LOYOLA COLLEGE (AUTONOMOUS), CHENNAI – 600 034

DB 21

M.Sc. DEGREE EXAMINATION – CHEMISTRY

FIRST SEMESTER – November 2008

           CH 1806 – ORGANIC REACTION MECHANISM & STEREOCHEMISTRY

 

 

 

Date : 04-11-08                 Dept. No.                                        Max. : 100 Marks

Time : 1:00 – 4:00

Part-A

                                                     Answer ALL questions.                                      (10 ´ 2 = 20)

1. R-CN is prepared by treating RCOO^– with BrCN. Where the carbon in RCN comes from? How?
2. How would you effect the following conversion? Explain with mechanism.
3. What is memory effect in rearrangement? Explain with an example.
4. What happens when phenyl azide is heated with aniline? Explain the formation of the product.
5. Explain the mechanism of ozonolysis of alkenes with an example.
6. What would be the preferred conformer of trans-1,2-dibromocyclohexane? And how its conformation changes with solvent polarity?
7. Explain why the optically active 2-benzoylpropionic acid undergoes racemization when treated with sodium ethoxide in ethanol?
8. Define enantiomeric excess. What is its percentage value for active and racemic products?
9. Solvolysis rate of cis-4-t-butyl cyclohexyl tosylate is greater than that of trans-isomer. Explain.
10. What is atrolactic acid rule? Give one example.

 

Part-B

                                                     Answer any eight questions.                              (8 ´ 5 = 40)

11. Explain the mechanism of Arndt-Eistert synthesis. Mention any one evidence for the mechanism.
12. How do isotopic labeling and stereochemistry help in predicting suitable reaction mechanisms? Give examples.
13. Explain kinetic and thermodynamic controls of a chemical reaction with a suitable examples.
14. ‘Diazotisation of aniline follows II order kinetics in nitrous acid’. Explain the mechanism.
15. How was the mechanism of von-Ritcher rearrangement confirmed? Explain the steps.
16. How would you effect ring cyclisation reaction in Fischer-Indole synthesis? Explain with mechanism.
17. What are the conditions to be fulfilled for a resolution process through diastereomers formation?
18. Why the deamination of (Ph)(Ph’)(OH)C-CH(NH₂)CH₃ does not follow Curtin-Hammet principle? Explain.
19. Discuss the steric course of the acetolysis reaction of 2-phenyl-3-pentyl tosylate &
    3-phenyl-2-pentyl tosylate.
20. Predict the stereochemical change of acetolysis of 3-methoxy-2-bromobutane in the presence of silver acetate in acetic acid.
21. Discuss the conformation analysis of 1,2 & 1,3-disubstituted cyclohexane.
22. Predict the Cotton effect for the following compounds
23. a)                                      b)                                      c)

 

 

Part-C

                                                     Answer any four  questions.                              (4 ´ 10 = 40)

23. a) How does the kinetic study in chain and parallel reactions help to design the reaction mechanism? Explain with suitable examples.
24. b) Explain the mechanism of Favorskii rearrangement. How is it useful in ring contraction reactions?
25. a) Explain the Wagner Meerwein rearrangement reactions. How does neighbouring group help in this rearrangement? How do elimination and substitution reactions compete in the product formation?
26. b) Explain the mechanism of the following rearrangement reactions. (2 ´ 2½)
27. a) Beckman rearrangement        b) Baeyer Villiger rearrangement
28. a) How do amines undergo dehydrogenation? Explain the mechanism.
29. b) Explain Gomberg-Bachmann Pinacol synthesis with a suitable example.
30. c) Explain the mechanism of cannizaro reaction.
31. a) Discuss the optical isomerism of allenes and spiranes.
32. b) Explain the reaction of erythro-3-bromo-2-butanol with HBr
33. Explain the following with example
34. a) Cram’s cyclic model
35. b) Atropisomerism
36. c) Chemical method of racemisation by anion intermediate formation
37. Discuss in detail the stereo selective synthesis of yohimbine.

 

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