Post Lab Experiment 3

Post lab experiment 3 Percent Yield Calculation. Mass of Magnesium (Mg) = 0.15 g Volume of 4-Bromoanisole (C₇H₇BrO) = 0.81 mL Density of 4-Bromoanisole = 1.491 g/mL Mass of 4-Bromoanisole= 1.491g/mL * 0.81mL = 1.208g Molar Masses: Magnesium (Mg) = 24.31 g/mol Moles of Mg = 0.15g / 24.31 g/mol = 0.00617 mol (limiting reagent) 4-Bromoanisole (C₇H₇BrO) = 187.04 g/mol Moles of 4 Bromoanisole= 1.208 g / 187.04g/mol = 0.00646 mol Actual Yield= 4-methoxybenzoic acid mass * Moles of limiting reagent 0.00617 moles * 152.13 g/mol = 0.94g %Yield= (actual mass / theoretical mass) * 100%= (0.2323g / 0.94g) * 100% = 24.71% Discussion: 1) Discuss the purity of product as determined from NMR and IR result. Is there evidence for biphenyl, anisole or any other impurity? The NMR and IR spectra show only the expected peaks for 4‐methoxybenzoic acid (broad O–H, aromatic signals, methoxy singlet), but I am seeing some peak at 10501150cm-1 in IR which may indicate residual ethyl acetate since I did added some excess residual ethyl acetate which may remain in the final product. 2) How do you know your reaction was successful based on NMR and IR? Please be very specific. The reaction’s success is confirmed by the presence of the characteristic carboxylic acid O–H stretch in the IR (2500–3000 cm⁻¹) and the distinct ¹H NMR signals (broad acid proton, methoxy singlet, and two aromatic doublets) consistent with 4‐methoxybenzoic acid 1.Could you monitor your reaction by TLC? What might be some problems trying to do TLC? It is possible to monitor the reaction by TLC to track the consumption of 4-bromoanisole and the formation of the benzoic acid derivative. However, problems is Grignard reagents are highly reactive and unstable in air and moisture, making it really difficult to analyze under standard TLC conditions. Also the polarity difference between the starting material and product could make it challenging to choose an appropriate solvent system for separation, with one might not travel at all and one who’s already all the way on the top. 2. Why do you first add HCl after your reaction is complete, then extract with NaOH and then add HCl again? The first HCl addition neutralizes any remaining Grignard reagent and reacts with unreacted magnesium which produce the magnesium salt that won’t dissolved. The NaOH extraction then deprotonates the benzoic acid product, forming its water-soluble carboxylate salt, which moves into the aqueous layer. The final HCl addition reprotonates the carboxylate, precipitating the benzoic acid product for isolation. 3. What would be the difference in the NMR of 4-bromoanisole and the product? The ¹H NMR of 4-bromoanisole would show signals for the aromatic protons, the methoxy (OCH₃) group, and possibly small peaks related to impurities. The product, a benzoic acid derivative, would have additional signals including a broad singlet around 10-12 ppm for the carboxyl (-COOH) proton. 4. Draw out the reaction scheme for your reaction in chemdraw and paste it into your postlab document. Do not copy the scheme from the procedure. 5. Provide the missing products in the boxes below. 6. Explain the following observation: Using one equivalent of the Grignard reagent leads to ketone formation, The Grignard reagent adds once to the ester or carboxyl derivative, forming a ketone after protonation. The newly formed ketone is still electrophilic, allowing a second Grignard addition, resulting in a tertiary alkoxide intermediate, which is protonated to yield a tertiary alcohol. NMR: referenced, peak picked, integrated. Remember you are using d6-DMSO so your reference is at 2.50 ppm. Draw the product and label the peaks directly on spectrum. 12.5-13.0ppm Carboxylic acid proton Very faint and broad peak, can be visible once zoom in 2.50ppm ~7.90-8.00 ppm aromatic protons ortho to carboxylic acid ~6.85 -7.00 ppm aromatic protons ortho to methoxy group DMSO ~3.80ppm -OCH3 Might be the residual diethyl ether 1.15-1.25ppm Might be OH in DMSO IR: Peaks labelled directly on spectrum: OH stretch, C-H stretch, C=O stretch, C=C stretch O-H (carboxylic acid) C-H Aromatic. ~ 30003100 cm-1 Broad band from 2500-3000 cm-1 C-H aliphatic. ~ 28502950 cm-1 C-O residual diethyl ether impurities ~ 1050-1150 cm-1 C=O ~ 17001720 cm-1 C=C ~ 14501600 cm-1 Unreacted 4-bromoanisole? ~ 500-600 cm-1 LC/MS: Product peak circled on LC and M-H peak circled on mass spectrum Overall, seeing a single dominant LC peak at 1.79 min and the [M – H]⁻ = 151 ion in the mass spectrum confirms that the major component in my sample is 4‐methoxybenzoic acid.