Problem Set 4: Conformations of alkanes and cycloalkanes ANSWER KEY
Chemistry 260
1.
Organic Chemistry
Order the following conformers from the most stable to the least stable.
H
H
H
H
H CH3
CH3
A
(1)
(2)
(3)
(4)
H
CH3
CH3
B
H
H
HH
C
D
C is staggered with no gauche interactions.
A is also staggered, but has one gauche interaction.
B is ecplised, but methyl groups are not eclisped with each
D has methyl groups eclisped.
Which of the following statements is true?
CH3
H3C
A
(1)
(2)
(3)
CH3
CH3
H
H
A, C, B, D
D, C, A, B
C, A, B, D
C, B, A, D
Answer: 3
Conformer
Conformer
Conformer
other.
Conformer
2.
H
H
H
H3C
CH3
H
B
Conformer A is less stable than conformer B
Conformer A is more stable than conformer B
Conformer A and conformer B are of equal energy
The answer is (1). Conformer B is more stable.
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3.
Given that the conversion to the lowest energy conformer in Q2 has an
equilibrium constant, K = 20, calculate the percentage of B in solution at 298
K.
Because B is lower in energy than A (i.e., more stable), B is the major isomer.
To calculate the percentage of B in solution:
K
= 20
K
= [B]/[A]
Let B = x, then A = 1- x
K
20
=
=
x/(1-x)
x/(1-x)
x
=
0.95
Thus, 95% of the solution is B
4.
Name the following compounds:
(a)
CH3
(b)
CH3
(a) methylcyclobutane
(b) methylcyclohexane
6
2 1 CH3
5
4
3
5.
4
CH3
1
2
3
Order the following molecules/conformers from the most stable to least
stable.
The answer is 3.
2
CH3
CH3
(H3C)2HC
(H3C)2HC
A
cis - smaller group axial
B
trans - both groups axial
CH3
CH3
(H3C)2HC
(H3C)2HC
C
cis - larger group axial
D
trans - both groups equatorial
6.
Draw all cyclopentane-containing isomers of dibromocyclopentane. All
isomers must contain a cyclopentane ring. Do not draw any compound
twice.
7.
Draw all of the constitutional isomers of compounds named
tribromocyclobutane (all structures contain a cyclobutane ring). Do not draw
any isomer twice.
8.
Draw compounds A and B in the lowest energy conformations.
3
9.
Menthol (below) is one of the most important flavouring chemicals used in
pharmaceuticals, cosmetics, toothpaste, chewing gum, etc. A number of
possible conformers of menthol exist. Draw the lowest energy chair
conformation of the naturally occurring menthol below.
10.
Represent the following conformer (shown as a Sawhorse projection) as a
Newman projection by completing the template provided. The two
projections must be in the same conformation.
Answer:
11.
An important tool in an organic chemists is the ability to draw various
representations of a single structure, particulary cyclic, or ring structures.
Below are representations of substituted cyclohexanes. Re-draw structures
(a), (b) and (c) as the lowest-energy chair conformation. Re-draw structures
(d) and (e) as flat.
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5