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Bryanna Tanase John Torres Geoffrey Gray Friedel Crafts Acylation of Ferrocene

Introduction Ferrocene is an organometallic compound in which two planar 5 membered rings sandwich an iron ion2. Ferrocene can be considered an aromatic compound because it is planar and both cyclopentadienide anions are aromatic because they each have 6 pi electrons2. Because ferrocene is aromatic it can readily undergo electrophilic aromatic substitution2. The Friedel Crafts Acylation reaction is a type of electrophilic aromatic substitution reaction in which benzene or another aromatic compound reacts with an acid halide (usually acid chloride) and aluminum chloride as the Lewis acid catalyst to produce the electrophile, which then reacts with the aromatic ring to produce an aryl ketone5. The general scheme is shown below in Figure 1.

Figure 1: Friedel Craft Reaction Scheme

The Friedel Crafts Acylation of benzene will be used as an example of how this reaction proceeds. In the first step, acid chloride reacts with aluminum chloride to produce the electrophile: the acylium ion5. Acylium is an exceptional nucleophile because it is stabilized by resonance as shown below in Figure 25.

Figure 2: Acylium resonance

In the second step, the pi electrons of benzene act as a nucleophile and attack acylium, destroying aromaticity and producing a carbocation intermediate, with the acylium ion now

attached to the ring5. In the third step, the leftover aluminum tetrachloride in solution attacks the hydrogen on the sp3 carbon holding the acylium ion, which restores aromaticity and generates hydrochloric acid5. The mechanism for the reaction is shown below in Figure 3.

Figure 3: Friedel Crafts Acylation of Benzene5

The benefits of Friedel Crafts Acylation over Alkylation are that there is no carbocation rearrangement involved due to the resonance stabilized acylium, and because the acyl chloride is a deactivating group, the aromatic ring will favor monosubsitution5. The objective for this experiment was to react ferrocene with acetic anhydride and phosphoric acid as a catalyst by Friedel Crafts acylation to produce acetyl ferrocene and to perform column chromatography to purify the product1. Phosphoric acid was used as the catalyst because ferrocene has a high pi electron density and can be acylated under mild conditions1. The mechanism for the acylation of ferrocene is shown below.

Figure 4: Friedel Crafts Acylation of Ferrocene

The first step in the mechanism shown above is the formation of the acylium ion. It is created when a lone pair on an acetic anhydride oxygen attacks alcohol from phosphoric acid, protonating the oxygen, and then a chain of electron movements occur on the protonated acetic anhydride as illustrated above. Once the acylium ion is formed, the acylation can begin. It is initiated when one of the pi bonds on the cyclopentadiene of ferrocene attacks acylium. This causes a pair of pi electrons on acylium’s triple bond to be transferred up onto the oxygen, so that the octet rule is not broken when acylium attaches to the ring. This also causes acylium to regain its neutral status. Phosphoric acid in solution then acts as a nucleophile and attacks the hydrogen on the same carbon as the ketone group, causing the electrons in the hydrogen bond to move

down into the ring and restore aromaticity to the cyclopentadiene ring. Thus, the product acetylferrocene is generated. A possible side reaction that could occur is that acetyl ferrocene could react with acylium again to produce a double acylated product.

Figure 5: Acylation side reaction

Procedure Test tube +.096 g ferrocene +0.383 g acetic anhydride + 0.215 g 85% phosphoric acid + TLC + warm in steam bath for 110 min +TLC Reaction mixture + cool in ice bath + 0.5 mL H2O (ice cold) dropwise

+ 3 M NaOH dropwise until neutral (check with litmus) + collect product by Hirsh funnel + wash solid/ crystals with water +dry + save a few milligrams for TLC Crude product= ferrocene + acetyl ferrocene + weigh Column chromatography +column (micro or Pasteur pipette) + wool, 1 g alumina + wet column with n-hexane + apply sample (crude product) + fill column with hexane + flush column with blub + see and collect orange yellow band Fraction 1= unreacted ferrocene + switch solvent in column to 1:1 hexane-diethyl ether + flash chromatography + see orange red band + collect it Fraction 2= product (acetyl ferrocene) TLC + evaporate solvent from both fractions

+ take TLC on crude and both fractions Rf values for both + evaporate fractions 1 and 2 Products Acetyl ferrocene + weigh + percent yield + melting point + HNMR Table of Chemicals

BT zards Point C

Mass (g/mol) 186.04

g Point C 294

173.5

Acetic anhydride

102.09

139.9

-73.1

Phosphoric acid

NA

158

21

Acetyl

228.07

160-

80-81

Ferrocene

Do not inhale, ingest, or put in contact with skin or eyes. Skin and eye irritant. Toxic to lungs, mucous membranes. Wear PPE. Do not inhale, ingest, or put in contact with skin or eyes. Skin and eye irritant. Toxic to lungs, mucous membranes. Wear PPE Do not inhale, ingest, or put in contact with skin or eyes. Corrosive. Skin an eye irritant. May be toxic to blood, lungs, bone marrow. Wear PPE. Do not inhale, ingest, or put in contact with

ferrocene

163

Toluene

92.14

110.6

-95

Ethanol

46.04

78

114.1

Hexanes

86.18

68

-95

skin or eyes. Skin, eye, respiratory tract irritant. Poison, may be fatal if swallowed. Wear PPE. Do not inhale, ingest, or put in contact with skin or eyes. May be toxic to blood, liver, nervous system, liver, brain, CNS. Wear PPE. Do not inhale, ingest, or put in contact with skin or eyes. Skin and eye irritant. May be toxic to lungs, heart, CNS, liver. Wear PPE. Do not inhale, ingest, or put in contact with skin or eyes. Skin and eye irritant. May be toxic to nervous system and skin. Wear PPE.

Table 1: Table of Chemicals

Results Amount Of Crude Product 0.22 g

Melting Point Rf Range

Percent yield

Percent recovery

Crude: 80 Pure: 87

185%

Unable to calculate as did not collect weight of pure product

Initial: 0.1 Fraction 1: 0.125 Fraction 2: 1.25 Mixture: 0.714

Table 2: Results

Methyl ketone

Methyl Unsubstituted cyclopentadiene

Figure 6: Product NMR

Calculations

Substituted cyclopenta diene

Theoretical yield .096 g ferrocene x

1 mol ferrocene x 186.04 g

1 mol acetyl ferrocene = 0.00052 moles acetyl 1mol ferrocene

ferrocene .383 g acetic anhydride x

1mol acetic anhydride x 102.09 g acteic anhydride

1 mol acetyl ferrocene =0.0038 1 mol acetic anhydride

moles acetyl ferrocene 1 mole of ferrocene gives 1 mol acetyl ferrocene 0.096 g ferrocene gives 0.00052 mol acetyl ferrocene= theoretical yield Actual yield= 0.22 g acetyl ferrocene x

1 mol acetyl ferrocene =0.00096 mol acetyl 228.07 g acetyl ferrocene

ferrocene Percent yield=

actual x 100= theoretical

Percent recovery=

0.00096 x100=185% 0.00052

grams pure product x 100= unable to obtain because the mass of crude grams impure product

product was not recorded Rf= a/b Initial= 0.5/5= 0.1 Ferrocene= 0.5/4=0.125 Mixture= 0.5/0.7= 0.714 Acetyl ferrocene=05/0.4= 1.25 Discussion The melting point of the product collected in the experiment was 87C (Table 1). Comparing this to the literature value of 85-86C, it is evident that the group obtained a pure compound with no

impurities1. If impurities did arise, it is suspected they would come from improper drying of the final product or undesired chemicals mixed in with the reaction mixture. In comparing the Rf values shown in Table 2, ferrocene had a value of 0.125, the mixture had a value of 0.714, and acetyl ferrocene had a value of 1.25. The value for acetyl ferrocene was at the lowest point on the TLC plate, and this matches what was expected because acetyl ferrocene is the most polar of all the substances due to the carbon- oxygen bonds in the carbonyl group3. Because acetyl ferrocene was more polar, it interacted better with the TLC plate, and traveled further down it than the other substances3. The calculated percent yield for the reaction was 185% as shown in Table 2, which is statistically impossible because this means that matter was somehow created. Reasoning for the high yield could be that the products were not dried properly following filtration or that there were impurities in the product because it was not filtered for long enough4. Those impurities added weight to the product and caused the unrealistically high yield4. In interpreting the HNMR spectrum of the product shown in Figure 6, it is evident that there are six different peaks correlating to the presence of 5 unique hydrogen molecules in the compound. The first peak is the furthest up field from 0-2 ppm which indicates the presence alkyl hydrogen, the second peak just past 2 ppm indicates the presence of a methyl ketone. The two shorter peaks just after that indicate the presence of the substituted cyclopentadiene ring, which has the ketone on it, and the last peak from 4-6 ppm indicates the unsubstituted cyclopentadiene ring that is a part of the product. In interpreting the HNMR spectrum of acetylferrocene on page 105 in the lab manual, there are four unique signals indicating the presence of four unique hydrogens. The first peak at 2.5 ppm is from the methyl group coming off of the carbonyl as denoted by the letter d, the

singlet peak at roughly 4.25 ppm stems from the unsubstituted cyclopentane ring where all five hydrogens are equal as denoted by letter c , and the identical peaks at 4.5 and 5 ppm indicate the presence of 2 different sets of hydrogen atoms on the substituted benzene due to diagonal symmetry, denoted by letters a and b. In interpreting the data the student received as recorded on page 106, it is evident that they obtained a different product than expected. Their melting point range was from 75-78C, which is much lower and also broader than the literature melting point range of 85-86C. This indicates that their product had impurities1. The HNMR spectrum looks identical to the one found on page 105, which would lead the viewer to think that the student obtained the desired product. However, there is a small deviation from the original NMR in that the singlet peak is caused by 15 hydrogen atoms instead of 5, and because there is a singlet they are all equivalent protons. From this difference, it seems that the student obtained a mixture of acetyl ferrocene and ferrocene because the symmetry in ferrocene makes all the protons equivalent, and ferrocene has no other protons that would give it a unique chemical shift. The mixture of both acetyl ferrocene and ferrocene accounts for the 5 extra protons on the singlet peak. Comparing the obtained HNMR with the literature NMR it is evident that the desired product was obtained because both NMR spectra exhibit the same splitting pattern and number of peaks. The melting point data was also very close to the literature value, with the exception that it was slightly higher most likely due to a slight problem with the equipment. Conclusion The objective for this experiment was to perform a Friedel Crafts Acylation of ferrocene to produce acetyl ferrocene and to purify the crude product by column chromatography. The data revealed that the desired product was obtained upon comparing the literature HNMR and that of

the obtained product to find that they were exactly the same. The data also revealed that acetyl ferrocene had traveled the furthest down the TLC paper because it was the most polar substance in the reaction. The group learned that it is important to make sure that all lab instructions are followed otherwise a key piece of data will be missing, such as the amount of pure product needed. Neglecting to weigh the pure product meant that the percent recovery of the product could not be calculated. Friedel Crafts Acylation can be applied to other situations in that it can be used to synthesize anti-HIV cosalane analogues6

References [1] Weldegirma, S. Experimental Organic Chemistry Laboratory Manual, 7th ed.; Procopy Inc: Tampa, Florida [2] University of Rhode Island. Acylation of Ferrocene Background [3] University of Syracuse. Interpretation of Experimental Data [4] Guidelines for Yield Reporting in Lab Reports http://course1.winona.edu/tnalli/s10/yieldreporting.htm (accessed Mar 1, 2018). [5] Hunt, I. Friedel Crafts Acylation http://www.chem.ucalgary.ca/courses/351/Carey5th/Ch17/ch17-2-4.html (accessed Mar 4, 2018). [6] Kurti, L.; Czako, B. Strategic Applications of Named Reactions in Organic Synthesis ; Elsevier , 2005....