Friedel-Drafts Acylation of Ferrocene PDF

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Friedel-Crafts Acylation of Ferrocene Nathaniel Miller CHM2211/L.904 3/1/21 Qi Tang

Introduction An example of a different type of electrophilic aromatic substitution reaction is the Friedel-Crafts reaction. This reaction can be used to add an alkyl or acyl group onto a ring (often referred to as alkylation or acylation). During this type of reaction, hydrogen is substituted on the ring via an electrophilic carbon species which forms a new C-C bond.1 Friedel-Crafts acylation reactions typically require aluminum chloride as a catalyst, however by using the compound ferrocene in this experiment, acylation can be performed using much milder conditions. In this case, phosphoric acid can be used as an acid catalyst which then creates an acylium cation to be used as an electrophile. This ion is created through protonation through acetic anhydride and the loss of acetic acid (Figure 1).2 During Friedel-Crafts alkylation, an alkyl halide is reacted with a Lewis acid in the presence of an aromatic ring. The process adds the alkyl group to the ring, losing a C-H bond and forming a new C-C bond. The role of the Lewis acid in this reaction is that it helps create a better leaving group by activating the electrophile as shown in Figure 2. In this case, a carbocation is formed. Once activated, the electrophile is then attacked by the ring (rate-determining step) and breaking the C-C pi bond in the ring, leading to the carbocation intermediate. The last part is the deprotonation of the C-H bond via weak bases to restore aromaticity (Figure 3). Friedel-Crafts acylation is a related process in that it also uses a Lewis acid but added to an acyl halide in the presence of an aromatic ring. An electrophilic aromatic substitution reaction also occurs where the acyl group is added to the aromatic ring after the loss of the H group. Lewis acids also vary during acylation, as stated previously, aluminum chloride is typically used during this process. The mechanism is quite similar to alkylation with first step being the activation of the electrophile. Afterward, the departure of a halogen results in the stable acylium

carbocation. This acylium ion is now the electrophile during the acylation reaction and is then attacked by the ring. Similar to the alkylation reaction, the final step is the deprotonation of the carbon on the ring to regenerate the pi bond and thus restoring aromaticity.3

Figure 1: Acylium generation

Figure 2: Activation of Electrophile

Figure 3: Alkylation

The objectives of this lab are to react ferrocene with acetic anhydride in the presence of phosphoric acid in a Friedel-Crafts reaction. Afterward, the product, acetyl ferrocene, will then be purified through column chromatography (Figure 4). The electrophile will be generated with acetic anhydride (Figure 1). In order to avoid diacylation, the side reaction, excess acetic anhydride and phosphoric acid should be avoided.2

Figure 4: Reaction Mechanism

Figure 5: Diacylation Side Reaction

Experimental

Table of Chemicals4

Results

Theoretical Yield: 0.093 g C 10 H 10 Fe x C 12 H 12 FeO

1mol C 10 H 10 Fe 1 mol C 10 H 10 FeO 228.07 g C12 H 12 FeO x x 1mol C 12 H 12 FeO 186.03 g C10 H 10 Fe 1 mol C10 H 10 Fe

= 0.114g

Percent Yield: 0.25 g Actual x 100=¿ 21.9% x 100= 0.114 g Theoretical Rf Values Rf Starting material:

4 cm 4.4 cm

Rf Crude ferrocene:

3.1cm 4.1 cm

3 cm 3.9 cm

Rf Pure acetylferrocene:

Figure 6: Product HNMR

= 0.76

1.8 cm 4.1 cm

Rf Crude acetylferrocene:

Rf Pure ferrocene:

= 0.91

= 0.44

= 0.77

1.7 cm 3.9 cm

= 0.44

Figure 7: Theoretical HNMR of Acetylferrocene

Discussion After column chromatography, the unreacted ferrocene was collected, weighed, and the melting point was recorded. The mass was measured to be 0.032g with a melting point value of 173-174C. Compared to the literature value for ferrocene’s melting point of 173C, it is safe to say that the unreacted ferrocene gathered in this lab was spot on. When compared to the crude melting point of 140-150C, this is an acceptable range when considering the crude mixture theoretically contained both ferrocene and acetylferrocene. The melting point of the product measured out to be 84-86C, which was well within the range of the literature value of acetylferrocene. This can be used to confirm that the product was a pure sample of acetylferrocene. After finding this value, further testing was needed to confirm this product. TLC was used to monitor the course of the reaction as well. The crude and pure mixtures were used to compare to the previously-taken TLC of the unreacted ferrocene. Using the Rf value of the unreacted ferrocene and the Rf value of the product, we can determine the polarity and

accuracy of the reaction by the calculate values alongside the distance travelled by each. It is known that ferrocene is less polar than acetylferrocene, it can be inferred that ferrocene would be higher in Rf value than acetylferrocene. After finding the ferrocene value to be 0.91 and the acetylferrocene value of 0.44, it is clear that the reaction was a success and the product was in fact acetylferrocene. For further discussion, the mass of the product was taken to determine the success of the reaction. The percent yield was then calculated after the theoretical yield was found. The yield was found to be 21.9%, well below the acceptable value for the reaction to be considered a success. With this less than acceptable value, it can be concluded that either most of the material was lost during the experiment or a side reaction occurred. It is possible that much of the reactants were lost during transfer, heat, or during TLC. It could also be inferred that diacylation occurred due to excess acetyl anhydride or phosphoric acid used during the workup. The last confirmation method used was HNMR, after the red band product was measured and removed it was placed into the apparatus for interpretation. According to the literature value given on page 105 of the manual, acetylferrocene should have 4 unique peaks. The first peak is downrange at 2.5ppm indicating the methyl group on the substituted ring. We can infer this because the methyl group has the 3 chrematistic protons of a methyl group and the integration on the spectrum matches with 3. The second peak is at approximately 4.3ppm and indicates the 5 protons on the unsubstituted ring because the integration value was also 5. The final 2 peaks are at approximately 4.5-5ppm and represent the protons on the substituted cyclopentyl group. When comparing this to the product obtained in Figure 6, it is clear that the product matches the literature value in the manual, further confirming that the product is indeed acetylferrocene. Question 1:

The HNMR spectrum provided in the lab manual contains 4 distinctive peaks presenting 4 different protons. The first peak is around 2.5ppm, meaning it is the deshielded proton group of a methyl. The integration of 3 confirms this. The second, and highest, is at 4.3ppm and shows the protons on the unsubstituted cyclopentyl ring. The 5 protons in this environment correspond to the integration of 5 given on the spectrum. The final two peaks are at approximately 4.5-5ppm and represent the protons on the substituted cyclopentyl group. Conclusion From the data measured and taken form the product acquired; it is clear that the reaction was a success yet a very small one. The most obvious reason is that the percent yield was very low which indicates the product was impure. The rest of the methods however do point toward a relative success. What can be taken away from these results is that sincere care must be taken when transferring the reactants between each reaction. Aside from this, the lab did accomplish what it set out to do.

Resources [1] Friedel-Crafts Acylation of Ferrocene. https://www.vernier.com/experiment/chem-o21_friedel-crafts-acylation-of-ferrocene/ (accessed Mar 2, 2021). [2] Weldegirma, S. In Experimental Organic Chemistry Laboratory Manual; University of South Florida: Tampa, FL, 2020; pp 96-99 [3] Ashenhurst, J. EAS Reactions (3) - Friedel-Crafts Acylation and Friedel-Crafts Alkylation. https://www.masterorganicchemistry.com/2018/05/17/friedel-crafts-alkylationacylation/#two (accessed Mar 2, 2021). [4] Acetic anhydride. https://pubchem.ncbi.nlm.nih.gov/compound/Aceticanhydride#section=Computed-Properties (accessed Mar 2, 2021). Acetyl-ferrocene ferrocene. https://pubchem.ncbi.nlm.nih.gov/compound/129730202#section=Chemical-andPhysical-Properties (accessed Mar 2, 2021). Chloroform-D. https://pubchem.ncbi.nlm.nih.gov/compound/71583 (accessed Mar 2, 2021).

Ethanol. https://pubchem.ncbi.nlm.nih.gov/compound/702#section=Experimental-Properties (accessed Mar 2, 2021). Ether. https://pubchem.ncbi.nlm.nih.gov/compound/3283#section=Experimental-Properties (accessed Mar 2, 2021). Ferrocene. https://pubchem.ncbi.nlm.nih.gov/compound/7611#section=Computed-Properties (accessed Mar 2, 2021). Phosphoric acid. https://pubchem.ncbi.nlm.nih.gov/compound/1004#section=PhysicalDescription (accessed Mar 2, 2021). Sodium hydroxide. https://pubchem.ncbi.nlm.nih.gov/compound/14798#section=ExperimentalProperties (accessed Mar 2, 2021). Toluene. https://pubchem.ncbi.nlm.nih.gov/compound/1140#section=Experimental-Properties (accessed Mar 2, 2021)....