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Cyclopentadiene reacted with potassium hydroxide under an inert nitrogen atmosphere to form potassium cyclopentadienide. This was treated with iron (II) chloride tetrahydrate to form ferrocene. The crude ferrocene was purified by vacuum sublimation and further reacted with acetic anhydride to produce acetylated ferrocene. Column chromatography was used to isolate the different fractions produced from the acetylation of ferrocene. The fractions obtained were non acetylated, monoacetylated and diacetylated ferrocenes.
Fig. 1: The correct molecular structure of ferrocene
In 1951 Pauson and Kealy inadvertently made ferrocene however their predicted structure of the compound was incorrect due to lack of spectroscopic techniques available today. Pauson and Kealy were trying to oxidatively couple two molecules together in order to form a 10Ï€ aromatic hydrocarbon. However iron (III) chloride was used and the product of the reaction was an orange powder that demonstrated great stability. The correct molecular structure of ferrocene (Fig. 1) was later determined by Fischer and Wilkinson using NMR spectroscopy and x-ray crystallography. The discovery and structure determination of ferrocene was responsible for the increased interest in d-block metal carbon bonds and organometallic chemistry in the 1950's and onwards.
Ferrocene is a metallocene. "*aIn these compounds the metal coordinates to the pi electron system of the ring, rather than to individual atoms." The stability of the ferrocene compound is due to two reasons. The first reason is that it obeys the 18 electron rule. The central metal ion has the oxidation state of +2 and therefore has 6 d electrons. Each cyclopentadienyl ring has a negative charge and has six Ï€ electrons, which obeys Hückel's rule of aromaticity, making each ring aromatic. The electrons on the two rings and the electrons on the central metal atom result in the complex having a stable 18 electron configuration. The second reason for the stability of ferrocene is because the two cyclopentadienyl ligands provide steric protection to the central metal ion. The orientation of the two cyclopentadienyl rings depend on the phase ferrocene is in. The two cyclopentadienyl rings will only eclipse each other when ferrocene is in the gas phase. "*bSolid ferrocene exists in several phases in which the rings are co-parallel but in different orientations."
The main use of ferrocene is as an additive for petrol and diesel. In diesel engines it prevents the formation of soot while it is used as an anti-knocking agent in petrol engines. The replacement of tetra-ethyl lead with ferrocene, as a knocking agent, is beneficial as it does not pose as many risks and side effects. While the uses of pure ferrocene are limited the derivatives of ferrocene are used in many areas of industry including pharmaceuticals and health. One method of producing ferrocene derivatives is by using the Friedel-Crafts reaction. The aim of this experiment is to successfully synthesise ferrocene, carry out tests to determine its redox behaviour and to carry Friedel-Crafts acetylation (Fig. 2) of the sublimed ferrocene. The structure of the derivatives produced by the acetylation will be determined and the abundance of isomers will be studied.
Fig. 2 Reaction showing the monoacetylation of ferrocene
*a reference: Oxford Dictionary of Chemistry, Oxford University Press, Fifth Edition, ISBN-13: 978-0-19-860918-6, pg 497.
*b reference: C.E Housecroft, A. Sharpe, Inorganic Chemistry, Prentice Hall, 2007, 3rd edition, pg 841
(1) Preparation of Ferrocene
Nitrogen gas was used to flush out a 3-necked round bottom flask and create an inert atmosphere for the reaction to take place. 1,2-dimethoxyethane (20cm3) and crushed potassium hydroxide (8.00g ) were added to the flask and stirred to ensure efficient mixing. The cyclopentadiene monomer (1.8cm3) was added slowly, against the nitrogen flow, and the suspension was stirred for 10 minutes. Iron (II) chloride tetrahydrate (2.01g) was dissolved in dimethylsulfoxide (DMSO, 8cm3) and was added to the reaction mixture, over a twenty minute period, using a pressure-equalising dropping funnel, which had been flushed out with nitrogen gas. The reaction mixture was stirred vigorously and left for a further 30 minutes. The nitrogen flow was disconnected and the reaction mixture was added to a slurry of hydrochloric acid (30cm3, 6 mol dm-3) and ice (30g) to neutralise any residual potassium hydroxide. The slurry was stirred for a further 15 minutes. The precipitate was collected through vacuum filtration and washed with cold water (4 x 10cm3). The filtrate was treated with tin (II) chloride and the colour changes were noted. The crude ferrocene was transferred to a watch glass and dried on a steam bath for five minutes producing orange crystals (0.771g, 40.99%).
(2) Purification of the ferrocene by vacuum sublimation
Crude ferrocene (0.771g) was added to the bottom half of the sublimation apparatus. The fully assembled sublimation apparatus was then placed in a silicone oil bath and heated to 110Â°C under vacuum. The sublimation was stopped at intervals to scrape the purified material off the finger of the apparatus and the mass of each batch was recorded. The product obtained were orange crystals (0.679g, 88.07%) mp 170 - 174Â°C Î´H (400MHz;CDCl3) 4.2 (10H, s, HA).
(3) Reaction of ferrocene: The ferrocinium ion
Ferrocene (0.01g) was dissolved in acetone (10 cm3). Iron (III) chloride hexahydrate (0.1g) was dissolved in water (5 cm3). For each of the following reactions the observations were recorded.
Ferrocene solution (5 cm3) was mixed with the iron (III) chloride solution and put aside.
A small amount of ascorbic acid was added to the solution (2 cm3) produced by mixing ferrocene and iron (III). A little dichloromethane was added and the mixture was shaken.
A few drops of dilute aqueous silver nitrate were added to the ferrocene solution (2 cm3).
(4) Friedel-Crafts acetylation
Phosphoric acid (0.25cm3, 85%) was added dropwise, with constant stirring, to a quickfit conical flask containing ferrocene (0.26g, 1.34 mmol) and acetic anhydride (5cm3, 53 mmol). The vessel was equipped with a CaCl2 drying tube and the reaction mixture was heated on a boiling water bath for 10 minutes. The reaction mixture was poured onto ice (20g) and neutralised by adding solid sodium hydrogen carbonate (NaHCO3) until effervescence stopped. The mixture was cooled in an ice bath for 30 minutes and any unwanted solids were filtered off at the pump and washed with ethyl acetate. The ferrocenes were extracted with ethyl acetate (2 x 50 cm3). The combined extracts were dried with magnesium sulphate, filtered and there volume reduced by three quarters of the original by rotary evaporation.
Analytical TLC's run on varied mixtures of petroleum ether and ethyl acetate (Fig. 5) showed three products were made in the acetylation. The different polarities of the mixture resulted in differences in the separation.
8:2 40-60 PE:EtOAC separated the first and second components well with the third component only moving a small distance from the base line.
7:3 40-60 PE:EtOAC showed a better separation of the third component however it still wasn't above the Rf value of 0.35.
6:4 40-60 PE:EtOAC separated the third component of the reaction well.
The diameter of the column was 20mm and was loaded with 15cm of silica gel for flash chromatography. 10ml fractions were collected.
300ml 8:2 40-60 PE:EtOAC eluted the first two bands however it had little impact upon moving the third band.
100ml 7:3 40-60 PE:EtOAC moved the third band down however it was not a vast improvement on the previous mixture of solvents.
100ml 6:4 40-60 PE:EtOAC was used to elude the final band.
TLC was used to determine which fractions contained which components (Fig. 6). Fractions that contained the same component were combined and the solvent removed in vacuo. Fractions that contained more than one component were discarded. First fraction was ferrocene (0.0516g, 19.85%) mp Vmax/cm-1 1767.2 (aromatic overtones), 1733.2 (aromatic overtones), 1666.8 (C=O). Î´H (400MHz;CDCl3) 4.2 (10H, s, HA).
Second fraction obtained was monoacetylated ferrocene (0.089g, 27.92%) mp 85 - 89Â°C Vmax/cm-1 1666.9 (C=O). Î´H (400MHz;CDCl3) 2.4 (3H, s, HD), 4.2(5H, s, HC) 4.53 (2H, t, 2.4Hz, HB), 4.795 (2H, t, 2.4Hz, HA).
Third fraction obtained was diacetylated ferrocene (0.021, 5.56%) Vmax/cm-1 1672.6 (C=O), 1605.3 (H2O). (400MHz;CDCl3) 2.3 (6H, s, HC), 4.48 (4H, t, 1.6Hz, HB), 4.75 (4H, t, 1.6Hz, HA).
Results and Discussion
(1) Preparation of Ferrocene
When tin (II) chloride was added to the filtrate the solution turned from dark blue to apple green and finally to yellow. The colour change in this test can confirm the presence of the ferrocinium ion and that ferrocene was produced in the reaction. The change in colour of the filtrate is due to the change in oxidation state of the iron in the ferrocene. Tin (II) chloride is a reducing agent and adding this to the ferrocinium ion results in it undergoing one electron reduction to form ferrocene. When tin (II) chloride is added to the filtrate the oxidation state of the iron, in the ferrocinium ion, changes from the +3 state to +2 and ferrocene is formed.
(2) Purification of the ferrocene by vacuum sublimation
The 13C NMR spectra (Fig. 10) shows one intense peak at 68.07ppm which is evidence that there all carbon atoms, in the ferrocene molecule, are in the same chemical environment. The chemical shift is lower than expected for an aromatic system because the two cyclopentadienyl rings are bonded to the central metal atom.
Fig. 14 shows the 1H NMR spectrum of sublimed ferrocene. All the hydrogens in the ferrocene molecule are in the same chemical environment therefore no coupling or splitting will occur and a singlet peak should arise. The peak at 4.2ppm corresponds to the hydrogen atoms on both cyclopentadienyl rings. These hydrogen atoms have a lower chemical shift compared to other aromatic shift values because the two cyclopentadienyl rings are bonded to a central metal atom.
(3) Reaction of ferrocene: The ferrocinium ion
The three experiments that were carried out showed that the iron ion in the ferrocene can easily undergo oxidation or reduction provided the right conditions and reagents are used.
In the first experiment the yellow ferrocene solution turned to a dark blue colour when the iron (III) chloride solution was added to it. The iron (III) chloride solution is an oxidising agent and resulted in the one electron oxidation of the ferrocene atom to produce the blue coloured ferrocinium ion. The oxidation state of the iron in this particular experiment changed from +2 to +3 showing the ferrocene can be easily oxidised.
The product of the previous reaction was the ferrocinium ion which is a good oxidising agent due to inert ferrocene being its redox product. The addition of ascorbic acid to the dark blue solution reversed the colour change brought about by reaction one and a yellow colour was resulting colour of the solution. This illustrates that the iron in the ferrocinium ion had been reduced from the +3 oxidation state to the +2 oxidation state. The addition of dichloromethane at the end of the reaction allowed the reformation of the iron (III) hexahydrate compound.
[Fe(Cp) 2]+ + 2H2O + C6O6H8 ïƒ Fe(Cp)2 + C6O6H6 + 2H3O+
The addition of silver nitrate to the yellow ferrocene solution resulted in the formation of a dark blue solution. The silver nitrate is an oxidising agent and the result of adding this to the ferrocene solution was the one electron oxidation to form the ferrocinium ion. The iron in the compound has changed the oxidation state from +2 to +3 and has therefore undergone oxidation.
[Fe(Cp) 2]+ + AgNO3 + H2O ïƒ Fe(Cp) 2 + AgNO3 + H3O+
The three experiments demonstrate the ease in which ferrocene can undergo redox reactions provided the right reagents and conditions are used. These three tests also help determine the composition of the filtrate tested earlier in the preparation of ferrocene. When tin (II) chloride was added to the dark blue filtrate it turned to an intermediate green colour and finally to a yellow colour. This shows that the filtrate contained the ferrocinium ion, in the +3 oxidation state, and when tin (II) chloride was added it was reduced to ferrocene, in the +2 oxidation state.
(4) Friedel-Crafts acetylation
The reaction of ferrocene with acetic anhydride produced monoacetylated and diacetylated ferrocene as dark brown crystals. Some unreacted ferrocene was also recovered at the end of the experiment. Analysis of the IR spectrum, 1H NMR spectrum and yield calculations showed that the major product of the reaction was the monoacetylated ferrocene while only a small amount of diacetylated product was produced.
The infra-red spectrum of recovered ferrocene (Fig. 7) has a C=O peak at 1666.8 cm-1. This peak arises because the recovered ferrocene is contaminated with some monoacetylated product.
Fig. 15 shows the 1H NMR spectrum shows the first component of the column, recovered ferrocene, was contaminated with some of the second component, monoacetylated ferrocene. The 1H NMR spectrum of the recovered ferrocene shows a large singlet peak at 4.22ppm which corresponds to the hydrogens on both cyclopentadienyl rings. All of these hydrogen atoms are in the same chemical environment therefore no splitting will occur and a singlet peak will be observed. The two triplet peaks at 4.54ppm and 4.815ppm show that the sample is contaminated with some monoacetylated ferrocene as these two peaks would not occur if only ferrocene was present.
Fig. 3 shows the reaction route for the monoacetylation of ferrocene.
The infra-red spectrum of monoacetylated ferrocene (Fig. 8) shows a peak at 1666.9 cm-1 which corresponds to the carbonyl group in the acetyl. This shows the acetylation of ferrocene was successful as a carbonyl peak is present in the spectra.
The 13C NMR spectrum (Fig. 11) of monoacetylated ferrocene shows the presence of six different carbon environments. The peak at 202.16ppm corresponds to carbon bonded to the oxygen, in the acetyl (C1). This will have the highest shift value as it is adjacent to an oxygen atom which will deshield the carbon. The peaks 79.23ppm, 72.39ppm and 69.61ppm correspond to the carbon atoms in the cyclopentadienyl ring which is bound to the acetyl group. The peak at 69.6ppm corresponds to the five carbon atoms (C4) in the bottom cyclopentadienyl ring. The peak at 27.46ppm corresponds to the carbon atom (C6) in the methyl group. The shift may be slightly higher than expected due to the carbon being deshielded by the oxygen atom on the adjacent carbon.
The 1H NMR spectrum of monoacetylated ferrocene (Fig. 16) shows four different hydrogen environments are present in the compound. The singlet peak at 2.4 ppm corresponds to the CH3 group (D). The three hydrogen atoms in this group are in identical chemical environments and there are no hydrogen atoms on the adjacent carbon atom therefore it will give the observed singlet peak. The chemical shift for this group is higher than expected because it is deshielded by the electronegative oxygen atom. The peak at 4.2ppm corresponds to the hydrogen atoms in the bottom cyclopentadienyl ring (HC). These five hydrogen atoms are in identical chemical environments and will give the singlet peak that is observed. These hydrogen atoms have a lower chemical shift compared to other aromatic shift values because the two cyclopentadienyl rings are bonded to a central metal atom. The peak at 4.53ppm corresponds to the two hydrogen atoms labelled HB. These two hydrogen atoms are in identical chemical environments therefore they will not couple with each other. However they will couple to the two hydrogen atoms HA to give the triplet splitting pattern that is observed. The hydrogen atoms will be split by the first HA and further split by the second HA to give the triplet splitting pattern. The peak at 4.795ppm corresponds to the two hydrogen atoms labelled HA. These two hydrogens will couple to the two hydrogens, labelled HB, to give the triplet splitting pattern that is shown. These hydrogens have a higher shift value than HB because they are deshielded by the electronegative oxygen atom. The coupling constants for HA and HB are the same (2.4Hz) implying that the interactions between the hydrogen atoms are mutual.
The infra-red spectrum of diacetylated ferrocene (Fig. 9) shows a peak at 1672.6 cm-1 which corresponds to the two carbonyl groups, in the acetyl groups. The signal at 1605.3 cm-1 is due to the presence of water in the dichloromethane solution.
Fig. 12 shows the 13C NMR spectra of diacetylated ferrocene. The spectrum shows that there are five carbon atoms that have different chemical environments. The peak at 201.06ppm corresponds to the two carbon atoms bonded to the oxygen, in the acetyl (C1). These carbon atoms will have the highest shift values as they are adjacent to an oxygen atom which will deshield them. The peak at 27.67ppm corresponds to the carbon atoms in the two methyl groups (C5). The peaks between 80.635ppm and 70.911ppm correspond to the carbon atoms in the cyclopentadienyl rings.
Fig. 4 shows the four possible isomers of diacetylated ferrocene.
Fig. 17 shows the first NMR spectrum of diacetylated ferrocene. There are three possible isomers of the diacetylated ferrocene (Fig. 4) and none of these can be eliminated from this NMR spectrum. The two singlet peaks at 4.54ppm and 4.80ppm show no splitting pattern and hence the structure of the major diacetylated ferrocene cannot be determined. No splitting pattern could have occurred for several reasons. The vagueness in this NMR spectrum means the major product of the reaction cannot be determined however the number of peaks in the spectrum indicates that more than one isomer was produced. One of these could have been because too much product was used to run the NMR spectra. An alternate reason could be that a higher frequency was needed to show the splitting pattern. The product could have been contaminated which would result in the distorted splitting patterns seen in the spectrum.
Fig. 13 shows the second NMR of diacetylated ferrocene. This NMR is tidier than the previous NMR and the splitting pattern of the peaks can be observed. It can be deduced, from the splitting pattern, that the major product of the reaction is isomer 1 (see Fig. 4). The 1H NMR spectrum of diacetylated ferrocene shows three different hydrogen environments are present in the compound. This immediately eliminates isomers 2 and 3 because they both have four hydrogen environments. The peak at 2.3ppm corresponds to the CH3 group (C). The three hydrogen atoms in this group are in identical chemical environments and there are no hydrogen atoms on the adjacent carbon atom therefore it will give the observed singlet peak. The triplet peak at 4.48ppm corresponds to the hydrogen atoms labelled HB. These hydrogen atoms couple to the hydrogens atoms labelled HA to give the triplet splitting pattern that is observed. The peak at 4.74ppm corresponds to the hydrogen atoms labelled HA. These two hydrogens will couple to the hydrogen atoms labelled HB to give the triplet splitting pattern that is shown. The hydrogens labelled HA have a higher shift value than HB because they are deshielded by the electronegative oxygen atom. The coupling constants for HA and HB are the same (1.6Hz) implying that the interactions between the hydrogen atoms are mutual.
After analysing all the experiment data and theory behind the acetylation of ferrocene it can be concluded that the major product of the reaction was the monoacetylated product, however some diacetylated product and unreacted ferrocene were also recovered. This was supported by the reaction yields which showed the largest amount of reaction component recovered was the monoacetylated form. The 1H NMR spectra and IR spectra also supported the experimental data.
Some product could have also been lost during filtration. This could have been avoided by ensuring the solvent, used to wash the filtrates, was as cold as possible.
The yield in the first part of the experiment is low which suggests that the reaction may not have gone to completion. The reaction was deemed to have gone to completion after half an hour of vigorous stirring. This was an assumption and could have been incorrect. The production of iron oxide in the first part of the experiment was a possibility which could account for the low percentage yield. The possibility of producing iron oxide was reduced by adding the iron (II) chloride in DMSO. The yield in the second part of the reaction was high which showed that the sublimation of ferrocene was successful. The ferrocene was sublimed in air which could be responsible for a decrease in the percentage yield. Some product could have also been lost during filtration. This could have been avoided by ensuring the solvent, used to wash the filtrates, was as cold as possible.
The acetylation of ferrocene was successful as both monoacetylated and diacetylated ferrocene was obtained. The percentage yield for this part of the reaction was low and this was due to two reasons. Unreacted ferrocene was obtained from the column which shows that the reaction had not gone to completion. The second reason is that air will have been able to enter the system and formation of alternate products would have occurred. This could have been prevented with the use of a schlenk line.
The melting points for the sublimed ferrocene and monoacetylated ferrocene were close to the documented literature value which illustrates a high level of purity for the final product. Any discrepancies could be accounted for by a non-calibrated melting point machine or impurities in the final product.
Water in the DCM solution is responsible for the non product peaks arising on the IR spectrum for monoacetylated and diacetylated ferrocene. Impurities or excess solvent may also be present and give rise to peaks that are not present in the final product. The non product peaks in the NMR spectrum for sublimed ferrocene are due to the solvent CDCl3 (7.3ppm) and impurities. The non product peaks in the NMR spectrum of recovered ferrocene arise because the fraction is contaminated with monoacetylated ferrocene. The peak at 7.3ppm is due to the solvent CDCl3. The non product peaks for the first NMR spectrum of diacetylated ferrocene occur due to impurities in the sample and because too much product was used to run the spectrum.