LOYOLA COLLEGE (AUTONOMOUS), CHENNAI – 600 034

WD 31

M.Sc. DEGREE EXAMINATION – CHEMISTRY

SECOND SEMESTER – April 2009

                    CH 2814 / 2808 – ORGANIC SUBSTITUTION, ADDITION & ELIMINATION RXNS

 

 

Date & Time: 20/04/2009 / 1:00 – 4:00  Dept. No.                                                       Max. : 100 Marks

 

 

PART – A

Answer ALL questions             (10 ´ 2 = 20 marks)

1. ‘2-Bromo-2-methylbutane with ethoxide ion gives 2-methyl-2-butene but with t-butoxide gives 2-methyl-1-butene as major products’. Explain.
2. How many stereoisomers are possible for 1,2,3,4,5,6-hexachlorocyclohexane? Which one will undergo slowest dehydrochlorination?  Why?
3. What happens when 2-methyl-1-propanol is treated successively with CS₂/NaOH, CH₃I and heated to 200°C? Explain.
4. Explain Sommelet-Hauser rearrangement with mechanism.
5. What are extrusion reactions? Give an example.
6. Define the following
7. a) Partial rate factor           b) Hammett equation
8. How does HNO₂ react with 1° and 2° amines?
9. Between CH₃CH=CHCH₂HgBr and CH₃CH₂CH₂CH₂HgBr which is more reactive with hydrochloric acid and why?
10. What are possible termination steps in the free radical substitution in aromatic substrates?
11. Arrange the following –HgR group in their decreasing order of leaving ability in the S_E1 & S_E2 reactions.

-HgMe,   -Hg is-Pr,   -HgEt,   -Hg-t-Bu

 

PART – B

Answer ANY EIGHT questions            (8 ´ 5 = 40 marks)

11. ‘Acetolysis of both 4-methoxy-1-pentylbrosylate and 5-methoxy-2-pentylbrosylate gave the same mixture of products’. Explain.
12. ‘Triphenyl methyl chloride reacts more rapidly with p-nitrobenzyl alcohol in benzene than with p-methylbenzyl alcohol’. Explain.
13. ‘p-methoxybenzhydryl hydrogen phthalate on solvolysis in methanol results a     racemic ether’. Justify your answer.
14. ‘Treatment of substituted anisoles with HI yields CH₃I and a substituted phenol, rather than methanol and a substituted iodobenzene’.  Explain
15. How does selenoxide undergo thermal syn elimination at room temperature? Explain with an example.
16. Is the nucleophilic substitution at a vinyl carbon easy? Justify your answer with a suitable example.
17. Give the mechanism of nitration of benzene. What are the various nitrating agents? Give evidences for the mechanism.
18. Explain the following with suitable example.
19. a) isoracemisation                        b) isoinversion
20. Explain why in the abstraction step of photochemical chlorination of ethane, ethyl radical is formed and not the hydride radical.
21. Give the mechanism of oxymercuration reaction. Also give the evidences for the free
    radical mechanism for the cleavage of Hg.
22. Give the evidences for the syn, anti, and non stereoselective electrophilic addition of olefins.
23. Arrange the following alkenes in their decreasing order of stereoselectivity of anti addition of Br₂ and explain your answer.

MeCH=CHMe,      PhCH=CHMe,      PhC(Me)=CHMe

 

PART – C

Answer ANY FOUR questions                        (4 ´ 10 = 40 marks)

23. a) Elimination of HBr from 2-bromobutane and elimination of trimethylamine from
    2-(N,N-dimethylamino)butane give different products in major amounts. Why?
24. b) The dehydrochlorination in cis and trans 2-chloromaleic acid , the trans isomer reacts about 50 times faster than the cis compound.
25. a) ‘The rates of hydrolysis of 0.1M alkyl bromides in 0.01M NaOH are in the order:
    t-BuBr > MeBr > EtBr > i-PrBr’.  Explain.
26. b) ‘Solvolysis of L-threo-3-phenyl-2-butyl tosylate in acetic acid gives threo product as the major’.
27. a) ‘For S_N1 mechanism, a branching increases the rate of the reaction’. Justify this statement with suitable examples.
28. b) ‘When the threo isomer of Ph-CO-CHBr-CHPh-COOH is treated with pyridine, it undergoes decarboxylative debromination, but erythro isomer under the same conditions undergoes dehydrobromination’.
29. a) Give the mechanism of the following:
30. i) Wohl-Ziegler bromination          ii) Eglinton reaction
31. b) Explain neighboring group assistance in free radical bromination reaction and discuss the steric course of the reaction.
32. a) Give the mechanism of free radical substitution at an aromatic substrate. Substantiate it with suitable evidences.
    b) Chlorination of 2, 3-dimethylbutane gives different ratio of products in aliphatic and aromatic solvents. Why?
33. Predict A, B, C, D, and E for the following reactions.

 

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