LOYOLA COLLEGE (AUTONOMOUS), CHENNAI – 600 034

M.Sc. DEGREE EXAMINATION – CHEMISTRY

GH 52

SECOND SEMESTER – APRIL 2008

CH 2801 – ORGANIC CHEMISTRY – II

 

 

 

Date : 23/04/2008            Dept. No.                                        Max. : 100 Marks

Time : 1:00 – 4:00

Part–A

Answer ALL questions.                                    (10 ´ 2 = 20)

1. Which reagent in each of the following pairs is the more nucleophilic? Justify your answer.
   a) Urea or thiourea     and      b) Phenoxide or p-nitrophenoxide
2. Elimination of HBr from 2-bromobutane and elimination of trimethylamine from
   2-(N,N-dimethylamino)butane give different products in major amounts. Why?
3. Debromination of meso-2,3-dibromobutane mainly gives trans product while
   (±)-isomer gives mainly cis product. Explain.
4. Explain Grunwald-Winstein relationship.
5. ‘For S_N1 mechanism, a branching increases the rate of the reaction’.
6. What is denaturation of protein?
7. Explain memory effect with an example.
8. Is Hofmann rearrangement inter or intramolecular?
9. What are coenzymes? Give the structure of one of the coenzymes.
10. List the major differences between DNA and RNA.

Part-B

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

 

11. Menthyl chloride on dehydrochlorination gives 100% of one isomer while
    neo-menthyl chloride gives 25% of that isomer and 75%4-methyl isopropyl cyclohexene. Explain.
12. List the evidences for S_N1’ mechanism with benzene diazonium salts and explain with suitable example.
13. How are the rates of nucleophilic substitution reactions are influenced by electron withdrawing and electron releasing groups? Explain with two examples each.
14. When 2,6-di-tert-butylphenol, a colorless solid, reacts with alkylperoxy radicals, a red  “dimeric” product is formed. Suggest a structure for this dimmer and provide a  mechanism for its formation.
15. On pyrolysis, erythro and threo isomers of 1-acetoxy-2-deutero-1,2-diphenylethane gave in each case trans-stilbene. Explain.
16. Explain the different types of RNA and their function with their structures.
17. Explain the mechanism of abnormal claisen rearrangment.
18. Write a note on biosynthesis of enzymes.
19. Explain the principle involved in the peptide synthesis with a suitable example.
20. Give the mechanism of Hoffmann rearrangement and suggest three evidences.
21. What happens when 2-methyl-1-propanol is treated successively with CS₂/NaOH, CH₃I and heated to 200°C?
22. What are the evidences for syn elimination reaction? Explain with suitable examples.

Part-C

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

23. a) Explain the recombinant DNA technology.
24. b) Determine the amino acid sequence of a heptapeptide (A) with the composition:

Leu₂, Ala₂, Tyr₂, Gly. A reacts with DNFB to give N-DNP tyrosine. A reacts with

carboxy peptidase to give alanine. On partial hydrolysis A yields the following

peptides: Leu-Ala, Tyr-Ala, Ala-Tyr, Gly-Leu-Leu, and Tyr-Gly.

24. a) When the elimination of HF from 1,1-dichloro-2,2,2-trifluroethane is carried out with sodium

methoxide in deuterated methanol, 1,1-dichloro-1-deuterio-2,2,2- trifluoroethane can be

recovered from the reaction mixture.  Suggest a suitable mechanism.

1. How is diethyl malonate converted into cyclopentane?
2. 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’.   Why?

1. b) Explain von-Ritcher rearrangement with mechanism.
2. a) What is ‘product spread’ in allylic nucleophilic substitution reactions? Explain.
3. b) ‘Acetolysis of both 4-methoxy-1-pentylbrosylate and 5-methoxy-2-pentylbrosylate give the same

mixture of products’. Explain.

27. a) List and explain the factors influencing enzyme activity with suitable examples.
28. b) What are the characteristics of free radical reactions?
29. Explain the following:
30. Cis-1,2-dibromecyclohexane undergoes dehalogenation with KI in ethanol at a slower rage than the trans– isomer does.
31. ‘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’.
32. The relative rates of solvolysis of tert-BuBr in 60% ethanol is 2.41 ´ 10⁴ while in water is 1.2 ´ 10⁶.
33. Br^– is displaced by the S_N2 mechanism more rapidly from CH₂BrCl than from CH₂Br₂.

 

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