LOYOLA  COLLEGE (AUTONOMOUS), CHENNAI-600 034.

B.Sc. DEGREE  EXAMINATION  – chemistry

second SEMESTER  – APRIL 2003

CH  2500/ che 502 organic chemistry – I

23.04.2002

9.00 – 12.00                                                                                        Max: 100 Marks

 

 

PART – A                      (10 ´ 2 = 20 Marks)

                                                 Answer ALL the questions.

 

1. Which alcohol of the following pair would you expect to be more

easily dehydrated? Why?

(CH₃) C(OH) CH₂ CH₃ (or)  (CH₃)₂ CH CH (OH)  CH₃.

2. State and explain Markonikov’s rule.
3. How would you distinguish 1-butyne from 2-butyne?
4. Identify the product in the following.

R-CH = CH₂  +  CCl_{4             ?}

5. Anti-Markonikov’s rule is possible only for the addition of HBr in presence of

peroxides to an unsymmetrical alkene  and not for HCl or HI.  Why?

6. ‘The presence of a small amount of O₂ slows down the chlorination of methane’.
7. In a study of Chlorination of  propane, four products of formula C₃ H₆ Cl₂

were isolated.  What are their structures?

8. Acidity of the following Carboxylic acids is in the following order.

Cl₂ CH COOH   >  F CH₂ COOH  >  Cl CH₂ COOH.  Explain.

9. Many -but not all molecules – that contain a cheral centre are chiral.
10. How do the properties of diasteromers compare?

 

PART – B                          (8 ´ 5 = 40 Marks)

Answer any eight questions

11. Identify the products of obtained in the dehydration of 3 -methy1-2 butanol.

Which one is major?  Justify your answer.

12. Arrange the following compounds in order of reactivity toward dehydrohalogenation  by strong base.   Account for your answer.

1-bromobutone, 1-brome-2,2 – dimethyl propane,

1-bromo-2 methylbutane, 1-bromo-3-methylbutane.

13. Account for the following

(i)         trans -alkene is more stable than cis -alkene.

(ii)        NBS is preferred for allylic halogenation.

14. 98% H₂ SO₄ is required to hydrate ethylene while 63% H₂ SO₄ is enough for

isobutylene.  Explain

15. Explain with mechanism the addition of HBr to 1,3-butadiene as 1,2-  Vs 1,4 addition.
16. How would you prepare cis and trans alkenes from alkynes?

Explain with an example.

17. Give the mechanism of chlorination of methane is the presence of uv light.

Explain the relative reactivities of F₂, Cl₂, Br₂ & I₂ towards methane.

 

 

 

 

 

 

18. How do you prepare the following?

(i)         n-nonane starting with CH₃ Br.

(ii)        3- methyl octane starting with sec-butyl chloride.

19.    In the halogenation of alkanes, bromine atom is much more selective than

chlorine atom.  Explain [ relative rate factors of  1600: 82:1 for Br₂ Vs 5.0: 3.8:1 for Cl₂ for the reaction at tertiary: secondary: primary hydrogen]

20.    Account for the following.

(a)        Chlorination of ethane is  400 times  faster than that of methane when         equimolar   amounts of  both are reacted with  a small amount of C1₂ .

(b)        chlorination of propane gives 45% of CH₃ CH₂ CH₂ Cl and
55%  of CH₃CH(Cl) CH₃.

21.    Draw and specify as  R or S, the enantiomers of

(i)  CH₃ CH (OH) COOH  (ii) Sec-butyl chloride
(iii)   Bromo chloro iodomethane

22.    What is a racemic modification?  Discuss any two methods of resolving a

recemic modification.

 

 

PART – C                           (4 ´ 10 = 40 Marks)

Answer any FOUR questions.

23. a) Explain the stability of conjugated dienes based  on hydrogenation data.
24. b) Explain the acidity of acetylene compared with water, ammonia and
25. Account for the following
26. Hydration of acetylene gives CH₃
27. Industrially acetylene is prepared from coal.
28. Propene reacts with HOCl gives propylene chloro hydrin.
29. 2-methyl-l-butanol on dehydration mainly gives 2-methyl 2-
30. Sec-butyl trimethyl ammonium ion on treatment with strong base              undergoes elimination.
31. a) Explain the mechanism of addition of halogens to alkene.
32. b) Explain the mechanism of anti-Markonikov’s rule.
33. Explain homogenous and heterogeneous hydrogenation of alkenes with an     Discuss the mechanism for both in detail.
34. Draw the different conformation of ethane and n-butane, considering the

rotations about the bonds shown.  CH₃-CH₃ and CH₃CH₂-CH₂CH₃.  Draw the

potential energy Vs rotation curve for both and explain the strain and rotational  barrier involved.

28. Draw the stereocherical formulas of the dichlorobutanes obtained by the

free-radical chlorination of (i) (S) -Sec-batyl chloride,  (ii) (R) -Sec-butyl chloride and  (iii)  racemic-Sec-butyl chloride.  Give  the ratios of the different 2,3-dichlorobutanes obtained in the case of  (i),(ii) & (iii) and Explain.  Also comment on the optical activity of the mixtures of 2,3-dichlorobutanes  obtained in (i)  ,(ii), & (iii).

 

               

 

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