LOYOLA COLLEGE (AUTONOMOUS), CHENNAI – 600 034

GH 30

M.Sc. DEGREE EXAMINATION – CHEMISTRY

SECOND SEMESTER – APRIL 2008

          CH 2808 – ORGANIC SUBSTITUTION, ADDITION & ELIMINATION RXNS

 

 

 

Date : 17/04/2008            Dept. No.                                        Max. : 100 Marks

Time : 1:00 – 4:00

PART – A

Answer ALL questions              (10 ´ 2 = 20 marks)

1. ‘Halocycloalkanes undergo nucleophilic substitution reactions slowly’. Why?
2. How would you prove that the transition state in S_N2 reaction must be linear? Explain.
3. ‘The rate of solvolysis of Ph-CH₂-CH₂-Ots in 75% in CF₃-COOH is 3040 times faster than the rate for CH₃-CH₂-OTs’.  Explain.
4. Identify the different products obtained on the pyrolysis of 2-butylacetate and justify your answer.
5. What is Chugaev reaction? Give an example.
6. In the free radical mechanism, abstraction of a univalent atom is energetically more favored. Explain why?
7. Give any three evidences for A-SE2 mechanism.
8. ‘S_N1 rates are greatly increased when there is a double bond in the b-position’. Explain.
9. What is partial rate factor? Explain.
10. Explain syn elimination with two suitable examples.

 

PART – B

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

 

11. Explain von-Ritcher rearrangement with mechanism. Give the main evidence for the mechanism.
12. Between menthyl chloride and neomenthyl chloride, which one undergoes dehydrochlorination at a faster rate? Account for your answer.
13. Identify the products obtained when erythro and threo isomers of 1-acetoxy-2-deutero-1,2-diphenylethane is subjected to pyrolysis. Justify your answer.
14. What happens when 4-methoxy-1-pentylbrosylate and 5-methoxy-2-pentylbrosylate are subjected to acetolysis?  Justify your answer.
15. Explain free radical rearrangement reaction with two suitable examples.
16. Explain the various products formed from the reaction of electrophile attacking at the ipso position in nitrobenzene with suitable examples.
17. Identify the product obtained on the solvolysis of L-threo-3-phenyl-2-butyl tosylate in acetic acid. Account for your answer.
18. Explain Sommelet-Hauser rearrangement with mechanism.
19. Explain the mechanism of Wohl-Ziegler bromination reaction with a suitable example.
20. Explain any two synthetic applications of selenoxide elimination reaction with mechanism.
21. Explain the stereochemistry of the products obtained in the oxymercuration-demercuration reactions on unsymmetrical olefins with suitable examples.
22. What are nonclassical carbocations? How are they formed?  List the evidences for their formation.

 

PART – C

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

23. a) ‘Solvolysis of syn-7-p-anisyl-anti-7-norborn-2-enyl p-nitrobenzoate was only 2.5 times faster than that of the saturated compound’. Explain.
24. b) Explain the stereochemical orientation of the epoxidation reaction of 4-methyl cyclopentene.
25. a) Explain Benzyne mechanism. What is the regioselectivity in the product obtained for the reaction of o-bromoanisole with potassium amide in liquid ammonia? Explain.
26. b) What is the stereochemistry of the product obtained on the acetolysis of cis and trans isomers of 2-acetoxycyclohexyl tosylate? Account for your answer.
27. a) ‘Cyclic transition states dictate the geometry of the products obtained in pyrolytic elimination reactions’. Justify this statement with suitable examples.
28. b) Explain the result of nitration in the following alkylbenzenes
    Compound                 ortho/para ratio
    Toluene                                   57
    Ethylbenzene                          0.93
    Isopropylbenzene                    0.48
    tert-butylbenzene                    0.22
29. a) Explain the mechanism and stereochemistry of hydroboration-oxidation reaction of alkenes with suitable examples.
30. b) Identify the major products obtained on the dehydrohalogenation of 2-fluorohexane and 2-iodohexane. Account for your answer.
31. a) Explain the reactions of ambident nucleophiles with suitable examples.
32. b) Identify the stereochemistry of the products obtained on the solvolysis of p-methoxy benzhydryl hydrogen phthalate in methanol. Justify your answer.
33. a) Explain neighbouring group participation in free radical reactions with two suitable examples.
34. b) Explain the following reactions with mechanism

(i) Gomberg reaction  and   (ii) Nazarov cyclization reaction.

 

 

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