LOYOLA COLLEGE (AUTONOMOUS), CHENNAI – 600 034

M.Sc. DEGREE EXAMINATION – CHEMISTRY

AD 14

FIRST SEMESTER – NOV 2006

CH 1806 – ORGANIC REACTION MECHANISM & STEREOCHEMISTRY

 

 

Date & Time : 28-10-2006/1.00-4.00     Dept. No.                                                       Max. : 100 Marks

 

 

 

PART A

Answer ALL questions.                                                                   (10 ´ 2 = 20 marks)

 

1. What is transition state theory? Mention its importance.
2. What are the various kinetics experiments to study the mechanism of a particular reaction?
3. Classify the rearrangement reactions and give example for each.
4. What is memory effect? Explain with an example.
5. Write a short note on catalytic dehydrogenation.
6. Explain Cram’s rule with a suitable example.
7. Explain why meso-stilbene dichloride on heating with pyridine at 200°C gives no product while dl-isomer gives alkene readily.
8. What is epimerisation? Give an example.
9. Explain Walden inversion with a suitable example.
10. Explain the racemization via cations with a suitable example.

 

 

PART B

Answer any EIGHT questions.                                              (8 ´ 5 = 40 marks)

 

11. How trapping of intermediate, cross-over experiments and isotopic labeling studies help to show that a reaction is intermolecular? Explain with suitable examples. (5)
12. How does the kinetics study in chain and parallel reactions help to design the reaction mechanism? Explain with suitable examples. (5)
13. Explain the mechanism of the following rearrangement reactions. (2 ´ 2½)
14. a) Beckman rearrangement b) Baeyer Villiger rearrangement
15. a) Why secondary amides do not undergo Hoffmann degradation? Give reasons. (2½)
16. b) Explain the mechanism of Claisen rearrangement with suitable example. (2½)
17. Explain the mechanism of NaBH₄ reduction. How is it chemoselective? Explain with an example.                                                                                                    (5)
18. How direct electron transfer can bring in reduction? Explain the mechanism with any two examples.                                                                                                    (5)
19. Predict the Cotton effect by the sign of the most occupied octant in the following compounds:

(i) (+)-trans-1-methyl-2-decalone            (ii) 5a-cholestan-6-one.   Explain.                  (2 ´ 2½)

18. Dehydrobromination of diastereoisomeric a-phenyl-b-bromo-b-benzoylpropionic acids with    hot pyridine give different products.  Explain.
19. Write notes on: a) asymmetric destruction  and  b) asymmetric synthesis
20. ‘The cotton effect curves of (+)-3-methylcyclopentanone in methanol shows only a 24%                 reaction upon addition of acid’.  Explain.
21. Give an example for first order and second order asymmetric transformations. Explain.
22. What are the different methods of resolution of racemic mixture? Explain any two methods.

 

 

PART C

Answer any FOUR questions.                                                          (4 ´ 10 = 40 marks)

 

23. a) How does the kinetics study help to design the reaction mechanism in the following      reactions?                                                                                                    (2 ´ 2½)
24. i) Aldol condensation ii) Benzidine rearrangement
25. b) How stereochemical study and isotopic labeling study help to design a reaction      mechanism as intramolecular? Explain with suitable examples.                                   (5)
26. a) Explain the mechanism of                                                             (2 ´ 2½)
27. i) Fischer Indole synthesis and      ii) Favorskii rearrangement
28. b) Predict the products in the following reactions. Give suitable reasons. (5)

 

25. a) Explain the reductive coupling of carbonyl compounds with any two examples. (5)
26. b) How does specific dehydrogenation in steroids take place? Explain the mechanism with an (5)
27. a) Explain the octant rule with a suitable example.
    b) Predict the Cotton effect by the sign of the most occupied octant in the following     compounds:

(i) 3-bromo-4-methoxycyclohexanone           (ii) 3-methylcyclohexanone

27. Explain the following:
    a)  trans-4-t-butylcyclohexane carboxylic acid is more stronger than cis-isomer.
    b) erythro-3-bromo-2-butanol with HBr gives meso product
    c) Dehydrochlorination of 1,2-diphenyl-1-chloroethane gives trans product fastly but cis     product slowly.
28. a) Explain the following with suitable example.
    a)      Asymmetric destruction
    b)      Bredt’s rule
29. c) Axial haloketone rule
30. Prelog’s rules

 

 

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