FERROCENE INTRODUCTION

Transcription

1 FERROENE INTRODUTION Ferrocene was accidently discovered in 1951 at Duquesne University in Pittsburgh, Pennsylvania by T.J. Kealy and P.L.Paulson. yclopentadienyl magnesium bromide (pmgbr) was reacted with iron (II) chloride in attempt to create a fulvalene. When an orange complex formed instead, the chemists hypothesized that the iron was bound to one carbon in each ring. A year later, in 1952, G. Wilkinson and R. B. Woodward correctly deduced the sandwich structure: two anionic cyclopentadienyl (p) rings each donating 6 π electrons to the Fe 2+ cation between them. fulvalene MgBr 2 + Fel 2 X Fe Kealy & Paulson's ypothesized Structure Fe Fe Actual Ferrocene Structure Figure 1. The First Synthesis of Ferrocene. 1

2 Ferrocene s stability and structure defied the conventional bonding descriptions of the time and the discovery significantly increased interest in organometallic chemistry (the study of transition metals directly bound to carbon). In fact, Wilkinson shared the Nobel Prize for his work in this area in Ferrocene s cyclopentadienyl rings are aromatic each containing 6 delocalized π electrons like benzene. If a strong base is used to deprotonate cyclopentadiene (p), the + is removed from the only sp 3 (tetrahedral) carbon in the structure. A lone pair of electrons is then formally assigned to a nonbonding p orbital of that carbon (which is now sp 2 hybridized). ow many resonance structures can be created for a cyclopentadienyl anion (p )? That lone pair of electrons joins the 4 electrons from the two π bonds to create a aromaticity in the ring. Figure 2. Aromaticity of p. Ferrocene easily undergoes acylation resulting in the addition of acetyl groups to one or both cyclopentadienyl rings. O O O O 3 O 3 + O Fe Fe 3 PO 4 3 Fe 3 3 acetylferrocene 1,1'-diacetyl-ferrocene Figure 3. Ferrocene Acylation. While conditions can be optimized so that more of one product is formed over the other, the crude product always has to be purified by column chromatography to separate the acetylated products from each other and from any unreacted ferrocene. 2

3 During this lab, you will perform this separation with solid-liquid chromatography. Make sure you have read the extra technique reading for chromatography. First, you will complete a TL analysis of a mixture of ferrocene, acetylferrocene, and 1,1 -diacetylferrocene to determine the eluent order needed to separate the mixture with column chromatography. You will use Spartan to determine the molecular dipoles of the 3 compounds and use this information to determine the identity of each TL spot and fraction that comes off the column. Once the mixture is separated into its 3 separate components with a column, absorption spectroscopy will be used in conjunction with standard addition to determine the concentration and then amount of each ferrocene in the mixture. SAFETY Safety goggles, aprons, and gloves must be worn at all times in the laboratory. eptane and acetone are extremely flammable and harmful by inhalation, ingestion, and when in contact with skin. Any container holding either heptane or acetone should be capped when not in use to prevent evaporation of the solvents, as they are harmful when inhaled. eptane and acetone solutions must be placed in appropriate waste bottles and can NEVER be poured down the drain. Before starting the experiment, the TA will asks you to do a quick demonstration or talk-through one of the following: 1) ow to prepare a TL plate and a TL developing chamber? 2) When do you need to remove the TL plate from the developing chamber and how do you calculate R f? 3) ow does column chromatography separate compounds? (compare it to TL) 3

4 PROEDURE Work in pairs. Answer all italicized questions in your observations. Part A. Thin Layer hromatography (TL) 1. Your TA will prepare the ferrocene mixture solution for spotting the TL by creating a saturated solution of the mixture in acetone. Why is the solution saturated and why is acetone used? 2. Prepare a solvent system by measuring about 5 ml of solvent into a TL jar or a small beaker as shown below. Line the side of the TL jar with a small piece of filter paper to envelope the whole jar or beaker in your solvent system. Keep the TL jar capped or the beaker covered to prevent the solvent from evaporating. Prepare the following solvent systems: a.) pure acetone, b.) 2:1 heptane:acetone, c.) 3:1 heptane:acetone, d.) 10:1 heptane: acetone, e.) pure heptane. 3. Obtain 5 TL plates and lightly draw straight pencil line about 1 cm from the bottom of the plate; this line is called the origin. Using the TL sample and a capillary tube spot your TL plates with the ferrocene solution at the middle of the pencil line you made. 4. Place a TL plate in each of the five TL jars. Be sure that the solvent is below the 1 cm pencil line you made. Allow the solvent to climb up the TL plate until it is about 1 cm from the top of the TL plate. Remove the TL plates and draw a line with a pencil to where the solvent climbed (this line is called the solvent front ), and allow the plates to dry. Take pictures of all plates and attach to your ELN. 4

5 5. In this case, the components of your mixture should be visible to the naked eye. If they are faint, you may be able to see them easier under a UV lamp. You can then calculate the R f (distance traveled by a compound/distance traveled by solvent, or solvent front) of each component of your mixture in each solvent system. Note: Less polar compounds have higher R f s than more polar compounds. 6. Obtain another TL plate. hoose the eluent that gives the best separation and spot each of the 3 pure ferrocenes separately on the same plate. Take a picture of the plate, record the color and R f of each ferrocene. 7. hoose 3 of the 5 eluents to use for part. Use the information from the TL plates and the information found from the Spartan assignment on Sapling to determine the order of the eluents when doing the column chromatography. heck with your TA about what 3 eluents you will be using (and what order you will be using the eluents in) for part before continuing. Note: In a column, the eluent is moving down rather than up as was in the TL plate. Thus, the component with the highest R f will move down the fastest in a column. Make sure that your first eluent selectively moves just the component with the highest R f first. Part B: olumn hromatography: Preparing a Silica Gel olumn Refer to the figure and photograph on the next page when assembling your silica gel column. The column of silica in the photograph is much shorter than what you should use. 1. Using a wire, pack a small ball of cotton through the top of the pipet until it is lodged into the narrow stem. Why is this cotton plug necessary? 2. lamp the pipet to a vertical stand in the fume hood and place an empty glass vial underneath. Using a spatula and a small funnel, add a layer of sand approximately 4 mm tall, making sure the level of sand is horizontal. Why is this sand layer necessary? 5

6 3. Silica gel is such a fine powder that it can be transferred using a pipet just like a liquid. In the fume hood, slowly transfer a 5 cm tall column of silica gel using a clean pipet. Be careful not to disturb the evenness of the sand layer. sand silica 4. Pour about 10 ml of eluant #1 into a beaker. Using a clean disposable pipet, add the eluant to the top of the silica gel column. Gently tap the side of the pipet so that the upper surface of the silica gel settles to an even level. sand cotton 5. Add another 3-4 mm layer of sand on top of the silica gel column and then add enough eluent to wet the sand layer. Part. olumn hromatography: Separating the Ferrocene Mixture heck with your TA about what 3 eluents you will be using and what order you will be using the eluents in before continuing. 1. Weigh out approximately 30 mg of the ferrocene mixture onto weighing paper. Record the exact mass. 2. Using a small funnel, carefully add the mixture to the pipet with the silica gel column. The ferrocene mixture should rest on the upper sand layer. 3. Using a pipet, fill the top of the column with eluant #1. If the liquid moves slowly, affix a pipet bulb on top of the pipet and gently apply pressure to the bulb, forcing the liquid downward. 6

7 At this point and throughout the entire experiment, DO NOT ALLOW TE SILIA GEL OLUMN TO RUN DRY! You will need to add more heptane often to prevent the column from drying out. Why is it important to not let the column run dry? 4. As the liquid passes through the column, a colored band should become visible in the silica gel column. As the color approaches the bottom of the column, replace the collection vial with Vial 1. In your notebook, record the color and appearance of the liquid you collect. ontinue running eluant #1 (about 12 ml) through the column until you have completely collected the colored material and the length of the silica gel column is once again white. If any colored material remains at the tip of the pipet, rinse it into Vial 1 with a small amount of eluant #1. 5. Place Vial 2 underneath the silica gel pipet. Using a pipet, add eluant #2 to the top of the pipet. A different colored band should begin to migrate down the column. ollect this material in Vial 2 and again record the color and appearance of the liquid. ontinue running eluant #2 (about 7 ml) through the column until you have completely collected all of the second colored material. If any of this colored material remains at the tip of the pipet, rinse it into Vial 2 with a small amount of eluent #2. 6. Place Vial 3 underneath the silica gel pipet, and add eluant #3 to the column using a pipet. Another colored band should begin to travel downwards through the column. ontinue adding eluant #3 and collecting all of the colored liquid in Vial 3 until the silica gel column is once again white; record the color and appearance of this liquid. If any colored material remains at the tip of the pipet, rinse it into Vial 3 with a small amount of eluant #3. 7. If you did not achieve a complete separation of three components perform Part again with a new unknown and a different order of eluants. 7

8 Part D. Standard Addition 1. Find two other groups to partner up with at this point. Each pair will do the standard addition procedure for one of the three fractions you obtained to calculate the mass of ferrocene in your fraction. You will combine data to determine the percentages of each ferrocene in the unknown. 2. Transfer your solution to a 25 ml volumetric flask. Rinse the original flask 3 times with small amounts of acetone, adding the rinse each time to the volumetric flask. Then dilute to volume with acetone, this is solution A. Label two clean, dry 25 ml volumetric flasks I and II. Transfer exactly 10 ml of your solution A to both flasks (10 ml in each). Which piece of glassware should you use? In addition, add one 10 ml portion of the ferrocene standard to flask II. Dilute both flasks I and II to volume with acetone. 3. Obtain a visible spectrometer from the stockroom. Use the USB cable to connect the visible spectrometer to the LabQuest2. The program should automatically recognize the instrument. 4. Prepare a blank by filling an empty cuvette with acetone. Why is acetone used as the blank? 5. alibrate the spectrometer by clicking. The calibration dialog box will display the message: Waiting seconds for lamp to warm up. (The minimum warm up time is one minute.) Note: For best results, allow the spectrometer to warm up for at least three minutes. Wipe the outside of the blank (made in the step above) with a kimwipe and insert the cuvette in the sample compartment. lick Finish alibration and then OK. 6. Measure each solution s absorbance in a cuvette. Make sure you record the path length! Use the arrow on the cuvette to alight it to the light source. Do not leave the solutions in the cuvette for longer than two minutes; extended exposure to acetone will make them foggy. Add enough of each solution to fill the cuvette ¾ full, take the measurement and record the absorbance at the maximum wavelength in your notebook, and then transfer the solution back into your flasks. 8

9 ALULATION & ONLUSION 1. Which variable(s) in the Beer s Law corresponds to the slope? ow do you determine a generic slope with 2 data points? What is that formula if you are using Beer s Law? 2. Use Beer s Law and the data from Part D to find the slope for the Beer s Law Plot for each ferrocene. (Note: You are not making a Beer s Law Plot, just finding it s slope.) Show one sample calculation. 3. Using the slopes calculated in the previous question, calculate the concentrations of the solution in the flask I for each ferrocene. 4. What is the mass of each ferrocene in flask I? 5. alculate the mass percent of each compound in the mixture by dividing the mass of each individual component divided by the total mass x 100. (Note: The total mass should be different for each ferrocene, as each ferrocene measurement was done by a different group.) 6. onsult Figure 3 for the structures of the three unknowns. Based upon the order by which they eluted from the silica gel column, what is the identity of ompounds 1, 2 and 3? Record your answer in your notebook and provide an explanation. ERROR ANALYSIS 1. What modifications could be made to the procedure to better account for random (indeterminate) errors? 2. List three potential systematic (instrumental, methodological, or personal) errors that could be made in this experiment. (Note: Be specific, systematic errors are in the details. For 9

10 example, losing your solution because you knocked over the cuvette is not a systematic error it s a gross one.) 3. Did any gross errors occur? Did you mess up? Did the equipment or instrumentation fail? Abstract writing An abstract is a brief summary of a research study, included in nearly all scientific writing situations. It describes the objectives of the study, the methods used, the major results, and their interpretation and implications. Typical abstracts are usually no more than 5 to 6 sentences long and include a single figure to help demonstrate the results. Write your own abstract for the entire experiment. 10