Lab Exam 2017

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School

University of California, Berkeley *

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Course

12A

Subject

Chemistry

Date

Dec 6, 2023

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pdf

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Uploaded by AdmiralRiverFish

Page 1 of 9 C HEMISTRY 12A F ALL 2017 L ABORATORY E XAM N OVEMBER 28, 2017 N AME - WRITE BIG ________________________________________ S TUDENT ID: __________________ S ECTION AND GSI: ___________ ________________________ • You will have 45 minutes in which to work. • BE NEAT! Non-legible structure drawings will not be graded. • Only answers in the answer boxes will be graded – you can write in other places, but we only grade the answers in the boxes. • All pages of the exam must be turned in. • No calculators • Molecular models may be used Problem Points (Maximum) 1 48 2 42 3 12 Total 100

Page 2 of 9 1. (46 points) You decide to carry out the following reaction: a. Write the mechanism for this reaction. Show the formation of both stereoisomers. b. Identify the relationship between the hydrogens in each indicated pair as enantiotopic, diastereotopic, or equivalent (homotopic). Pairs of hydrogens Relationship Ha and Hb Hc and Hd Hf and Hg Hh and Hi H OH OH H OH + H + nerol α -terpineol OH H OH Ha Hf Hc Hh Hj He Hb Hg Hi Hd

Page 3 of 9 c. The 1 H NMR spectrum for nerol is shown below. Label all hydrogen atoms with letters – label the hydrogen on the OH as Ha and proceed with your labeling counterclockwise around the molecule. Hydrogen atoms that are magnetically equivalent should have the same letters. Assign the peaks in the NMR spectrum. You may not be able to assign all peaks definitively. d. An expansion of the region between 5 and 5.5 ppm is shown below. What causes the peak at 5.05 to be broad? OH Integration 1 1 1 2 3 3 3 4 5

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Page 4 of 9 e. The spectrum of terpineol is shown below. Assign the following three peaks: i. 5.35ppm ii. 4.49 ppm iii. 1.20 ppm Integration 1 1 1 3 3 1 1 1 6 H OH

Page 5 of 9 f. If you had a mixture of nerol and terpineol, which two peaks would you use to determine the relative ratio of nerol and terpineol in your crude reaction mixture? i. Circle these peaks on the spectrum of the mixture shown below: ii. Explain your choices briefly. iii. If the integration of the peak you chose from nerol is A and that from terpineol is B, show the equation you would use to calculate the ratio of nerol to terpineol.

Page 6 of 9 2. (42 points) You decide to carry out the following reaction: a. You decide to monitor this reaction by TLC. What do you use to visualize the plates? Explain your choice briefly. b. You run your TLC in 1:1 hexanes:ethyl acetate. You spot both starting materials (A and B) and the product mixture (P) on the plate. Fill in the spots in approximate positions. Assume your reaction went to approximately 75% completion. Br is more electronegative than S. Explain your sketch briefly. O Br HO O S HO SH + A B p K a = 5 p K a = 10 A B P

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Page 7 of 9 c. You decide to purify your product using extraction. Choose from the following solutions and write a flow chart for your extraction. Finish the extraction with all compounds in organic solvent so that they can be isolated by simply evaporating the solvent i. NaOH, aqueous at pH = 12 ii. buffered aqueous phosphate buffer, pH = 7.5 iii. HCl, aqueous at pH=2 iv. Ethyl acetate d. Before evaporation, you decide to add Na 2 SO 4 to your final organic layer to remove any water that may be present. How can you tell when you’ve added enough? e. Will your product be pure at the end of the extraction? Why or why not? If not, what additional method of purification could you use?

Page 8 of 9 f. You decide to evaporate the solvent from the first organic layer of your extraction and are surprised to find the following compound. Write the mechanism for its formation and explain why is it in the organic layer. g. Because of the impurity shown in part f, you decide to purify the reaction mixture using reverse phase HPLC rather than extraction. i. In what order do you expect the staring materials and product to elute? ii. Explain why you observe this order of elution in HPLC. Discuss the intermolecular interactions involved. O O

Page 9 of 9 3.5 4.0 4.5 5.0 5.5 ppm 3.5 4.0 4.5 5.0 5.5 ppm 45 3. (12 points) Consider the following 2D NMR spectrum shown below. Indicate which protons are coupling to each other. A couples to ________________ B couples to ________________ C couples to ________________ D couples to ________________ E couples to ________________ F couples to ________________ A B C D E F A B C D E F

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