(2) After dissolving a solid in a solvent at high temperature, the solution is not filtered.

Transcription

1 Name Key 216 W13-Exam No. 1 Page 2 I. (10 points) The goal of recrystallization is to obtain purified material with a maximized recovery. For each of the following cases, indicate as to which of the two goals, (A) maximum recovery and (B) purity, would be adversely affected by the recrystallization method indicated. Write (A) or (B) for each of the questions. (1) In the initial dissolution step, an unnecessarily large volume of solvent is used. (2) After dissolving a solid in a solvent at high temperature, the solution is not filtered. Answer (A) Answer (B) (3) The hot solution obtained by gravity filtration after dissolving a solid in a solvent at high temperature is cooled to room temperature using an open beaker. Answer (B) () Crystallization is accelerated by immediately placing the flask of hot solution in an ice-water bath. Answer (B) (5) The crystals obtained after recrystallization are collected by gravity filtration. Answer (B) II. (11 points) In connection with our research directed at probing the molecular mechanism of chemical carcinogenesis, we carried out a series of synthetic reactions shown below. Consider the relative R f values of the compounds shown in the following reaction scheme when analyzed by thin-layer chromatography (TLC) on silica gel-coated plates. NaB cat. Cl 1. AgCC 3 I 2, Δ C 2 5 C 3 2. NaC 3 Δ C (1) (2 points) Which one should show the lowest R f among all of the compounds 1 through? Answer (2) (2 points) Which of compounds 1 and 2 should show a higher R f? Answer 1 (3) (2 points) Which of compounds 1 and 3 should have a higher R f? Answer 3 () (2 points) Which of compounds 2 and 3 should have a higher R f? Answer 3 (5) (3 points) Compound 1 is shown to have a high R f value (~0.8) when a mixture of 3 : 1 ethyl acetate/hexanes is used as an eluent. ow would you change the solvent system in order to have a considerably lower Rf value for this compound? Answer: Increase the fraction of hexanes in the mixture of the two solvents, e.g., 1 : 3 ethyl acetate/hexanes

2 Name Key 216 W13-Exam No. 1 Page 3 III. (17 points plus 3 bonus points) N-Benzylaniline can be prepared by reaction of benzyl chloride and aniline in the presence of aqueous sodium bicarbonate (NaC 3 ). Cl + N 2 + NaC 3 N + NaCl C 2 benzyl chloride aniline N-benzylaniline The procedure calls for 372 g of aniline, 127 g of benzyl chloride, and 105 g of sodium bicarbonate dissolved in 100 ml of water, and produces 160 g of N-benzylaniline. Write an answer in the box provided to each of the following questions. (1) (3 points) Calculate as to how many moles of each of the reagents are used in this procedure. Use the following atomic weights: C = 12, = 1, N = 1, = 16, Na = 23, Cl = 35.5 benzyl chloride: _1.00 moles; aniline: _.00 moles; NaC 3 : _1.25 moles (2) (3 points) What is the limiting reagent? benzyl chloride (3) (3 points) What is the theoretical yield of N-benzylaniline? Show your work. 183 g; 1.00 moles x 183 g/mole = 183 g () (2 points) What is the percentage yield of N-benzylaniline? Show your work. 87 %; (160 g / 183 g) x 100 = 87% (5) (3 points) About 260 g of aniline can be recovered from the reaction. Does this affect the calculation of theoretical yield? Explain. The recovered amount of aniline corresponds to 2.80 moles. Since in this experiment, mol equiv of aniline is used relative to the limiting reagent, benzyl chloride, it does not affect the calculation of theoretical yield. (6) (3 points) What volume of benzyl chloride (density 1.10 g/ml) is required for the reaction? 115. ml (7) (3 points; bonus question) Why is it necessary to use such a large excess of aniline for the reaction? Explain. As N-benzylaniline is more nucleophilic than the starting aniline, it can undergo multiple benzylations. By the use of a large excess of aniline, the overall reaction can be directed toward the monobenzylation of aniline.

3 Name Key 216 W13-Exam No. 1 Page IV. (15 points) n the basis of its spectroscopic information provided below, answer the following questions given and draw the structure of an unknown compound (C ; a strong IR absorption in the cm -1 region) in the box below. In addition, assign all 1 and 13 C peaks to the corresponding nuclei. Please note that one of the 1 peaks may be assignable to the hydrogen(s). 300 Mz 1 NMR spectrum in CDCl 3 (δ ppm) Proton-decoupled 13 C NMR spectrum in CDCl 3 (δ ppm) integration values The letter s, t, or q on top of each peak denotes the splitting pattern, singlet, triplet, or quartet, respectively, when the spectrum is run in the proton-coupled mode.! (1) (1 points) What is (are) the unit(s) of unsaturation of this compound? zero (2) (2 points) Does this compound have an ether (R--R ) group? N (3) ( points) What is the structure of () (8 points) Assign all 1 and 13 C chemical shifts to the compound? (no partial) the corresponding and C nuclei in the proposed structure. 2.8 ppm 3.55 ppm 1.35 ppm C C C 68 ppm C ppm ppm 1 C ppm C C 0.8 ppm 3.55 ppm 1.35 ppm 1 pt for each chem shift assignment 8

4 Name Key 216 W13-Exam No. 1 Page 5 V. (15 points) An unknown compound, C 9 12, has the following spectral properties. IR (neat): (broad and strong), (many medium strength peaks); no absorption peaks in the cm -1 region. 1 NMR in CDCl 3 : δ 1.89 (2, quintet, J = 7.0 z), 1.98 (1, s), 2.65 (2, t, J = 7.0 z), 3.60 (2, t, J = 7.0 z), and ppm (multiplets, 5 s). 13 C NMR in CDCl 3 : δ (t), 3.15 (t), (t), (d), (d), (d), and (s) ppm. The letters s, d, and t indicate splitting patterns when the spectrum is run under a proton-coupled mode. (1) (1 points) What is (are) the unit(s) of unsaturation of this compound? (2) (2 point) Does this compound have a benzene ring? Yes No (circle the one that applies) (3) (2 point) What does the IR spectrum tell you in terms of the presence of an -containing functional group in this molecule? (or hydroxyl) group () ( points) What is the structure of the (5) (6 points) Draw in the box below the structure compound? (no partial) you drew in () and assign all non-aromatic 1 peaks and the 13 C peaks at and ppm to the corresponding nuclei in the proposed structure ppm 2.65 ppm (t) 3.60 pm (t) ppm 1.98 ppm (s) 1.89 ppm (quintet) 1 pt for each chem shift assignment VI. (15 points) n the basis of its spectroscopic information provided below, answer the questions given on page 6 and draw the structure of an unknown compound (C ; IR a strong peak at 173 cm -1 ; no peaks in the cm -1 region) in the box on page 6. In addition, assign all 1 peaks to the corresponding nuclei. 6

5 Name Key 215 W13-Exam No. 1 Page 6 VI. (continued) 13 C NMR spectrum in CDCl 3 (1) (1 points) What is (are) the unit(s) of unsaturation of this compound? one! (2) (2 point) What does the IR spectral information tell you in terms of the presence of an -containing functional group in this molecule? (3) (2 points) What do the 13 C NMR spectral data tell you in terms of the presence of an -containing functional group in this molecule? carbonyl group ester group () ( points) What is the structure of the (5) (6 points) Draw in the box below the structure compound? (no partial) you drew in () and assign all 1 peaks and the 13 C peak at 6.38 ppm to the corresponding nuclei in the proposed structure ppm.0 ppm 0.9 ppm 3 C C ppm 1.6 ppm 6.38 ppm 1 pt for each chem shift assignment 6

6 Name Key 215 W13-Exam No. 1 Page 7 VII. (8 points) For each of the following reactions, draw in the box provided the structure of the expected major product. (1) 1. AlCl 3 Cl work-up (2) 1. Cs 2 C 3 C 3 C 2 -I 50 C, DMF work-up C 2 C 3 VIII. (9 points) The 1 NMR data for and 5 of diol 5 are shown below. Please note that two hydroxy s are not involved with 1 1 couplings. δ 3.88 ppm (dd, J = 3. and 2.5 z) δ 3.72 ppm (dd, J = 12.0 and 3. z) Si C 3 C Si (1) n the basis of the NMR data above, show the stereochemistry of alcohol 5 at C- and C-5 by writing in the boxes above right s and s (6 points). (2) Indicate below the coupling constants for each of the following (3 points): 3 J 3, = 12.0 z; 3 J,5 = 3. z; 3 J 5,6 = 2.5 z