Chem 112 -internal 2002

Universal Science College (B. Sc. Biochemistry) First Semester 2002 Second Term Exam Subject: Organic Chemistry Date: 19 Jan, 2003

Full Marks: 100 Pass Marks: 50 Time: 3 hours Group - A

Answer any three questions. Questions are of equal value. 3 × 14=42 1. Discuss kinetic and non-kinetic methods for determination of reaction mechanism. 2. What is conformational analysis? How do you distinguish conformational isomers from configurational isomers? What is absolute configuration and relative configuration? 3. Predict the relative stabilities of the cis and trans- isomers of following compounds on the basis of 0.19 kcal/mole for 1,3-diaxial hydrogen interactions of butane gauche interaction assuming that stereoisomers exist in their more stable conformation. I. 1,3-dimethyl cyclohexane II. 1,4-dimethyl cyclohexane 4. Define inglet carbene. How does carbene take part in concerted and non-concerted way during addition reaction? Give with mechanism. Group B Answer any six questions. Questions are of equal value. 6 × 7 = 42 1. How stability of carbocations were found in the following ways. Explain. CH3+ < (CH3)2 CH+ < (CH3)3C+ 2. Define hyper conjugation. How the theory is used to explain the stability of 1°, 2° and 3° carbon free radicals? 3. Explain the factors affecting the stability of conformation. Illustrate your answer with example for each factor. 4. Considering only rotation about the bond shown, draw a potential energy versus rotation curve for: i. (CH3)2CH-CH-(CH3)2 ii. (CH3)2CH-CH2CH3 5. Why Bayer strain theory fails to explain the stability of higher ring compounds? 6. Draw the chair conformations of cis-1, 2-dichlorohexane. Explain why cis-1, 2dichlorohexane cannot be obtained in an optically active form. 7. Predict the major product of the following reaction. 3 − KNO3 i. Me3CH FeSO  → ? O

ii.

8.

R H2N

O O

O

hν R → ?

)4 iii. Pb (OAc  →? hν iv. RN3 → ? Give an example of reaction of carbene involved in ring expansion.

Group – C Answer any eight questions. Questions are of equal value. 8 × 2 = 16 1. What is activation energy? 2. What do you mean by heterocyclic bond and dissociation energy? 3. Define reaction mechanism. Write its one importance in organic chemistry. 4. Give Neumann projection formula for cis and trans 1,2-dimethylcyclohexane. 5. What are reaction intermediates? Give name of any three such intermediates. 6. A meso-tartaric acid is optically inactive. Why? 7. Which of the following compounds are conformational isomers and why? I. Boat form of cyclohexane II. Eclipsed form of ethane 8. What do you understand by Van der waal strain? 9. Write the product of: R-N=C=O ? 10. Give an example of photolytic decomposition of isocyanate? 11. What do you obtain by photolysis or pyrolysis of aliphatic diazo compounds or ketenes? 12. What happens when sulfonyl amine is heated in gas phase? ***