LOYOLA COLLEGE (AUTONOMOUS), CHENNAI – 600 034
M.Sc. DEGREE EXAMINATION – CHEMISTRY
|
SECOND SEMESTER – APRIL 2007
CH 2808 – ORGANIC SUBSTIT., ADDITION & ELIMINATION RXNS
Date & Time: 17/04/2007 / 1:00 – 4:00 Dept. No. Max. : 100 Marks
PART – A
Answer ALL questions (10 ´ 2 = 20 marks)
- What are p-complexes in an aromatic electrophilic substitution reaction?
- Define partial rate factor and explain.
- The rate of acid-catalyzed hydrolysis of benzamide is -0.298 and the relative electron withdrawing effect of -NO2 substituent is + 0.710. Calculate the relative reactivity of the
nitrobenzamide.
- How does nitrous acid react with aromatic 1° and 2° amines?
- Explain why iodination of benzene requires HNO3?
- Explain Hofmann’s elimination rule with a suitable example.
- Among acetic anhydride and n-propyl alcohol, which one is more powerful nucleophile? Justify your answer.
- What happens when threo and erythro DL pair of 3-bromo-2-butanol is treated with HBr? Explain.
- ‘SN1 rates are greatly increased when there is a double bond in the b-position’. Why?
- What happens when isobutyl acetate is strongly heated?
PART – B
Answer any eight questions (8 ´ 5 = 40 marks)
- Explain the effect of electrophile in the reactivity of aromatic substitution reactions.
- Describe the various products formed from the reaction of electrophile attacking at the ipso position in nitrobenzene.
- Predict the mechanism for the following reaction.
- Explain neighboring group assistance in free radical reactions.
- Explain why in the abstraction step of photochemical chlorination of ethane, ethyl radical formed but not the hydride radical.
- Predict the products with mechanism for the following:
a) Wohl-Ziegler reaction b) Eglinton reaction - Identify the products obtained when erythro and threo isomers of 1-acetoxy-2-deutero-1,2-diphenylethane is subjected to pyrolysis. Explain.
- ‘Acetolysis of both 4-methoxy-1-pentylbrosylate and 5-methoxy-2-pentylbrosylate gave the same mixture of products’. Explain.
- ‘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.
- ‘Menthyl chloride undergoes dehydrochlorination at a slower rate comparing neomenthyl chloride’. Why? Identify the products obtained and explain the reaction.
- ‘Dehydrohalogenation of 2-fluorohexane mainly gives Hofmann product while
2-iodohexane gives Zaitsev’s product as major’. Explain. - Explain von-Ritcher rearrangement with mechanism. Give the main evidence for the mechanism.
PART – C
Answer any four questions (4 ´ 10 = 40 marks)
- a) Explain the mechanism of bimolecular aliphatic electrophilic substitution with examples.
- b) Describe the effect of solvent on the reactivity of chlorination reaction of 2,3-dimethylbutane.
- a) Give the mechanism of sulphonation of benzene. What are the various sulphonating agents? Give evidences for the mechanism.
- b) Give the evidences for the syn, anti, and non stereoselective electrophilic addition of olefins.
- Predict the product for the following
- a) How does the presence of a cyclopropyl neighbouring group enhance the rate of nucleophilic substitution reaction? Explain with two examples.
- b) 2-butylacetate on pyrolysis gives a mixture of products with different yields. Why?
- a) ‘p-methoxybenzhydryl hydrogen phthalate on solvolysis in methanol results a racemic ether’. Justify your answer.
- b) Explain Bucherer reaction taking b-naphthol as an example.
- a) Identify the stereochemistry of the products obtained on the dehydrobromination of different conformers of 1,2-dibromo-1,2-diphenylpropane.
- b) How does selenide elimination help in the synthesis of a,b-unsaturated ketones?