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Electrophilic Aromatic Substitution Lab Report Grace Schulte 006, Kaitlyn Gruber & Cat Lim 2 October 2019

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Abstract This experiment focused on determining the regioselectivity of electrophilic aromatic substitution reactions. Between three different carbocation intermediates, one major product was predicted to be formed the most based on energy levels for the intermediates. Bromine was reacted with bromobenzene in the presence of a catalyst (FeBr3) for this specific trial. The final product was examined by 1H NMR to identify the product(s). Results were then compared with classmates and discussed. Introduction Electrophilic aromatic substitution is a very important reaction within the chemistry field. The reaction can be used to synthesize important intermediates that are helpful precursors for pharmaceutical products.1 When looking into these types of reactions, it is common for different isomers to also be synthesized along with the desired product due to regioselectivity; however, this regioselectivity can sometimes be predicted based on common chemistry patterns and principles. Outcomes of reactions like these can vary in difficulty to predict, but things such as energy diagrams can help direct the experimenter in the right direction. In this specific electrophilic aromatic substitution reaction, the purpose is to predict which final product will be produced from the reaction and use 1H NMR and comparisons with classmates to determine whether or not the results were conclusive. The predicted reaction is shown in equation 1. Eq. 1

Materials and Methods Equipment: Round bottom flask

Stir bar Gas trap Separatory funnel Erlenmeyer flask Chemicals: Table 1. Chemicals used during the experiment Reagent Amount used (mmol) Amount used (g or mL) C6H5Br 1.00 0.105 mL CH2Cl2

n/a

FeBr3

n/a

NaHSO3 NaHCO3

n/a n/a

10.00 mL Amount added by instructor 5.0 mL 5.0 mL

MgSO4

n/a

n/a

Procedure: 1 mmol of bromobenzene (0.157g, 0.105 mL) was added to a 25 mL round bottom flask. Dichloromethane (5 mL) was then added and swirled in the flask. Since this specific reaction called for a catalyst, 1M solution of bromine with FeBr3 in dichloromethane (CH2Cl2) was added to the round bottom flask with the help of instructors. The gas trap was attached to the round bottom flask and the reaction was stirred at room temperature for one hour. After one hour, 10% aqueous sodium bisulfite solution (5 mL) followed by saturated sodium bicarbonate solution (5 mL) was added to the reaction mixture and it was stirred for an additional five minutes. The reaction mixture was then transferred to a separatory funnel and the aqueous and organic layers were separated by using two portions of CH2Cl2 (5 mL each). The organic layers were combined, dried with MgSO4, and filtered into a clean 25 mL Erlenmeyer flask. The solvent was then evaporated. Finally, the end product was put into an NMR tube so that an 1H NMR spectrum could be obtained and observed.

Other Observations Table 2. Classmate outcome comparison chart Compound

Catalyst?

Reaction?

Ratio

of

products Anisole

Toluene

Yes

Yes

100 % para

No

Yes

100% para

Yes

Yes

Benzyl bromide was formed

No

Yes

1:1

para

and

ortho Bromobenzene

Yes

Yes

100% para

No

No

100%

starting

product Methyl

Yes

No/Yes

Benzoate

100%

starting

product 51% meta, 41% ortho No

n/a

n/a

Results and Discussion During this lab, bromobenzene was reacted with bromine and a catalyst (FeBr 3). It was stirred for an hour while waiting for the reaction to happen. What was seen was that the reaction did not actually lose much color, indicating that the Bromine was likely still present in the final product. The organic layer was the layer focused on in this lab. It contained the final product which was predicted to be para formation of dibromobenzene. There was an error that occurred somewhere during this lab in which I could not receive my own 1H NMR data so I was allowed to examine someone else’s data from a different lab section. There might be a chance that my NMR data could not be read because the amount of product I obtained was so small that it was too diluted and therefore failed to analyze four times.

After Rachel Lindren’s 2 results were examined, it was determined that the final product of this specific reaction does favor the para formation of dibromobenzene. What was seen while assessing the proton NMR spectra is that the most prominent peaks of importance were visible in the regions of 5.32 ppm and 7.28 ppm. The peak seen at 7.28 ppm is actually a signal coming from the solute, chloroform. That leaves the large singlet peak at 5.32 ppm to be the major comparison between the predicted products. When looking each of the predicted products, it is blatant that the only spectra even close to resembling the actual product is that of the para formation. The ortho and meta spectra are so drastically different that there is very little chance that they are present in the final product. More evidence that points towards the para formation is that of the tall singlet occurring at 7.38 ppm. These peaks do occur at quite different parts of the spectra but the final products peak occurs more upfield most likely due to impurities. The full reaction is shown in Fig. 1.

Fig. 1 Table 2 above shows the collective data from the class’s results. For the results they were organized in a way that there are two rows for each aromatic compound because each compound had trials in which they were reacted with and without a catalyst. Two people were assigned to do each reaction and it was found that most of the results were conclusive with each other. As for

an overall trend, it was tested that catalysts make the reaction more likely. The results, however, are only consistent with this assumption with the bromobenzene reactions. The distribution of products for each of the substrates can be explained individually. For Anisole, it was seen that a reaction occurred regardless of the catalyst and both yielded the same product. This can be explained through reaction energy diagrams. It is most likely that the lowest energy product will form and in this condition the para formation was predicted to form. For Toluene, the reaction with the catalyst actually formed benzyl bromide instead of any of the expected products. This is observed because when FeBr 3 is exposed to UV light for too long the Bromine molecules actually react with themselves creating a free radical reaction. The Toluene reaction without the reaction had a product distribution of 1:1 para and ortho formations. This can be explained through reaction energy diagrams again. The difference in energy between the ortho and para carbocation intermediates is so small that it is nearly negligible in which a 50/50 split of ortho and para formations is produced. The Bromobenzene reaction with a catalyst reacted and resulted in a nearly 100% ratio of para formation. The results of this reaction follow the common principles of energy again as the para formation has the lowest energy carbocation intermediate. The Bromobenzene without a catalyst did not react. This is because bromobenzene is nonreactive in SN1 and SN2 conditions. This is because in the ring, the C-Br bond is relatively strong, and the structure does not allow for molecules to come in from the back side. For Methyl Benzoate, it was observed that with the catalyst there were two results. One experiment had a reaction and the other did not. For the reaction that did not happen the Methyl Benzoate simply did not react with the Bromine and catalyst. For the reaction that did, the two formations that came back were meta and ortho. This does not agree with the reaction energy pattern; however, it was determined through overlapping the proton NMR peaks in which the ortho and meta spectra peaks were consistent with the final product spectra peaks. The data for the reaction of Methyl Benzoate without a catalyst was not shared with the class. When data was shared among classmates, it was discussed that most of the reactions occurred at a medium to slow pace. There really didn’t seem to be a trend into whether or not this had anything to do with whether or not the reaction had a catalyst for the particular data taken. During the lab there were significant issues in which errors occurred for students. For multiple students with Toluene, the reaction ended up not being any of the expected products but a completely different one because the NMR tubes were exposed to UV light for too long. For

others, some of the experiments were spilled while others did not receive any NMR data back at all. This could be due to human error as well as malfunctions with the NMR machine. Conclusions The purpose of this lab was to predict the final product of a specific electrophilic aromatic substitution as well as use comparisons of 1H NMR and classmate’s data to decide whether or not the results were conclusive. The Bromobenzene reaction that was reacted with Bromine and a catalyst proved to yield a 100% para formation and was expected. 1H NMR was utilized to compare the predicted products to what was actually produced, and it was obvious that the para formation was absolutely dominant. The total results of the classes separate experiments were compared with a table. Each of the products had explanations as to why they were produced from their specific substrates based on common chemical principles. There were significant errors that occurred during this lab. One experiment that might be worth looking into includes how these organic molecules may be manipulated to produce something that can be used in a pharmaceutical way that is environmentally friendly. For example, figuring out how to make a pharmaceutical product in such a way that is less harmful to the environment. Supplemental Information 1. Assign all of the signals in the NMR for both the starting material and all possible products for your aromatic substrates. Use the instrumentation manual, and TopSpin tutorial videos (see link on Moodle) for guidance. You will find the integrations of the various signals to be quite helpful in assigning the signals in some of these compounds. References 1

El-Hiti, Gamal A., & Smith, Keith. Use of zeolites for greener and more para-selective electrophilic aromatic substitution reactions. Royal Society of Chemistry. Green Chemistry, 2011.

2

Rachel Lindren, Data for reaction

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