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)
- Which of the following compounds are aromatic?
(i) cyclobutadiene (ii) cyclopentadienylcation
(iii) cyclooctatetraene (iv) cycloheptatrienyl anion
- Hammett equation is not applicable to aliphatic reactions and ortho-product. Why?
- How would you prove that the transition state in SN2 reaction must be linear? Explain.
- What are the evidences for SN1’ mechanism with benzene diazonium salts?
- State and explain Zaitsev’s rule with an example.
- Identify the stereochemistry of the products obtained on the dehydrobromination of different conformers of 1,2-dibromo-1,2-diphenylpropane.
- Give any two examples for each long-lived and short-lived free radicals.
- Give the products of the following.
- How does selenoxide undergo thermal syn elimination in the formation of a,b-unsaturated ketone?
- Explain the mechanism of hydroboration-oxidation reaction with a suitable example.
PART – B
Answer any eight questions (8 ´ 5 = 40 marks)
- What is ipso substitution? Explain with mechanism.
- What is partial rate factor? Explain its importance in ArSE reactions with a suitable example.
- Explain the following electrophilic substitution reaction mechanism with any one example.
(i) Direct amination (ii) Insertion by nitrenes
- What are ambident nucleophiles? Explain their role in reactions with suitable examples.
- How does the presence of a neighbouring group enhance the rate of nucleophilic substitution reaction? Explain with typical examples.
- Explain the mechanism of Bucherer reaction taking b-naphthol as an example.
- Explain (1) syn elimination with atleast three suitable examples and (2) the mechanism of addition of HBr to 3,3,3-trichloropropene.
- How many stereoisomers are possible for 1,2,3,4,5,6-hexachlorocyclohexane? Which one will undergo (1) fastest and (2) slowest dehydrochlorination? Why?
- Explain the factors influencing the different types of free radicals.
- How is ESR spectroscopy used to detect short-lived free radical? Explain.
- How does the stereochemistry of the products differ on the hydroxylation of 2-butene using (1) alkaline KMnO4 and (2) perbenzoic acid? Explain.
- 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)
- a) Discuss the orientation and reactivity of aniline towards bromination.
- b) Explain the mechanism of SE2 reaction with an example.
- Explain the following:
(i) 3-phenyl-3-methylbutan-1-al with ditertiarybutylperoxide.
(ii) Photolytic cleavage of 5-methyl-2-hexanone
- (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.
- (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
- (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.
- (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
Latest Govt Job & Exam Updates: