Chem 112-final 2005

(B. Sc. Biochemistry) First Semester 2006 Subject: Chem 112 (Organic Chemistry)

Full Marks: 100 Pass Marks: 45 Time: 3 hours

Candidates are required to give their answers in their own words as far as practicable. The figure in the margin indicates full marks. Group (A) Long Questions (Any three)

(3× ×14 = 42)

1. Discuss SN2 reaction with reference to mechanism, stereochemistry and reactivity.

2. Explain the kinetic and non-kinetic methods of determining reaction mechanism giving examples. 3. a. Give suitable examples to show the difference between configuration and conformation. The specification of configuration (R or S) is based on a set of sequence rules. Justify this statement with appropriate examples. b. The concentration of cholesterol dissolved in chloroform is 6.15 g per 100 ml of solution. A portion of this solution in a 5 cm Polarimeter tube causes an observed rotation of -1.20. Calculate the specific rotation of cholesterol. 4. Outline the preparation of alcohols by following methods by suitable mechanism. a) Oxymercuration – demercuration b) Hydrolysis of alkyl halides c) Hydroxylation of alkenes d) Hydroboration - oxidation

Group (B) Short Questions: (Any six) (6× ×7 = 42) 1. Describe the preparation of properties of free radicals and carbanion intermediates.

2. Write a short note on organic acids and bases. 3. Compare stabilities of the possible chair conformation of: a. Cis – 1, 2-dimethyl cyclohexane b. trans -1, 2 dimethyl cyclohexane On the bais of 0.9 KL cal/mol per 1,3- diaxial methyl hydrogen interaction, predict the potential energy difference between the member of each pair of conformations. 4. Cleavage of optically active sec-butyl methyl ether by anhydrous HBr yields chiefly methyl bromide and sec –butyl alcohol. The sec – butyl alcohol has the same configuration and optical activity as the starting material. How do you interpret this result? 5. Give four reactions for the preparation of ether. 6. Explain the mechanism of dehydrohalogenation. 7. Show the mechanisms and orientation of free radical addition of alkene. 8. Explain the mechanism of E1 reaction.

Group (C) Very Short Questions: (Any eight) 1. Give definition of reaction mechanism.

2. Name the electrophiles and nucleophiles. 3. What do you mean by functional group? 4. What is stereo-chemical inversion? 5. How can you tell from its formula whether or not a compound can exists as enantiomer?

(2× ×8 = 16)

6. Draw the potential energy relationship among conformations of cyclohexane showing each conformer. 7. Give the product of free radical halogenation of isobutane with mechanism. 8. What are the products do you expect on ozonolysis of 2,3-dimethyl -2- pentene on ozonolysis. Show the ozonide for this reaction. 9. What are Grignard reagents? 10. What do you mean by orientation in reaction? 11. What is peroxide effect? 12. Give the mechanism of hydroboration.