent spend more time moving in the mobile phase in chromatography. For Chromatography, you might want to remember Polar Dissolves More, not like dissolves like. Some Common Solvents: (listed by increasing polarity) Petroleum ether (C5 hydrocarbons) (non-polar) Ligroin (C6 hydrocarbons) (non-polar, higher BP) Diethyl ether (CH3CH2OCH2CH3) (slightly polar) Dichloromethane (CH2Cl2) (polar) Ethyl acetate (ester, CH3C(O)OCH2CH3) (more polar) Methanol (alcohol, CH3OH) (really polar) Acetic acid (carboylic acid, CH3C(O)OH) (etremely polar) And of course mitures in any ratio of any of the above can be used. Consider the following eample: Compound A is a non-polar compound and compound B is a very polar compound. Add Petroleum ether to the miture. Which will dissolve? Non-Polar A, because a non-polar solvent can only dissolve non-polar compounds. What if you added methanol mied with acetic acid instead? Which will dissolve? Both! The more polar the solvent, the more compounds it can dissolve, starting with the non-polar ones and increasingly dissolving more polar ones. Keep in mind that the more polar the solvent, the more the compounds will dissolve into the mobile phase. This means the compounds will move faster and further through the stationary phase.

2 2. Stationary Phase Alumina (Al2O3) or Silica (SiO2) on a solid support The more polar the compound, the stronger the compound adheres ( sticks ) to the adsorbent. Functional Groups (listed by increasing polarity): Alkanes (nonpolar, non-stick) Alkenes and aromatics (still pretty nonpolar) Ethers (little more polar) Esters (even more polar) Ketones and aldehydes (yes, holding on tighter) Amines (reaching the polar end of the options) Alcohols (even more polar than amines better at H bonding) Carboylic Acids (the most polar, holding on like its super-glued!) Partitioning Effect: Generally, polar compounds spend more time on the stationary phase ( stuck ) and nonpolar compounds spend more time in the mobile phase (moving along with the solvent). The goal is to use a solvent system that moves ONLY the least polar compound, until it is completely removed from the system THEN use a more polar solvent to move the net compound and so on, until all components are separated out one at a time Just remember that non-polar compounds move in ALL solvents but polar compounds need more polar solvents in order to enter the mobile phase and be moved through the stationary phase. Column Chromatography: Used for separation (purification) of organic compounds (solids or liquids) Size of separation can range from milligrams to kilograms Packing of the Column: -demo- Must remove air bubbles pockets of air allow compounds to travel in the mobile phase faster through air pockets resulting in separation that is not uniform Column must be assembled in a vertical fashion, so bands will travel downwards in an even and uniform fashion for the best separation 2

3 alumina or silica gel Crooked bands tend to stay overlapped slightly alumina or silica gel Sample must be loaded in the most concentrated method o need the narrowest band possible for best separation o wide bands tend to stay wide (or get wider) and thus stay overlapped alumina or silica gel Not Separated Again, you may use a single solvent or you may change solvents during column by allowing solvent level to drain to the top of the sand in column, THEN refill with new solvent. Use gravity to push your solvent through or you can use gas pressure to move the mobile phase quicker through the stationary phase ( flash column chromatography ) Collect fractions containing the bands and evaporate the solvent. 3

4 Any crystals growing on the bottom of the column should be rinsed into the appropriate vial. Take a look at the following columns. compound. a. b. Determine which compound is the less polar Thin-Layer Chromatography (TLC): Used to identify compounds (like GC) Used to check the purity of a compound Used to check a reaction s progress Only need micrograms of material to do this process. Truly, the only major difference between column chromatography and TLC is the direction of the flow of the mobile phase! Similarities between Column Chromatography and TLC: Mobile Phase: Same Solvents Stationary Phase: Same Alumina or Silica applied on some solid support Same partitioning effect based on compound polarity Process Summary: -demo- 1. Spotting the Plate (application of the compounds) TLC plate is marked with a pencil (not pen) to show point of origin (where compound is applied). This point of origin must be about 1 cm above the bottom of the plate (above 4

5 the level of the solvent used as the mobile phase). This avoids having the mobile phase wash the compounds off. Don t spot too close to the end of the plate either The microcapillary tube uses capillary action to pull liquid into tube and then capillary action will release the liquid onto the adsorbent by gently tapping the microcapillary tube to the TLC plate. Plate 1: Plate 2: F AF R V1 V2 2. Developing the Plate (movement of eluent through adsorbent) Developing Chamber glass container, such as beaker or jar, with a cover. Mobile phase uses capillary action to climb the plate in a vertical fashion, thus often a wick is added to the chamber to provide saturated atmosphere throughout, so the mobile phase does not evaporate away as it climbs the plate. Must be sure the plate does not touch any wick or the side of the chamber as the plate develops. This would cause the mobile phase to move in a sideways fashion also, ruining the lanes of travel by the compounds and preventing any identification. Plate should always be monitored watch the solvent front and removed prior to the solvent eceeding the distance of the plate. MARK the distance the solvent has traveled when removing! Probably a good idea to pour out old solvent from first TLC plate and add in fresh solvent before doing second TLC plate or your solvent system (1:1) may not still be 1:1 which will skew your results. What will happen to your peaks if your solvent system evaporates and becomes, for instance, more polar? 3. Visualization Typically organic compounds are colorless and not seen by the human eye. To visualize, you may use iodine, a UV lamp, or one of many organic stains. Circle the spot with a pencil. Record the appearance of the spot on the plate. In today s eperiment, ferrocene and acetylferrocene are colored compounds so you will be able to see without any stain or UV light. Draw each TLC plate into your notebook LIFE-SIZED. Trace around the plate and transfer all details to your drawings origin, shapes/sizes/colors of spots, solvent front distance. No thumbnail sketches please! 4. Calculations (For EACH spot): 5

6 Rf value (Retardation Factor): Relative amount the spot traveled when compared to the solvent. The Rf value is calculated by dividing the distance the spot travels by the distance the solvent travels (from the origin to the solvent front), as shown below: Y X R f = X/Y All measurements should be done in metric units (cm or mm, not inches). The Rf value will always be between 0 and 1 and is NOT epressed as a percentage and has no units. What problem occurs if you forget to draw the solvent line when you remove your TLC plate from its developing chamber? no solvent front Rf values are commonly used for identification purposes. the same compound with always have the same Rf value under the same conditions (i.e. same solvent system). Apply standard compounds on the same TLC plate as an unknown compound and run the compounds side-by-side to compare. (Your Plate #1) Keep in mind that the spots for the same compounds will always have the same appearance, as well as the same Rf value. Same Compound or Not? TLC can also be used for checking the purity of a compound. A pure compound will always appear as a single spot (because it is a single compound). (Your Plate #2) 6

7 If the Rf value is too low (<0.2) or too high (>0.8), try a different solvent system. You may be using a system that is too polar or too non-polar to differentiate how many compounds (spots) you are seeing. Pure or Not? Can you even tell? non-polar solvent polar solvent Probably the most popular reason for doing TLC: Following the Progress of a Reaction: Use TLC to monitor the status of a reaction s progress. Always spot the beginning starting material on the same plate as the reaction miture. If the starting material is still present, you will see starting material (same appearance, same Rf value) as a spot for the reaction miture. If the starting material is completely gone (i.e. Reaction is Finished!), then you should not see any spot that corresponds to starting material in the reaction miture. O LiAlH 4 OH sm rn sm rn sm rn sm rn sm rn T = 0 min T = 15 min T = 30 min T = 45 min T = 60 min 7

8 Consider the following questions: Which compound is the most polar? Which compound is the least polar? In what order would these compounds come out of a column? A B C One more time: Which compound is the most polar? Which compound is the least polar? In what order would these compounds come out of a column? X Y Z What would this TLC plate look like, if it was developed in an even more polar solvent? more polar solvent How about a less polar solvent? less polar solvent What problem is associated with determining the purity of the following spot? Today in Lab: Each pair of students must separate ferrocene and acetyl ferrocene using column chromatography. 8

9 Each pair of students need to run one TLC plate with the two known compounds on it and a sample of the miture in order to identify which of the spots is which compound. You will need to measure to obtain Rf values for all four of those spots. (Four separate calculations) You need to run a second TLC plate on the contents of Vial 1 and Vial 2 from your column separation. This is to check the purity of those separated compounds. You will also need to measure to obtain Rf values for both of those spots, even though Rf has nothing to do with purity. Upon completing the second TLC plate, see your instructor about where to place the two vials so they may evaporate off solvent to form crystals for net weeks MP analysis. Net Lab Period: Mass and Melting Point Analysis of Components (Day Two) a. In the net lab period, remove any caps that were placed on your vials and determine the mass of vial 1 and vial 2 (if required). Observe what the crystals look like in each vial. b. Then, working with your lab partner, obtain a melting point of each compound (you will need to use a spatula to scrape the materials loose off the sides of the vial). Place both melting point capillaries in the Digimelt and input the following parameters: If melting separately: Vial 1: START: 140ºC, RAMP: 2ºC/min STOP: 190ºC Vial 2: START: 50ºC, RAMP: 2ºC/min STOP: 100ºC Data Table should include all weights, TLC plate measurements, and melting point information, including Digimelt settings. 9

10 Data Table Melting Point Information Weight (g) Distance (cm) Start (ºC) Ramp (ºC/min) Stop (ºC) Miture Vial 1 empty (no cap) Vial 2 empty (no cap) Vial 1 with crystals (no cap) Vials 2 with crystals (no cap) Plate 1, Ferrocene Plate 1, Acetylferrocene Plate 1, Top Spot Plate 1, Bottom Spot Plate 1, Solvent Front Plate 2, Vial 1 spot Plate 2, Vial 2 spot Plate 2, Solvent Front Ballpark, Vial 1 Crystals Slow run, Vial 1 Crystals Ballpark, Vial 2 Crystals Slow run, Vial 2 Crystals 2 90 MP Range (ºC) 10