Chapter 3: Conformations of Alkanes and Cycloalkanes

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Chapter 3 Conformations of Alkanes and
Cycloalkanes
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I. Conformations of Alkanes
A. Ethane torsional strain
rotate 60º
Chem3D
staggered conformation
eclipsed conformation
in between skewed
Newman projections sight along C-C bond
Stereoisomers Isomers with the same
connectivity, but different 3-D orientation of
their atoms in space.
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I. Conformations of Alkanes
A. Ethane torsional strain
DG 3 kcal/mol K 0.01 torsional strain
lower energy
higher energy
Ea 3 kcal/mol barrier to free rotation (but
at room temp most molecules have KE gt Ea so
rotation is essentially free) krot 106 s-1
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DG
0
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I. Conformations of Alkanes
B. Butane steric repulsions
CH3CH2CH2CH3
I anti (180º)
II
III gauche (60º)
IV
VI
V gauche (60º)
gauche 0.8 kcal higher energy than anti - van
der Waals repulsions steric strain eclipsed 3
kcal torsional strain 0.3 kcal each CH3-H
eclipse 3 kcal each CH3-CH3 eclipse
Chem3D
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I. Conformations of Alkanes
B. Butane steric repulsions
I
II
III
IV
VI
V
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4
DG
6
2
3.6
3.6
0.8
0.8
0
I II III IV V VI
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II. Conformations of Cycloalkanes
A. Stabilities of cycloalkanes
Ring strain bond angle strain torsional
strain (eclipsing) steric strain (van der
Waals)
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II. Conformations of Cycloalkanes
A. Stabilities of cycloalkanes
Total ring strain
DHcomb per CH2
166.6 kcal 31.5 kcal 162.7 26.4 157.3 7.0 156.1
0 157.0 6.3 157.3 9.6 156.2 1.2
small
angle strain and torsional strain
minimal strain
normal
transannular steric strain
medium
gt C12
large
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II. Conformations of Cycloalkanes
A. Stabilities of cycloalkanes
poor overlap bond angle strain (i.e., 109.5º
sp3 in 60º triangle)
plus ,
all Hs eclipsed torsional strain
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II. Conformations of Cycloalkanes
A. Stabilities of cycloalkanes
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planar, 90º but all eclipsed
puckered, 88º slightly more angle strain, but
less eclipsing strain
envelope relieves eclipsing
planar, 108º but all eclipsed
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II. Conformations of Cycloalkanes
B. Conformations in cyclohexane
1. chair and boat conformations
flagpole interaction
DG 7 kcal
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chair conformation - all staggered - no
eclipsing - no steric strain ? no ring
strain (99.99 at room temp.)
boat conformation - eclipsing 4 kcal -
steric strain 3 kcal ? ring strain 7 kcal
skewed boat 1.5 kcal more stable than
boat (0.01 at room temp.)
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II. Conformations of Cycloalkanes
B. Conformations in cyclohexane
2. equatorial and axial positions
axial positions
equatorial positions
3. chair-chair interconversion
Ea 10 kcal
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II. Conformations of Cycloalkanes
B. Conformations in cyclohexane
4. drawing cyclohexane chairs
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II. Conformations of Cycloalkanes
Ray
C. Substituted cyclohexanes
1,3-diaxial repulsions
DG 1.8 kcal (or 0.9 kcal per CH3-H repulsion)
equatorial (95) no steric strain (anti)
axial (5) steric repulsions (gauche)
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II. Conformations of Cycloalkanes
C. Substituted cyclohexanes
More pronounced effect with larger groups
DG 5.5 kcal
(99.99)
(0.01)
locked in equatorial conformation
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II. Conformations of Cycloalkanes
D. Disubstituted cyclohexanes
trans- cis-1,4-dimethylcyclohexane
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II. Conformations of Cycloalkanes
D. Disubstituted cyclohexanes
DG 3.6 kcal
diequatorial no repulsions
diaxial 4 1,3-diaxial repulsions 4 x 0.9
3.6 kcal
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II. Conformations of Cycloalkanes
D. Disubstituted cyclohexanes
DG 0 kcal
equatorial-axial 2 x 0.9 1.8 kcal
axial-equatorial 2 x 0.9 1.8 kcal
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II. Conformations of Cycloalkanes
D. Disubstituted cyclohexanes
DG 2.7 kcal
1 gauche interaction 0.9 kcal
4 1,3-diaxial repulsions 4 x 0.9 3.6 kcal
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II. Conformations of Cycloalkanes
D. Disubstituted cyclohexanes
DG 5.4 kcal
no repulsions
2 1,3-diaxial CH3-H 1.8 kcal 1 1,3-diaxial
CH3-CH3 3.6 kcal
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II. Conformations of Cycloalkanes
D. Disubstituted cyclohexanes
Larger groups predominate in determining
conformation
DG 3.7 kcal
1.8 kcal
5.5 kcal
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II. Conformations of Cycloalkanes
D. Disubstituted cyclohexanes
Question 3-1. Draw the most stable chair form of
the following compounds. Explain. Click on the
arrow to check your answers.
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II. Conformations of Cycloalkanes
D. Disubstituted cyclohexanes
Answer 3-1. Draw the most stable chair form of
the following compounds. Explain. Click on the
arrow to check your answers.
All groups can be equatorial. This chair form is
more stable than the other, where all are axial.
Isopropyl is bigger than a methyl group, so more
stable chair is where larger group is equatorial.
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III. Polycyclic Rings
decalin
borneol
adamantane
prismane
bicyclic
tricyclic
tetracyclic
Bicycloalkanes
bicyclox.y.zalkane (x ? y ? z) numbering
starts at a bridgehead, proceeds around the
largest bridge first, then around successively
smaller bridges
Cy
C
Cz
Cx
C
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III. Polycyclic Rings
bicyclo4.4.0decane bicyclo2.2.1.heptane
bicyclo4.1.0heptane bicyclo3.2.1octane
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IV. Heterocyclic Compounds
ethylene oxide oxirane oxacyclopropane oxetane ox
acyclobutane tetrahydrofuran oxacyclopentane t
etrahydropyran oxacyclohexane
pyrrolidine azacyclopentane piperidine azacycl
ohexane
furan pyran