UNIVERSITY OF ZAMBIA
NATURAL SCIENCES
CHEMISTRY DEPARTMENT
CHE 2511 BASIC ORGANIC CHEMSTRY
31st March 2021
TUTORIAL SHEET 2
QUESTION ONE
(a) Draw the two chair conformations for each of the following di-substituted cyclohexanes.
 Circle the more stable one.
 For convenience, you may abbreviate the substituents (Me, Et, Pr, Bu, iPr, tBu, or the like
rather than drawing out methyl, ethyl, propyl, butyl, isopropyl, t-butyl….)
 Assume that a halogen, OH, or NH2 is smaller than a CH3 or any other alkyl group. •
Remember to draw in the hydrogens on each of the “substituted” carbons, also show all
the hydrogen atoms for the first three questions, from a-c.
a)
b)
c)
d)
e)
Cis-2-bromo-1-methylcyclohexane
Cis-3-isopropyl-1-methyl cyclohexane
Trans-2-butyl-1-isopropylcyclohexane
Cis-4-ethyl-1-hydroxylcyclohexane
Which of the following correctly lists the conformations of cyclohexane in order of
increasing energy?
(i)
chair < boat < twist < half chair
(ii)
half chair < boat < twist < chair
(iii)
chair < twist < half chair < boat
(iv)
chair < twist < boat < half chair
(v)
half chair < twist <boat < chair
(f) What is the energy difference between axial and equatorial conformers of
methylcyclohexane?
(h) From the perspective of viewing the C-2-C3 bond, draw the Newman projection of the most
stable conformation of 2,3-dimethylbutane.
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QUESTION TWO
(a) Consider the chair conformers shown above. The energy costs for each one is given:
(i)
Which one is more stable? Briefly explain your answer.
(ii)
Use a suitable formula to estimate the amount of conformer (in percent) at
equilibrium at a temperature of 298o K.
(iii)
What is the meaning of your calculated percentage of this particular conformer?
QUESTION THREE
(a) Define the term conformation.
(b) Draw a Newman projection of the most stable conformation of 2-methylpropane.
(c) Describe the source of angle strain and torsional strain present in cyclopropane.
(d) View a butane molecule along the C2-C3 bond and carry out a conformational analysis of
the molecule:
(i)
(ii)
(iii)
Plot a graph of potential energy (PE) versus the dihedral angle.
Calculate the energy costs for all the projections you drew in (a).
Give the Newman projection of the lowest energy conformer.
(e) Consider the molecule 2,3,4-tribromohexane.
(i)
Draw a fisher projection (C-1 on top, C-4,5 on bottom) of the 2(S),3(R), 4(R)
stereoisomer and label as A.
(ii)
Name (B) and draw (C) molecules, show an pertinent stereochemistry:
B
Br
CH2CH2CH=CH2
CH2CH CH2
C
(S)-4-cyclopropyl-4-methyl-2-hexyne
CH2CH3
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(f) Give the relationship between the following structures given below as; conformational
isomers, constitutional isomers, enantiomers, diastereomers, cis-trans isomers, meso
compounds or not related.
(i)
CH3
H
H
H
H
H
H
CH3
H
CH3
H
(ii)
CH3
CH3
CH3
H
Cl
Cl
H
H
Cl
H
Cl
CH3
CH3
(iii)
CH3
H
Br
Br
H
H3C
F
H
F
H
CH3
H3C
(iv)
HO
H
H3C
CH3
Cl
CH3
Cl
NH2
H
NH2
OH
CH3
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QUESTION FOUR
(a) What does a polarimeter measure?
(b) A 0.5 g of a chiral stereoisomer was dissolved in 20 mL solution. The observed rotation was
measured using a polarimeter was found to be -3.0 degrees at 25 oC using a wavelength of
589 nm. The sample tube was 20 cm long. What is the specific rotation of the stereoisomer?
(c) If a solution of a compound (30.0 g/100 mL of solution) has a measured rotation of +15° in a
2 dm tube, what is the specific rotation?
(d) What is the percent composition of a mixture of (S)-(+)-2-butanol, α= +13.52°, and (R)-(-)-2butanol, α = -13.52°, with a specific rotation α = +6.76°?
END OF QUESTIONS
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