CHEM/I/01 Student's Copy
2 0 1 8
Pre-CBCS
1st Semester
CHEMISTRY
FIRST PAPER (CHEM-111)
Organic Chemistry—I
Full Marks 55
Time 2½ hours
PART A—OBJECTIVE
Marks 20
The figures in the margin indicate full marks for the questions
SECTION—A
Marks 5
Tick the correct answer in the brackets provided 1×5=5
1. The hybridization state of carbon atom in CH

3 molecule is
sp sp2
sp3 dsp2
2. Which of the following is an electrophile?
&&NH3 BF3
:C..l:


3. A pair of stereoisomers which are mirror images to each other, is called
enantiomers diastereomers
metamers racemates
/186 1 Contd.
4. The isomers converted into each other by rotation around a single bond are
called
position isomers
enantiomers
conformers
diastereomers
5. SN2 reaction takes place with
strong nucleophile and 1º-alkyl halide
weak nucleophile and 3º-alkyl halide
racemization
two steps
SECTION—B
Marks 15
Answer the following questions 3×5=15
1. "Alcohols have higher boiling points than their corresponding alkanes."
Explain.
2. Differentiate between homolytic and heterolytic bond cleavages of a
chemical bond.
3. Explain chemoselective and regioselective reactions using suitable
examples.
4. Draw the conformers of n-butane and indicate the most stable and least
stable thereof.
5. Differentiate between nucleophile and bases.
PART B—DESCRIPTIVE
Marks 35
The figures in the margin indicate full marks for the questions
1. Based on VSEPR theory, predict the geometry with probable bond angle
of the following 1×4=4
AlCl3, CH3, H3O
Å Å
Define inductive effect. 'Chloroacetic acid is stronger than acetic acid.'
Explain.
CHEM/I/01/186 2 Contd.
OR
2. Define hybridization. Draw the molecular orbital diagram of ethane
molecule and explain the type of hybridization involved in it.
Explain intermolecular and intramolecular hydrogen bondings taking
suitable examples. 1½+1½=3
3. Explain the following (any two) 2×2=4
Electromeric effect
Mesomeric effect
Electrophiles and nucleophiles
Arrange the following carbocations in their increasing order of stability
with suitable explanation 3
CH3 CH2, (CH3)2CH
Å Å Å
OR
4. State Hückel's rule of aromaticity. Indicate whether the following
compounds are aromatic or not
Write short notes on (any two) 1½×2=3
Carbenes
Free radicals
Carbanions
Arynes
5. What are geometrical isomers? Assign and Z -nomenclature for the
following compounds
Assign and S-configuration for the following molecules 1×3=3
CHEM/I/01/186 3 Contd.
N
H

Å
..
C=C
CH3
C2H5
Br
Cl

C=C C=C
C2H5 CH=CH2
COOH H
Br H
I H3C

COOH
CH3
O2N H

HO CH3
H
COOH

COOH
H3C
H
CH2OH
OR
6. Write notes on (any two 2×2=4
Stereogenic centre
Centre of symmetry
Meso-compounds
Racemization
Draw the enantiomers of 2-methyl-1-butanol and assign them or Snomenclature.

7. What is meant by conformational analysis? Draw the chair- and boatconformations
of cyclohexane. Which conformer is more stable and
why?
Draw the chair conformations of methyl cyclohexane and explain their
stability. 3
OR
8. Draw the conformational isomers of ethane (with energy diagram).
Show the most stable and least stable conformers with proper
justification.
Explain 1,3-diaxial interactions taking suitable example of monosubstituted
cyclohexane and how it affects stability of a molecule. 3
9. The reaction of methyl bromide with hydroxide ion follows SN2
mechanism. Explain the mechanism, stereochemistry and kinetics of
this reaction. 4
Explain SNi reaction with suitable example. 3
OR
10. Explain the mechanism of E1 and E2 reactions taking suitable
examples.
Explain Saytzeff's rule for E2 reaction with suitable example. 3