Chem 112 -final 2002

(B. Sc. Biochemistry) First Semester 2002 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)

Group (B) Short Questions: (Any six)

(6× ×7 = 42)

1. What do you mean by reaction mechanism? Write and explain different types of reaction in organic chemistry with one example of each. 2. what is the necessary and sufficient condition foe enantiomerism? What is necessary but not sufficient condition for optical activity? What conditions musy be met for the observation of optical activity? How can one tell from its ormula whether or not a compound can exist as enantiomer? How do you decide a molecule is having R or S configuration?. 3. What is conformational analysis? Discuss the factors affecting stability of conformations. 4. what are carbocation? Explain the structure of a carbocation. Account for the fact that “The greater the number of alkyl groups, the more stable the carbocation”.

1. What do you mean by Homolytic and Heterolytic bond cleavage? Explain with suitable examples. 2. What do you understand by conjugate acid-base pair? Show that a strong acid has a weak conjugate base and a weak acid has a strong conjugate base. “Acetic acid in water acts as a weak acid but it acts as a strong acid liquid ammonia”. Explain. 3. Draw and specify R or S enantiomers (if any) of: i. 2-chloropentane ii. 3-chloro-2-methyl pentane iii. 3-chloro-3-methyl pentane 4. When 1.5 gm of an optically active compound is dissolved in 25 ml of chloroform at 25°C. The optical rotation of the solution observed in a 2 dm polarimeter tube using a sodium D-lime light is -4.8°. what is specific rotation of the compound? 5. Give the relationship of axial and equatorial bonds as a function of their location on chair cyclohexane. 6. How do you establish that equatorial methyl cyclohexane is more stable by 1.8 Kcal than axial methyl-1-cyclohexane? 7. How does nitrene take part in concerted and non-concerted way during addition reaction? Explain with mechanism.

8. Give the possible product of following reaction: Br

CH3 NH NaNH  / → ? 2

R

R

H

H

2CH3COOC2H5 +

N

N

NH

O N O

↑↑ NCOOEt  → ? NaOC H  → ? 2

O

R

3

5

hν →

rearrangem   ent → B

A

5

NH2

+ Pb(OAc)4

Group (C) Very Short Questions: (Any eight)

? (2× ×8 = 16)

1. What do you obtain by photolytic or thermal decomposition of azides? Give one example. 2. Why the trimethyl amine is less basic than dimethyl amine although the methyl group are more in number in dimethylamine than trimethylamine? 3. Distinguish between configuration and conformation. 4. Define electrophile and nucleophile. 5. What are epimers? Give example. 6. Define conformation isomer. Give example. 7. Explain Gauche and Staggereed conformations. 8. Draw a energy profile diagram of a two step exothermic reaction. 9. What happens when anthracene is treated with o-bromofluorobenzene in the presence of lithium? 10. Dimerization of carbene intermediate is less possible. Why? 11. What is torsional strain? Explain. 12. How do you know that cyclohexane is a stable molecule?