Chem 281 2004-2 Final Examination August 7, 2004 Name: Student Number: Note: You are allowed to use models for this exam. Notes, textbooks and calculators are strictly prohibited. Write your final answers in pen in the space provided. Useful data (periodic table, IR frequencies and NMR shifts) can be found on the last page. Scrap paper will be provided. Good luck! TTAL: 120 marks Page 1
Part I: Multiple Choice Questions. Place the correct answer in the space provided on below. Questions 1-15 are worth 2 points each. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 1. What is the degree of unsaturation for a molecule with the formula C 6 12 N? (a) 0 (b) 1 (c) 2 (d) 3 2. Which of the following represents the most stable conformation for (1R,3R)-1,3-dichlorocyclohexane? 3. The following pair of molecules have what relationship to one another? F C 3 C 3 4. Rank the following in order of increasing stabilities. C 3 F (a) Identical molecules (b) constitutional isomers (c) enantiomers (d) diastereomers C 3 C C 3 3 3 C C 3 C 3 I II C 3 III IV C 2 C 3 3 C C 2 C 3 C 2 C 3 C 3 (a) III < I < IV < II (b) II < I < IV < III (c) II <I < III < IV (d) II < IV < I < III 5. If a mixture contains 25% of the R-enantiomer of a compound and 75% of the corresponding S- enantiomer, what is the e.e. for the mixture? (a) 75% (b) 50% (c) 3 (d) 25% (a) (b) (c) (d) 3 C 3 C C 3 C 2 C 3 6. What effect will doubling the concentration of potassium tert-butoxide have on the rate of the following reaction? a) No effect - K + b) The rate will double c) The rate will decrease Ts d) The rate will quadruple Page 2
7. What are the relative rates of the following reactions via an S N 1 mechanism? (a) I > II > III > IV (b) III > I > IV > II (c) IV > III > I > II (d) I >II > IV > III I NaCN Et CN II C 2 C 3 Et III F LiF hexane IV CN NaCN Et 8. Which of the following systems will most favour elimination over substitutiont? a b NaEt Et NaEt Et?? c d NaC 3 C 3 9. Which of the following molecules should have the highest heat of hydrogenation? (a) (b) (c) (d) 10. Which of the following compounds is least likely to form from the radical bromination of 2-hexene? (a) (b) (c) (d) 11. ow many constitutional isomers are there with the formula C 4 10? (a) 4 (b) 7 (c) 10 (d) none of the above 12. Which of the following compounds will be converted into an achiral p roduct by treatment with potassium tert-butoxide? K?? a b c d Page 3
13. Which of the following molecules has the lowest pka? a b c d 14. Which of the following structures is NT a resonance structure of I? I N N N N 15. Which of the following molecules has the configuration (2R, 3S)? C 3 F F F F a b c d 3 C 2 N F 3 C C 2 C 3 C 3 C 2 C 3 C 3 C 2 C 3 Part II: True/False Label each of the following statements as either true or false. (10 marks) 16. All strong bases are good nucleophiles 17. The conjugate bases of strong acids are good leaving groups. 18. Cations are stabilized by electron-withdrawing groups. 19. A molecule with an absolute R-configuration will always have a (+)-optical rotation. 20. The boiling points of a pair of diastereomers will always be the same. C 3 N F Page 4
Part III: Fill in the blanks. 21. The reactions below are missing some or all of reagents necessary to carry out the indicated transformations. Place the letter of the necessary reagents from the list below in the appropriate box. Each set of reagents should be used exactly once. (8 marks) Reagents I. II. III. IV. Part IV: Short Answer A. (i) C 3 C 3 (ii) E. 2 S 4, heat B. F. 2, C 4 C. 2, UV-light G. (i) g(ac) 2, 2 ; (ii) NaB 4 D. 2, 2. (i) B 3 -TF; (ii) 2 2, - 22. Compound A reacts with to yield three products, B, C and D. B is a six memebered ring, while C and D are both seven membered rings. Draw the structures of B, C and D. (10 marks) A V. VI. VII. VIII. B + C + D Page 5
23. Draw the major organic products for the following reactions. Be sure to indicate the correct stereochemistry of all products. (15 marks) I. 2 C 4 II. III. 2 Pd NaC 3 C 3 24. Suggest a sequence of reactions that will allow us to carry out the following transformation. (10) Page 6
25. The reaction of compound A with leads to a mixture of two enantiomers (R)-B and (S)-B. The isolated reaction mixture has specific optical rotation α= +15. Pure (R)-B has an optical rotation of α=-30. Na 2 A (R)-B (S)-B (a) (b) (c) (d) What is the ratio of R:S for this reaction? Which enantiomer is favoured? (2 marks) Explain this ratio in terms of the mechanism for the reaction. (5 marks) Suggest two ways to improve the e.e. of this reaction. (5 marks) Addition of catalytic amounts of potassium iodide (KI) to this reaction mixture leads to the same final products, but the ratio of R:S is different: in this case, α= -25. Explain why. (10 marks) Page 7
26. An unknown compound with formula C 8 16 2 was found to have the 1 -NMR shown below. a) Draw the structure of this compound. A table of NM R chemical shifts can be found on the last page of this exam. (10 marks) b) What further experiment could you perform to help confirm the identity of this compound? Be specific as to what information this experiment would give and how it would help in the identification. (5 marks) Doublet int=2 Doublet int=6 Septet int=1 Nonet (9) int=1 Doublet int=6 Page 8
Approximate 1 -NMR Chemical shifts Type of ydrogen Chemical Shift, d (ppm) 1 alkyl, RC 3 0.8-1.0 2 alkyl, RC 2 R 1.2-1.4 3 alkyl, R 3 C 1.4-1.7 alkyl iodide, RC 2 I 3.1-3.3 alkyl bromide, RC 2 3.4-3.6 alkyl chloride, RC 2 3.6-3.8 ether, RC 2 R 3.3-3.9 alcohol, C 2 R 3.3-4.0 alcohol hydroxyl, R 0.5-6.0 aromatic, Ar 6.0-9.5 amino, R-N 2 1.0-5.0 phenolic, Ar 4.5-7.7 allylic, R 2 C C C 3 1.6-1.9 R vinylic, R 2 C C 2 4.6-5.0 vinylic, C 5.2-5.3 R 2 C R aldehyde, R C 9.5-10.5 carboxylic acid10.3-13 R C Selected IR Absorptions Functional Group Band Position (cm -1 ) alkenes =C- 3020-3100 C=C 1640-1680 alkynes C- 3300 C C 2100-2260 ethers C- 1100-1300 alcohols C- 1050-1150 - 3400-3650 alkyl halides C- 600-800 C- 500-600 C-I 500 amines N- 3300-3500 C-N 1030-1230 carbonyl compounds C= 1670-1780 ketone R C C 3 1.8-2.6 acetylenic, RC C 2.5-3.1 carboxylic acids - 2500-3100 nitriles C N 1540 Page 9
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