GH 50

FIRST SEMESTER – APRIL 2008

CH 1801 – ORGANIC CHEMISTRY – I

Date : 16-04-08 Dept. No. Max. : 100 Marks

Time : 1:00 – 4:00

Answer ALL questions. (10 ´ 2 = 20)

Explain Prelog’s rule with a suitable example.
What is mutarotation? Give an example.
How would you resolve racemic alcohols?
Dehydrobromintion of threo-1-bromo-1,2-diphenylpropane is faster than erythro. Explain.
‘Aryldiazonium salt involves both in electrophilic and nucleophilic substitutions’. Justify.
What is first order asymmetric transformation? Give an example.
How do inductive and field effects affect the second substitution in aromatic systems?
In 4-t-butylcyclohexanecarboxylic acid, trans form is more acidic than cis isomer in 66% aquesous DMF. Explain.
How is the existence of NO₂⁺ proved in nitration reaction?
How does trapping method help in determining the mechanism of a chemical reaction?

Answer any EIGHT questions. (8 ´ 5 = 40)

The diazotization of aniline in moderately concentrated acid has been found to be

second order in HNO₂. Explain.

How will you determine the configuration of
a) (-) mandelic acid with respect to (+) lactic acid?
b) (-) lactic acid with respect to (+) tartaric acid?
How will you correlate the configuration of (+)-a-phenyl-a-methylbutyric acid and (-)-2-phenyl-2-butanol by Baeyer-Villiger rearrangement?
How do cross over experiments help in determining the mechanism of a chemical

reaction. Explain with an example.

Explain the following:
a) Asymmetric destruction b) Biochemical asymmetric transformation
What is Bucherer reaction? Explain the mechanism of the reaction.
Explain the mechanism of sulphonation of benzene in presence of oleum? Why is sulphonation of benzene reversible?
Dehydrobromination of diastereoisomeric a-phenyl-b-bromo-b-benzolypropionic acids with hot pyridine give different products. Explain.
Acetolysis of diastereoisomeric 3-phenyl-2-pentyltosylate and 2-phenyl-3-pentyl tosylate give different products. Account for your answer and explain the formation of the products.
What are consecutive reactions? Give two examples. Apply conditions and derive specific rate expression.
Explain the conformation of n-butne(C₂-C₃ rotation) with potential energy diagram.
What are the different stereoisomeric aldopentoses? Draw their structures and give their names.

PART-C

Answer any FOUR questions. (4 ´ 10 = 40)

a) How do isotopic labeling and stereochemistry help in predicting suitable reaction mechanisms? Give examples.
b) ‘meso form of stilbene dichloride has a dipole moment 1.27D while the dl-pair has 2.75D’.
a) Explain the racemization via cations and anions with suitable examples.
b) Prove that Hammett equation is LFER. How is the orientation and reactivity of

electrophiles normally studied? Briefly explain with examples.

a) Discuss the similarities and differences in Friedel Crafts alkylation and acylation

reactions with suitable examples.

b) Which one among 2,6-Dimethyl-4-nitroaniline and 2,5-dimethyl-4-nitroaniline is a weaker base? Justify your answer.
a) Explain the octant rule with a suitable example.
b) The cotton effect curves of (+)-3-methylcyclopentanone in methanol shows only a 24% reaction upon addition of acid
a) Compare the ipso-attack in aromatic electrophilic substitution with the S_NAr mechanism.
b) ‘The reaction between ethylene oxide and HBr in water is a third order reaction’. Explain.
Explain the following:
a) meso-stilbene dichloride on heating with pyridine at 200°C gives no product while dl-isomer gives alkene readily.
b) erythro-3-bromo-2-butanol with HBr gives meso product
c) Dehydrochlorination of 1,2-diphenyl-1-chloroethane gives trans product fastly but cis product slowly.
d) Benzyne intermediate can be trapped using anthracene.

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