Experiment D-E - Lab Worksheet 2023

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Dec 6, 2023

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Lab Worksheet for Experiment D-E NMR and chiral HPLC analysis CHEM 12 A, 2023 Name: ____________________________________ Section Number: _________ Ensure you have corrected any outstanding errors in your NMR spectra for experiment D. You need to bring the following spectra for analysis in this lab – if your GSI said the spectra you handed in last week were fine, you can just bring those again. You need all the following spectra: - Adipic acid standard - Salicylic acid standard - Crop A - Crop B For each spectrum, you need: integration, peak pick, enlarged regions to see all peaks and coupling and TMS set to 0 ppm or that DMSO set to 2.50 ppm. Bring your calculator and previous laboratory reports. Part I – Prelab: Section 1: Preparation for Analysis (1) Provide a sample calculation for how you will determine the relative amount of salicylic acid and adipic acid in crops A and B using 1 H NMR. Hint: Suppose you identify a peak unique to salicylic acid and a peak unique to adipic acid that correspond to the same number of hydrogens. What does it indicate about the relative amounts of adipic and salicylic acid if these two peaks have the same integration? (2) Provide sample calculations for how you will use the HPLC chromatograms to determine the relative ratio of acetophenone and 1-phenylethanol as well as for how you will use them to determine the enantiomeric excess (% ee) of the products.

Section 2: NMR practice problems: (3) You have made a mixture of 1,1-dimethylcyclohexane and 1-bromo-1-methylcyclopentane. The 1 H NMR spectrum of the mixture shows two singlets; one at 0.9 ppm and the other at 1.8 ppm, with relative integral values of 1:4. What is the ratio of 1,1-dimethylcyclohexane to 1-bromo-1-methylcyclopentane in the mixture? Draw the molecules, label the 1 H’s corresponding to these two peaks, and s how your work. (4) The 1 H NMR spectrum of N,N-dimethylformamide acquired at room temperature is shown below. Explain why the methyl groups (~3 ppm) display two peaks at room temperature and only one peak at 150 °C. Hint: consider the resonance forms of amides.

(5) Two products are formed from the following acid-catalyzed dehydration reaction. The spectra of products M and N are shown to the right. Using these spectra, identify the structures of compounds M and N . Assign the peaks in each 1 H NMR spectrum to the protons in the molecules you drew below. There may be peaks that you are not able to assign exactly.

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Part II – During and after Lab: Section 1: Calculate the relative concentrations of each component in each sample submitted for HPLC analysis. Your samples were run on a Chiralpak IB column. The column was run at 1mL/min flow rate, 0.5 μL injection, with an eluent of 96% hexanes and 4% isopropanol run for 7 min. The elution was monitored at 254 and 214 nm, and the integration of peaks was included in report. If you did not get HPLC results, ask to share results with a neighboring group. A set of standard samples was run with each section’s samples. - acetophenone: 0.0202 M - R -phenylethanol: 0.0189 M - S -phenylethanol: 0.0196 M - Mix acetophenone: 0.0177 M, R -phenylethanol: 0.0212 M, S -phenylethanol: 0.0197 M (6) Fill in the table at the bottom of the page by calculating the appropriate values. Only write down one example calculation for each of the following questions. When recording the %ee, be sure to mark which enantiomer was in excess (R or S). (7) Use the data from the HPLC standards to calculate the molar ratio of each of the three compounds formed in both the NaBH 4 reduction and the catalytic transfer hydrogenation product mixtures. Perform both calculations in the space below, then fill in the table. (8) What is the % yield of 1-phenylethanol (the sum of both ( R )- and ( S )- enantiomers) for each reaction? (9) Use the data from the HPLC chromatograms to calculate the %ee of each reaction.

Reaction Integrated Areas Acetophenone: (R) phylethanol: (S)-Phenylethanol Molar Ratios Acetophenone: (R)-phylethanol: (S)-Phenylethanol Percent Yields Phenylethanol (both R and S) Percent Enantiomeric Excess (% ee) (Label R or S for the excess enantiomer) NaBH 4 reduction Transfer hydrogenation (10) Compare the yield and selectivity of the NaBH 4 reduction reaction to that of the transfer hydrogenation reaction. Section 2: NMR practice problems Consider the acid-catalyzed hydration of 1-hexene below: (11) Below are the structure and 1 H NMR spectrum of 1-hexene in CDCl 3 . Each hydrogen atom in the structure has been labeled with a letter (H A -H G ). After completing the rest of this section, you should write the letters near each resonance (or overlapping groups of resonances) on the spectrum below to indicate peak assignments.

(12) Predict the Multiplicity for H a , H b and H c based on the structure. Multiplicity H a Multiplicity H b Multiplicity H c (13) Draw a splitting diagram for H c below. Assume J ac = 16Hz, J bc = 10Hz, and J cd = 6 Hz. Using this diagram, draw a sketch of your predicted appearance of the spectrum for H c .

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Below is an expanded region of 1-hexene 1 H spectrum (4.8-6.0 ppm). The peaks between 5.8-6.0 ppm are assigned to H c . Peak Table: ppm Hz 5.93 2372.31 5.91 2365.63 5.90 2362.14 5.90 2358.94 5.89 2355.37 5.87 2348.59 5.86 2345.02 5.85 2341.82 5.84 2338.33 5.83 2331.64 (14) Using the expanded view of the 4.8-6.0 ppm region of the 1-hexene NMR spectrum and the peak table provided on the previous page, calculate the following coupling constants. Place your answers on the lines and show your work. J ac =______________Hz Show work: J bc = _____________Hz Show work: J cd = _____________Hz Show work:

(15) How did you decide which coupling constants were which (which one was J ac , which one was J bc etc.)? (16) Based on your answers above, assign the peaks between 4.95-5.10 ppm. Is there anything surprising about the appearance of these peaks? (17) The region for 0.8-2.3 ppm is expanded below. Assign all peaks.

(18) Consider the 1 H NMR spectra of 2-hexanol and 3-hexanol shown below. Draw the correct molecules on each spectrum, mark chemically equivalent 1 H’s, and assign the peaks in both spectra. Section 3. Analyze the 1 H NMR spectra of adipic acid, salicylic acid, Crop A, and Crop B. (19) Assign the spectra of pure adipic acid and pure salicylic acid and attach the assigned spectra to this report. Give a brief description of your justification/reasoning for your assignments of adipic acid and salicylic acid 1 H NMR spectra. (20) Compare your spectra of Crop A and Crop B to the pure spectra. Briefly explain any differences between your spectra and the standards (2 sentences). (21) What are the relative concentrations of adipic and salicylic acid in Crops A and B? Show your work. 3-hexanol 2-hexanol

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Section 4: Discussion (22) Suggest two changes to the experimental procedure that you predict would improve the yield of the transfer hydrogenation reaction? (1-2 sentences) (23) If your 1 H NMR spectra of Crop A and B contained other peaks besides those of adipic and salicylic acids, suggest what impurities might be in your product mixtures. Discuss in any other abnormalities of your spectra. (24) Compare the relative concentrations of adipic and salicylic acid in Crops A and B that you determined using NMR to those you determined by melting point several weeks ago. Do the two techniques produce similar results? (25) If any of your product mixtures contained NaCl you would not be able to determine this using either NMR or HPLC. Why not? Give one reason per technique. (26) Summarize, in one sentence, the experimental and analytical work that you did today; and in another sentence, summarize what you have learned. Materials to Include with your Laboratory Report HPLC chromatograms of the NaBH 4 and transfer hydrogenation reactions. Labeled and assigned 1 H NMR spectra of Crop A and Crop B of recrystallization. You will write the labels and assignments by hand during lab.