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Sch 303: Stereochemistry, Conformational Studies And Reaction Mechanism Question Paper
Sch 303: Stereochemistry, Conformational Studies And Reaction Mechanism
Course:Bachelor Of Science
Institution: Kenyatta University question papers
Exam Year:2011
Page 1 of 6
A Constituent College of Kenyatta University
UNIVERSITY EXAMINATIONS 2012/2013 ACADEMIC YEAR
3
RD YEAR EXAMINATION FOR THE DEGREE OF BACHELOR OF
SCIENCE
COURSE CODE/TITLE: SCH 303: STEREOCHEMISTRY,
CONFORMATIONAL STUDIES AND
REACTION MECHANISM
END OF SEMESTER:I DURATION: 3 HOURS
DAY/TIME: TUESDAY:9.00 TO 12.00 NOON DATE:11/12/2012
INSTRUCTIONS
Answer ALL questions in Section A and any other TWO questions in section B.
SECTION A: (40 MARKS)
Question One
a) Explain each of the following terms:
i) Stereogenic centre
ii) Enantiomer
iii) Diastereoisomer
iv) Meso compound (4 marks)
b) Using the Cahn-Ingold-Prelog rule, assign priorities to the following set of
substituents:
i) –CH3, -Br, -Cl, -OHPage 2 of 6
ii) –CH2CH3, -CH2CH2OH, -CH2CH2Br, -CH2-O-CH3
iii) O O
II II
- C-CH3 , -C-OH , -CH2CH3 , -I
iv) -C=CH , -CH=CH2, - CH3, - CH2CH3 (4 marks)
c) Sighting down the C-2, C-3 bond, draw Newman projection formula for the most
stable conformation of n-butane. (1 mark)
d) i) Draw two different chair conformations of Cis-1, 3-dimethylcyclohexane
(2 marks)
iii) Which of the two chair conformations of Cis-1, 3-dimethylcyclohexane is
more stable. Explain. (2 marks)
e) Give mechanism for each of these reactions and state whether the products will
be single enantiomer or not.
i) O OH CH2CH3
II
1. CH3CH2MgBr
CH3
H 2. H+
/H2O CH3
H (4 marks)
ii) CH3CH2 CH2CH3
C = C + Br2 CH3CH2C(CH3)BrCHBrCH2CH3
CH3 HPage 3 of 6
f) Draw tetrahedral representations of the following molecules
i) (R)-2-chlorobutane
ii) (S)-3-bromo-l-pentene (4 marks)
g) Arrange the following cyclohexane conformer in decreasing order of
stability; twist boat, chair, boat. Explain your answer. (5 marks)
h) Coniine, derived from poison hemlock (Conium maculatum) and taken by
Socrates when he was sentenced to die, has the structure given below:
N
I
H Coniine
i) Using asterisk(s), identify the chiral centre(s) on the Coniine.
(1 mark)
ii) A 4.5g of the coniine was dissolved in 30ml of ethanol and placed
in a sample tube with a path length of 5 cm. The observed rotation
at the sodium D line was +1.200
. Calculate the specific rotation
[?]D for coniine. (4 marks)
i) Ribose, an essential part of ribonuclesic acid (RNA), has the
following structure:
H OH H OH
HO CHO
OH H OH H
Ribose
i) How many chirality centres does ribose have? Identify themPage 4 of 6
ii) How many stereoisumers of ribose are there?
iii) Draw the structured of the enantiomer of ribose. (5 marks)
SECTION B: (30 MARKS)
Question Two
a) Draw clear conformational Isomers of the most stable chair forms of each of the
following molecules, labeling each substituent as axial and equatorial.
i) CH3 (ii) CH3
(5 marks)
OH CH3
OH
CH3 CH3
b) Cis-1, 2-dimethylcyclohexane is optically inactive even though it has two chiral
centres. Explain. (2 marks)
c) Describe how you would separate a racemic mixture into its two optically active
components by chemical resolution. (8 marks)
Question Three
a) Draw Fischer presentations of:
(i) the two enantiomers of the amino acid serine
(HOCH2CH(NH2)CO2H) and identify each as R or S. (5 marks)
ii) The meso form of 3,5-heptanediol (2 marks)
b) Give the structure of the product(s) formed in each of the reaction below,
indicating the stereochemistry in all cases.Page 5 of 6
i) CH3 CH2CH3 Pt
+ H2
C = C
D
D
O H
ii) 1) CH3MgBr
2) H + / H2O
iii) CH2 = CHCH2CH3 + HCl
O O
II II
iv) CH3 –C-CH=PPh3 + C-H (8 marks)
Question Four
a) State and explain the condition(s) for optical Isumerism.
b) Assign R or S configurations to the chirality centres in the following molecules.
i) NH2 H (ii) CHO
CH3CH2 H
CH3 C02H BrPage 6 of 6
iii) COCH3 (iv) CH3
CH3 F OH
CH3O CH2OH
H ( 4 marks)
c) Some data on (IR, 2S)(-) ephedrine (A), (1S, 2R)-(+) ephedrine (B), (1S,2S)-(+)
ephedrine (C), and (IR, 2R) – (-) ephedrine (D) is given in the table below:
Study the table and state the value for (a) and (b)
Table: Ephedrine Data
(A) (B) (C) (D)
Mpt (O
c) 40-40.5 40-40.5 (a) 117-118
(?) -6.3 o
(b) +52 o
-52 o
(2 marks)
d) Sighting along the C1-C2 bond of 2-methylpropane
i) Make a graph of energy versus angle of rotation around the Cl-C2 bond.
ii) Draw a Newman projection of the most stable conformation.
iii) Given that an H ? H eclipsing interaction costs 4.0Kj/mol and an
H ?CH3 eclipsing interaction costs 6.0 KJ/mol, assign relative values to
the maxima and minima in your graph. (6 marks)
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