1 SCHOOL OF CHEMISTRY SENIOR CHEMISTRY CHEM3119 Materials Chemistry Generic skills plus PROJECT E: Ferrocene EXPERIMENT E1: PREPARATION OF FERROCENE AND A DERIVATIVE EXPERIMENT E2: SPECTROSCOPIC STUDIES OF FERROCENE AND ACETYLFERROCENE EXPERIMENT E3: FURTHER INVESTIGATIONS OF FERROCENE 2 3 CONTENTS Page E1: Preparation of Ferrocene and a derivative 5 E2: Spectroscopic Studies of Ferrocene and Acetylferrocene 11 E3: Further Investigations of Ferrocene – Electrochemisty of Ferrocen 17 – Your own Investigations 21 Appendices 23 4 5 EXPERIMENT E1: PREPARATION OF FERROCENE AND A DERIVATIVE Introduction Organometallic chemistry has developed into a major branch of inorganic chemistry. Defined as the chemistry of the metal-carbon bond, its area of interest extends from various aspects of valence theory to chemical technology, from bonding in metal π-complexes, for example, to the catalytic synthesis of industrial alcohols and aldehydes. Organometallic chemistry owes much of its present status to the discovery of ferrocene (Ref. 1) in 1951 and to work done subsequently on the chemistry of cyclopentadienyl compounds (Ref.2). Since the preparation of ferrocene involves a number of techniques typical of organometallic chemistry it is practically and historically an appropriate synthesis for senior chemistry students to undertake. Experimental Method Before commencing this experiment, you must complete a HIRAC form and submit it to a Demonstrator, along with your Name/SID, to be assessed. Your HIRAC marks will be entered into the system. You may get your HIRAC assessed on any day prior to the session/experiment that you are about to start. CAUTION: Cyclopentadiene exists in equilibrium with its dimer. The equilibrium strongly favours the dimer at and above room temperature. Cyclopentadiene is therefore stored in the freezer in the service room. Both ferrocene and acetylferrocene, however, are air and water-stable solids at room temperature. i. Synthesis of Ferrocene, (η5-C5H5)2Fe, di-η5-cyclopentadienyliron. Obtain the kit for this preparation from the appropriate bench (lab 439, check WHEREIS list). Make sure that the apparatus is clean and dry. The Ferrocene workstations have been set up for you in the fume hoods near the instrument rooms, lab 439. Workstations consist of hotplate stirrer (although heating is not required), nitrogen line with an adaptor and the bubbler line. Please make sure that the workstations remain intact after use; reassemble all apparatus and wipe any spills from the hotplate stirrer and the adaptors. Clean the glassware and return your kit to its designated location. 6 Figure 1. Experimental Setup for Ferrocene Synthesis Add 25 g of powder potassium hydroxide using a powder funnel. Set up the apparatus shown above in Figure 1. Place the magnetic stirrer bar in the flask, attach dropping funnel, nitrogen and bubbler lines. Flush the system with nitrogen for a few minutes before adding the reagents. Add 60 mL of 1,2-dimethoxyethane to the flask through the middle neck and commence stirring. Reattach the dropping funnel and flush the system with nitrogen for another 10 min. The flow of nitrogen should be adjusted so that a steady flow passes through to the bubbler. CAUTION: See a demonstrator before you use the nitrogen. Obtain iron(II) chloride tetrahydrate (FeCl2·4H2O) from the desiccator. Iron(II) chloride should be green-yellow in colour, if it is more like orange it has oxidised into iron(III) chloride and should not be used. Make sure that exposure of Iron(II) chloride to the air is reduced to a minimum. Use 6.5 g of Iron(II) chloride. Crush any lumps and very quickly dissolve in 25 mL of dimethylsulfoxide (DMSO). Ensure that dropping funnel’s tap is closed. Transfer the solution of Iron(II) chloride in DMSO to the dropping funnel and stopper the funnel. Do not add the solution to the flask at this stage. 7 Obtain 6 mL of freshly distilled cyclopentadiene from the service room. Keep this on ice during collection and addition. Remove the nitrogen line going to the bubbler and (with the nitrogen still flowing) add 5.5 mL of cyclopentadiene dropwise from a Pasteur pipette. Replace the adaptor to the nitrogen bubbler and stir the mixture vigorously. The empty cyclopentadiene vial should be capped and placed in the broken glass bin. Do not wait for too long before adding the next reagent. The solution will warm up and the cyclopentadiene will polymerise. Slowly, over 15 min, add the iron(II) chloride solution to the stirred mixture in the flask. Stir the mixture for a further 20 min. Turn the nitrogen off and pour the reaction mixture into a mixture of 90 mL of 6 M hydrochloric acid and 100 g of ice (in 500ml beaker). Rinse the reaction flask with some of the acid/ice slurry. Stir for 15 min over an ice-bath and collect the precipitate on a filter paper using a Buchner filter and by filtering at the pump. Wash the precipitate with four 25 mL portions of water. Transfer up the crude product it to a 250 mL beaker and extract it with boiling petroleum spirit (petroleum benzine 40-60) as follows: add 50 mL of the solvent to the beaker containing the crude ferrocene and bring to the boil on a steam bath. Decant the hot solution into a 500 mL conical flask containing 15 g (or more if required) of anhydrous sodium sulfate. Repeat with further 50 mL portions of the solvent. Continue collecting the portions of dissolved ferrocene in hexane (orange/yellow in colour) into a conical flask until the decanted solvent becomes colourless. This means you have extracted all ferrocene and might be left with some insoluble impurities in the flask. Combine the extracts and vigorously shake for 5 min with sodium sulfate. Carefully decant the solvent from the conical flask or filter through cotton wool to remove the sodium sulfate. Place the filtrate in a weighed evaporating basin (crystallization dish) and allow to evaporate to dryness at the back of the fume hood. DO NOT HEAT SOLUTION (to speed up the process) – your ferrocene will sublime along with hexane. Solution can be left in the evaporating basin until your next session. Record your yield. Reserve 0.3 g for further purification by sublimation and use some of the remainder for the preparation of the acetyl derivative. ii. Purification by Sublimation. Collect a pyrex petri dish and a cover from the appropriate bench. Spread a sample (~ 0.3 g) of the crude ferrocene on the bottom of the petri dish. Put the cover on the dish. Half fill a 500 mL beaker with crushed ice. Place the petri dish containing the ferrocene on the hot-plate, preheated to 50 °C, in the fume hood and immediately place the beaker of ice on top of the petri dish. Increase the temperature to 100 °C. DO NOT PLACE AN EMPTY PETRI DISH ON THE HOTPLATE – IT WILL CRACK. The ferrocene will start to sublime and deposit on the glass cover dish immediately below the ice in the cold zone. When the ferrocene has all transferred to the top (which means not much solid left on the bottom part of the petri dish), slide off the beaker of ice and using a pair of tongs carefully remove the petri dish off the hotplate. When it has cooled to a room temperature carefully remove the top cover and collect the sublimed ferrocene. Record your yield and submit the sample. (consider 0.3g is your 100%, calculate % yield from the mass you have collected) 8 iii. Synthesis of acetylferrocene, (η5-C5H5)(η5-C5H4COCH3)Fe To a mixture of 1.5 g of ferrocene and 5 mL of acetic anhydride in a small dry 50 mL round bottom flask, add dropwise with constant stirring, 1 mL of 15 M (syrupy) phosphoric acid. Stopper the flask with a calcium chloride drying tube. Heat the mixture on a hotplate at 50°C while stirring vigorously for 25 min, then pour onto 20 g of ice in a tall 250 mL beaker. Avoid washing out too much of the intractable tar from the 50mL round bottom flask. When the ice has melted, neutralize the mixture by slowly adding 50% sodium hydroxide solution, dropwise. CAUTION: Adding sodium hydroxide too quickly can overshoot the pH change. Cool the mixture in an ice bath and filter off the solid using a vacuum filtration. Wash with 250 mL of water. The solid contains a mixture of ferrocene, acetylferrocene, diacetylferrocene and an intractable tar. These will be separated using column chromatography. To select an appropriate solvent for column chromatography, trial different solvent systems using TLC. The aim is to find a solvent system in which your compound is well separated from others, and moves at least one third of the way up the TLC plate. First test the separation using 100% hexane as the solvent, to give you a reference point for your other trial solvents. Think about how including some ethyl acetate in your solvent system will change the polarity relative to 100% hexane. And how will this change in solvent polarity effect how your compounds move up the TLC plate Conduct a minimum of four TLC trials, using the following solvents. You may wish to improve the separation by trialing other mixtures or adjusting the solvent ratios. (a) hexane (b) ethyl acetate (c) hexane:ethyl acetate 9:1 More information about how to perform TLC analysis is given in the Appendix to this lab manual. Take photographs or make schematic diagrams of your TLC plates to submit with your sample. On the basis of these tests select a suitable solvent or combination of solvents to purify a sample of the impure acetylferrocene by column chromatography. Note that it is common to use one solvent system to elute one analyte, and then increase the polarity of the solvent system to elute another analyte. Check with a demonstrator that you have made an appropriate choice. Take a photograph of your TLC plates or make schematic drawings for inclusion in your electronic notebook. iv. Purification by Column Chromatography Read the general instructions for column chromatography in the Appendix to this lab manual. Weigh 1.0 g of impure acetylferrocene into a 50 mL conical flask, and dissolve in minimum amount of hot hexane while heating on a steam bath. If sample doesn’t dissolve completely add a few drops of ethyl acetate. You will need to add the solution to the column hot. Prepare a bed of silica in hexane in a B29 chromatography column with a cotton wool plug. To dothis, pour a small volume (about 10 mL) of hexane into the column and run out the air bubbles in the cotton wool and stopcock region. Pour a slurry of silica in hexane through a small funnel placed in the top of 9 the burette. Rinse the silica into the column with hexane from a Pasteur pipette and, at the same time, run hexane from the bottom of the column at a steady rate. Never allow the solvent level to fall below the top of the adsorbent. When the bed depth is about 15 cm, run the solvent down to the top of the silica and load the impure acetylferrocene solution dropwise onto the top of the column until the remainder can be poured in without disturbing the surface of the adsorbent. Avoid adding any tar that may have accumulated in your crude product mixture. With a small amount of pressure (N2) push the solution down the column until its surface is level with the top of the silica. Carefully add 0.5 mL of the eluent from a Pasteur pipette and again run the liquid down to the bed level. Repeat this procedure until all eluent has been added and then proceed with the elution using the solvent(s) or solvent mixture you have selected. Further, the elution once started should be completed as soon as possible, as blue oxidation products are formed on standing. Elute the components through the column and collect fractions (in a rack of test tubes) until the ferrocene begins to be eluted. You should see a slow moving reddish-orange band staying on top while a fast moving yellow band is collected into the first test tube. The yellow band is ferrocene and the reddish-orange band is acetylferrocene. Collect ferrocene and acetylferrocene fractions in pre-weighed 250 mL beakers and reduce the volume to approximately 10 mL on a steam bath. You must monitor this reduction of volume carefully – DO NOT allow the products to dry completely or they will sublime off the beakers and you will have no product. Allow the remaining solvent to evaporate in a fume hood at room temperature. Weigh the crystals from each fraction and record the yields. To confirm the purity of the acetylferrocene and ferrocene after the column, run a single TLC plate for both samples, using the same solvent as was used to elute the acetylferrocene. Take a photograph of your TLC plate or make a schematic drawing for inclusion in your electronic notebook. v. Infrared Spectra Record the infrared spectra of ferrocene and acetylferrocene using the ATR-IR (see spectroscopy manual) for the range 4000 to 400 cm-1. Assign as many peaks as possible. Save your infrared spectrum as an ASCII file (.csv) and replot using Excel or other graphing software. vi. Melting points Determine the melting points of the sublimed ferrocene and chromatographed products and compare these with the literature values. Submit the following samples to the Service Room (see “E” assessment document on Canvas): – crude ferrocene (marked on appearance and yield calculations) – sublimed ferrocene (marked on appearance and yield calculations) – crude acetylferrocene (not marked on appearance or yield), can still submit if you like. – column purified acetylferrocene (marked on appearance and yield calculations) – column purified ferrocene (marked on appearance and yield calculations) 10 11 EXPERIMENT E2: SPECTROSCOPIC STUDIES OF FERROCENE AND ACETYLFERROCENE Experimental Method Before commencing this experiment, you must complete a HIRAC form and submit it to a Demonstrator, along with your Name/SID, to be assessed. Your HIRAC marks will be entered into the system. You may get your HIRAC assessed on any day prior to thesession/experiment that you are about to start. Please notify the Service room staff the day before you wish to carry out metal decomposition. A large hot plate will be set up in the first, designated fume hood for Wet Ashing. i. Determination of metal content (wet ashing and spectroscopic analysis) Wet Ashing is an important preliminary step in many metal determinations. It is used to decompose organic and other matter and to bring metals into solution in a form which is suitable for analysis and in a comparable matrix to the standards. Analyze sublimed ferrocene and acetylferrocene for iron using ICP-OES or AAS spectroscopic techniques and compare the results with expected (stoichiometric) iron content. Prior to starting Wet Ashing, consult a demonstrator regarding your calculations for the required amounts of samples and final concentrations of just Fe metal Ions after the dilution. AAS/ICP standards are provided for you by the Service Room (2-10ppm). The solutions have been prepared in 0.1 M HCl to improve stability. Metal Cr Mn Fe Co Ni Cu Concentration range 2-6 1-6 2-10 2-10 2-10 1-8 (mg metal L-1) Proceed with metal decomposition by wet ashing before diluting your samples for analysis. Read through the method. Calculate the weight of sample required and the subsequent volume of the first dilution (for the final dilution to end up within the concentration range of the standards 2-10ppm). Check your calculations with a demonstrator prior to making solutions to avoid overdiluting your samples passed the lower range. It is good practice to analyze a compound as soon as possible after synthesis followed by decomposition and to perform the analysis in duplicate. Accurately weigh duplicate samples of each Ferrocene and Acetylferrocene (0.05-0.1g) for analysis. Record the mass. Add samples to 50 – 100 mL conical flasks. When wet ashing the samples, take extreme care and add the acids very slowly. In the designated fume hood, place a small stemless funnel on top of each conical flask (do not put them on the hot plate at this stage). Add approximately 2 mL of 18 M sulfuric acid to your sample. Then add approximately 2 mL of 15 M nitric acid and swirl gently to mix. 12 Transfer conical flasks onto hot plate and heat, set the temperature to 250 0C. Take care as oxides of nitrogen will vigorously evolve. Continue heating until all the water has been evolved. This is indicated by the distillation of oily sulfuric acid up the walls of the conical flask and the appearance of white SO3 vapor. NOTE: This stage might take 24 h or longer. Add all acids before leaving the conical flasks on the hot plate. Service room staff will monitor and continue with digestion until your next session. If the solution is still dark (black or brown) or if dark particles persist after the initial treatment with 15 M nitric acid /18 M sulfuric acid, cool to room temperature andadd more 15 M nitric acid (~5 mL) and heat again on the hot plate. Repeat this procedureuntil all organic matter is oxidised and the solution is clear. Some anhydrous metal saltsmay appear on the bottom of the flask (as white/greenish precipitate). These will be dissolved later by adding deionized water while making up your solution. Allow the conical flask and contents to cool, then carefully dilute the concentrated acid solutions with approximately 20 mL of deionised water. If the white anhydrous metal salts do not immediately dissolve (this is usually the case with nickel and iron salts), it should happen at later stages of dilution (no visible solid present). From this point you will need to calculate and work out whether to dilute your samples in 500 or 1000ml of the deionized water. Then, pipette 10.0 and 20.0mL aliquots of each of the resulting solution into 100 mL volumetric flasks and adjust to volume with 0.1 M HCI. Label all solutions. Your final concentrations should be within the range of the standards from 2-10mg/L. Metal Determination Two common methods of metal analysis are available for use in the second and third year teaching labs: Microwave Plasma Atomic Emission Spectrometers (MP-AES) and ICP-OES. Check with the Service Room about availability of the instruments. When reporting the results, quote the percentage metal found in your sample, and the value calculated for a pure compound. ii. NMR Spectra Collect 1H NMR spectra of your two compounds (dissolve approximately 20 mg in 0.5-1 mL CDCl3). Identify the hydrogen atoms that give rise to the peaks. iii. Mass Spectra You will be provided with spectra. Also provided is a bar graph showing the abundance of the important peaks relative to the most intense one (taken as 100%). Make a table of these peaks and the relative abundance of each. Where possible, identify the species represented by each peak. 13 References Properties, Bonding and Chemistry: 1. G. Wilkinson and F. A. Cotton, in Progress in Inorganic Chemistry, Vol.1, F.A. Cotton (ed.), Interscience, New York, 1959, pp 1-124 (a more advanced reference). 2. J. E. Huheey, Inorganic Chemistry, Harper and Row, New York 2nd edn, 1975, pp 465- 478. Mass Spectra: 3. J. Charalambous in Mass Spectroscopy of Metal Compounds, J. Charalambous (ed. Butterworth, London, 1975, pp 19-27. 1. V. M. Parikh, Absorption Spectroscopy of Organic Molecules, Addison-Wesley Publishing Company, Reading, 1974, pp 152-153. Infrared Spectra: 2. K. Nakamoto, Infrared and Raman Spectra of Inorganic and Coordination Compounds, Wiley-Interscience. New York, 3rd edn., 1978, pp 388-393. 3. E. Maslowsky, Vibrational Spectra of Organometallic Compounds, Wiley-Interscience, New York, 1977, pp 308-321, 365-367. Electronic copies of these references are available on Canvas. 14 Sample: Ferrocene Acetylferrocene Solvent: CCl4 CS2 Concentration: Saturated Saturated Cell path: 0.1 nm 0.1 nm 15 16 EXPERIMENT E3: ELECTROCHEMISTRY OF FERROCENE AND ITS DERIVATIVE Introduction Ferrocene Ferrocene is well known to undergo reversible, one-electron oxidation to its ferrocenium ion: This oxidation process occurs at a potential of E° (Fc+/Fc0) = 0.40 V vs. SHE (standard hydrogen electrode). This, along with its good solubility in common organic solvents, has led to the widespread use of ferrocene as an internal standard in electrochemical analyses, particularly in non- aqueous electrolytes.1 Different structural factors such as change in the metal ion and substitution of the cyclopentadienyl ring alters the formal oxidation potential (E1/2) of the one-electron oxidation process. Cyclic Voltammetry (CV) A common electrochemical technique known as cyclic voltammetry will be used to investigate the change in E1/2 of ferrocene caused by acetylation of the cyclopentadienyl ring. Cyclic voltammetry sweeps the potential of the working electrode linearly and measures the resulting current.2 As suggested in the name, the sweep direction is reversed at a switching potential and provides information on the reversibility of the observed redox processes. As the applied potential approaches that of a redox process, the faradaic current increases (reduction process) or decreases (oxidation process) giving rise to a peak in the cyclic voltammogram (Figure 1). The non-zero current observed in a cyclic voltammogram in the absence of any redox processes is called the non- faradaic or capacitive current and is due to a build-up of charge at the working electrode. 17 Figure 1. An example of a typical cyclic voltammogram involving one reversible redox process. Peak currents ipa and ipc, and peak potentials Epa and Epc are annotated. Figure reproduced from Reference 3. The formal redox potential (E1/2) can be determined by: , + , 1/2 = 2 where Ep,a is the potential of the anodic peak (in Volts) and Ep,c is the potential of the cathodic peak (in Volts). Scan rate dependence The Randles-Sevcik equation describes the relationship between scan rate and peak current: 1 2 = 0.4463 ( ) where ip is the peak current (Amp), n is the number of electrons transferred (for ferrocene n = 1), F is the Faraday constant (C mol-1), A is the surface area of the working electrode (cm2), C is concentration (mol cm-3), D is the diffusion coefficient (cm2 s-1), v is scan rate (V s-1), R is the gas constant (J K-1 mol-1), T is temperature (K). The diameter of working electrode is 3mm. In cyclic voltammetry, the current which passes through the working electrode is limited by diffusion of the analyte to and from the electrode surface. Thus, the concentration of analyte at the electrode surface changes with scan rate, i.e., larger peak currents are observed at faster scan rates. In the case of a reversible redox process, the peak current is governed solely by diffusion. By plotting ip vs. v1/2, the diffusion coefficient (D) can be determined from the slope. Experimental Method Before commencing this experiment, you must complete a HIRAC form and submit it to a Demonstrator, along with your Name/SID, to be assessed. Your HIRAC marks will be entered into the system. You may get your HIRAC assessed on any day prior to the session/experiment that you are about to start. 18 i. Setting up the electrochemical apparatus Prepare a 0.1 M stock solution of tetrabutylammonium bromide (supporting electrolyte; 25 mL) in dry acetonitrile. Dry acetonitrile can be obtained from the Puresolv solvent purification system – please ask a demonstrator to assist you with this. Use this electrolyte to make up 10 mM solutions of ferrocene (10 mL) and acetylferrocene (10 mL), separately. Note: use sublimed ferrocene and column-purified acetylferrocene. Keep the rest of the supporting electrolyte to run a background for your voltammetry experiment. Ask your demonstrator to gently polish a glassy carbon working electrode (measure its diameter for area calculation) and Ag/AgCl reference electrode using 0.05 μm alumina on a microfibre cloth. Rinse these electrodes with deionised water and acetone, and allow to dry. Rinse the Pt-coated counter electrode and electrochemical cell with acetone and dry under a flow of nitrogen. Figure 2: electrodes and cell Pipette approx. 2 mL of electrolyte solution into the glass cell and screw on the Teflon lid. Place all three electrodes: the glassy carbon working electrode, Pt-coated counter electrode and Ag/AgCl reference electrode, into the glass cell through the designated holes in the Teflon lid. Ensure that approx. 1 cm of the electrodes are immersed in the electrolyte and you attach the clips as shown in Figure 3 (the connection is very sensitive). Clip the electrodes to the potentiostat using the alligator clips in the following order: GREEN to the glassy carbon working electrode, RED to the Pt/Ti counter electrode and YELLOW to the Ag/AgCl reference electrode. Make sure the alligator clips of each electrode are not touching any neighbouring electrodes or clips and that the three electrodes are not touching – this is to avoid short-circuiting the system! If you get results that appear to be no signal or noise it is likely that the electrodes are not connected correctly. Readjust and rescan. Use a N2 line equipped with a drying tube followed by an acetonitrile bubbler, syringe and needle to degas the electrolyte (approx. 5 min). Connect the N2 line and acetonitrile bubbler to a retort stand for stability. Once degassed, raise the needle out of the solution but keep it within the cell to maintain a headspace of N2. 19 Figure 3. Attachment of electrodes for cyclic voltammetry. ii. Cyclic voltammetry Run a cyclic voltammogram (CV) of the electrolyte itself (background measurement). This should show no peaks, indicating there are no electrochemically-active species present. Discard the electrolyte into the appropriate waste container and wash the cell with acetone and dry under a flow of N2. Introduce approx. 2 mL of the ferrocene solution into the cell and degas under N2 for 5 min. Raise the needle out of solution and run a CV under the same parameters as for the background. Collect CVs at the following scan rates: 10, 20, 50, 100, 200, 400 and 800 mV/s. Run cyclic voltammetry on the acetylferrocene solution in the same way. References 1. Astruc, D. Eur. J. Inorg. Chem. 2017, 2017, 6-29. 2. Wang, J., Fundamental Concepts. In Analytical Electrochemistry, John Wiley & Sons, Inc.: 2006; pp 1-28. 3. Electrochemistry, http://okbu.net/chemistry/mrjordan/inorganic1/electrochem/ECHEM1.HTML, accessed 28 Jul 2017. 20 PART 2: YOUR OWN INVESTIGATIONS Before commencing this experiment, you must complete a HIRAC form and submit it to an Academic, along with your Name/SID, to be assessed. Your HIRAC marks will be entered into the system. You may get your HIRAC assessed on any day prior to the session/experiment that you are about to start. In your remaining lab sessions, together with your lab partner, design and implement further investigations into reaction kinetics, the reactions studied in experiments D1–D3, or similar related systems. Before beginning your investigative experiment, you should follow the investigative experiment checklist on Canvas. This involves checking with a demonstrator and the service room that your proposed experiment is feasible in the laboratory and that appropriate equipment and chemicals are available. You should then check with the academic supervisor (or whoever they suggest) that your experiment is sensible and can be performed within an appropriate time-frame. You must then prepare a HIRAC for approval before beginning any new experimental work. The HIRAC must be signed off on by the academic in charge of the laboratory and must be submitted together with your report. Your additional investigations must relate in some way to at least one of experiments completed as part of this project. A number of suggestions are given below but you are not limited to these. Examples of investigative experiments you might consider are: How does varying the reaction conditions for the synthesis of acetylferrocene (e.g. temperature, solvent) affect the product and yield What other characterisation measurements of ferrocene and acetylferrocene could be undertaken Can 1,1′-diacetylferrocene be prepared and what is the effect of a second acetyl group on the cyclic voltammetry of the compound Investigate the cyclic voltammetry of other ferrocene derivatives and compare the effect of substitution on redox potentials. What is the effect of different solvents on the redox potentials of the compounds 21 APPENDIX APPENDIX A: GENERAL METHOD FOR TLC To get a good separation between your products and to use as little solvent as possible, the key is to test potential solvent systems before setting up your column. To do this, you will need to run thin layer chromatography (TLC) using aluminium sheets coated with silica. TLC is a method that allows you to see what is happening in your reaction – how many products are forming and whether your reagents are used up. It does this by separating compounds based on their polarity. Compounds are loaded to the bottom of the stationary phase (the silica), and solvent is run up the plate. More polar compounds interact more with the silica, so take longer to move up the plate. You can change the polarity of your solvent system (for example from hexane to dichloromethane) to make everything move faster. In order to run a TLC: 1. Obtain a silica TLC plate (aluminium-backed) and a capillary tube (glass or plastic), plus a 50 mL beaker to act as your “developing tank”. 2. Mark a baseline on the TLC plate with a soft pencil and mark off and label the points you will spot. (e.g. starting materials, reaction mixture) 3. Repeatedly spot a small volume of your dissolved compounds/reaction mixture with the capillary, making sure not to put too much on that the spot to let it spreads into the others. The resulting spot should be 1–2 mm in diameter. NOTE: It doesn’t matter what solvent your compounds are dissolved in, as long as it has evaporated by the time you run the TLC plate. 4. Pour some solvent into the bottom of your beaker so that the level is below the baseline of your TLC plate. 5. Carefully stand the TLC plate in the beaker and place a watch glass on the top, as shown in Figure Figure 1: TLC set-up. 6. Once the solvent is ~ 1 cm from the top, remove the plate, mark the position of the solvent front (how far the solvent’s got to) with a pencil, and allow it to dry. 7. Depending on your compounds, you can visualise them under the UV light, or with a TLC stain. Your TLC should look something like the one in Figure 2. 22 Figure 2: Schematic of a TLC plate. 8. TROUBLESHOOTING: a. If your TLC shows a big long streak up the plate, you spotted on too much compound at the bottom. Try diluting your sample and/or spotting less times and run it again. b. If your spots have remained at the bottom of the plate, try increasing the polarity of your solvent to obtain and Rf of 0.3. (Rf = distance compound travelled/distance solvent travelled). c. If your spots have moved very quickly and are all at the top, try reducing the polarity of your solvent, as this will give you better separation between compounds and potentially reveal some new spots that have run very quickly together. The ideal Rf is around 0.3. (Rf = distance compound travelled/distance solvent travelled). You can also watch demonstration that have been posted to YouTube on how to run your TLC plates: https://www.youtube.com/watch v=Ah9lPqHz7FY 23 APPENDIX B: GENERAL METHOD FOR COLUMN CHROMATOGRAPHY Column chromatography is a technique that allows you to separate compounds by their polarity in order to purify them. It is like a large-scale version of a TLC (see appendix A). Compounds are loaded to the top of the stationary phase (the silica), and solvent is run through the column. More polar compounds interact more with the silica, so take longer to move through the column. You can change the polarity of your solvent system (for example from hexane to dichloromethane) to make everything move faster. You can choose an appropriate solvent system for y
