LOYOLA COLLEGE (AUTONOMOUS), CHENNAI – 600 034

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.

Go To Main Page

      LOYOLA COLLEGE (AUTONOMOUS), CHENNAI – 600 034

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.

Go To Main Page

LOYOLA COLLEGE (AUTONOMOUS), CHENNAI – 600 034

M.Sc. DEGREE EXAMINATION – CHEMISTRY

SECOND SEMESTER – APRIL 2012

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

Date : 17-04-2012             Dept. No.                                        Max. : 100 Marks

Time : 9:00 – 12:00

PART – A

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

1. Which of the following compounds are aromatic?

(i) cyclobutadiene           (ii) cyclopentadienylcation

(iii) cyclooctatetraene      (iv) cycloheptatrienyl anion

2. Hammett equation is not applicable to aliphatic reactions and ortho-product. Why?
3. How would you prove that the transition state in S_N2 reaction must be linear? Explain.
4. What are the evidences for S_N1’ mechanism with benzene diazonium salts?
5. State and explain Zaitsev’s rule with an example.
6. Identify the stereochemistry of the products obtained on the dehydrobromination of different conformers of 1,2-dibromo-1,2-diphenylpropane.
7. Give any two examples for each long-lived and short-lived free radicals.
8. Give the products of the following.
9. How does selenoxide undergo thermal syn elimination in the formation of a,b-unsaturated ketone?
10. Explain the mechanism of hydroboration-oxidation reaction with a suitable example.

PART – B

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

11. What is ipso substitution? Explain with mechanism.
12. What is partial rate factor? Explain its importance in ArSE reactions with a suitable example.

13. Explain the following electrophilic substitution reaction mechanism with any one example.

(i) Direct amination                  (ii) Insertion by nitrenes

14. What are ambident nucleophiles? Explain their role in reactions with suitable examples.
15. How does the presence of a neighbouring group enhance the rate of nucleophilic substitution reaction? Explain with typical examples.
16. Explain the mechanism of Bucherer reaction taking b-naphthol as an example.
17. Explain (1) syn elimination with atleast three suitable examples and (2) the mechanism of addition of HBr to 3,3,3-trichloropropene.
18. How many stereoisomers are possible for 1,2,3,4,5,6-hexachlorocyclohexane? Which one will undergo (1) fastest and (2) slowest dehydrochlorination?  Why?
19. Explain the factors influencing the different types of free radicals.
20. How is ESR spectroscopy used to detect short-lived free radical? Explain.
21. How does the stereochemistry of the products differ on the hydroxylation of 2-butene using (1) alkaline KMnO₄ and (2) perbenzoic acid? Explain.
22. Explain the following:
    a) meso-2,3-dibromobutane reacts faster with iodide ion than dl isomer.
    b) gauche conformation of ethylene glycol is more stabler than anti form.

PART – C

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

23. a) Discuss the orientation and reactivity of aniline towards bromination.
24. b) Explain the mechanism of SE₂ reaction with an example.
25. Explain the following:

(i) 3-phenyl-3-methylbutan-1-al with ditertiarybutylperoxide.

(ii) Photolytic cleavage of 5-methyl-2-hexanone

25. (a)How is benzyne formed? What are the evidences for the benzyne mechanism?

(b)‘Acetolysis of both 4-methoxy-1-pentylbrosylate and 5-methoxy-2-pentylbrosylate give the same mixture of products’. Explain.

26. (a)What is ‘product spread’ in allylicnucleophilic substitution reactions? Explain.

(b)Explain the stereochemistry of the product obtained in the rearrangement reaction of 2-amino-1-anisyl-1-phenyl propanol reacts with nitrous acid

27. (a)‘Menthyl chloride undergoes dehydrochlorination at a slower rate comparing neomenthyl chloride’. Why?   Identify the products obtained and explain.

(b)‘Dehydrohalogenation of 2-fluorohexane mainly gives Hofmann product while 2-iodohexane gives Zaitsev’s product as major’.  Justify this statement and explain.

28. (a)Explain the stereochemistry of the products obtained on hydration of
    cis- and trans-2-pentene with (1) water in the presence of an acid and
    (2) Oxymercuration-demercuration methods.

(b) 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

Go To Main page