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8/10/2019 11.Konformasi Alkana Dan Sikloalkana
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Conformations of Alkanes and Cycloalkanes
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I. Conformations of Alkanes
A. Ethane: torsional strain
C C
H
HH
H
HH C C
H H
H HH H
rotate 60
staggeredconformation
eclipsedconformation
in between:skewed
Chem3D
Newman projections: sight along C-C bond
H
HHH
HH H
HH
H
HH
Stereoisomers:
Isomers with thesame connectivity,
but different 3-D
orientation of their
atoms in space.
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I. Conformations of Alkanes
A. Ethane: torsional strain
H
HHH
HH H
HH
H
HH
lower energy higher energy
DG~ 3 kcal/mol
K ~ 0.01
torsional strain
DG
0
3
H
HH H
HHH
HH H
HHH
HH
H
HH
Ea~ 3 kcal/mol
= barrier to free rotation
(but at room temp most
molecules have KE >Easo
rotation is essentially free)
krot~ 106s-1
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I. Conformations of Alkanes
B. Butane: steric repulsions CH3CH2CH2CH3
CH3
CH3
CH3
H3C
CH3H3C
CH3
CH3 CH3CH3
CH3
CH3
I
anti
(180)
II III
gauche
(60)
IV VIV
gauche
(60)
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 V VI
0
2
4
6
DG
3.60.8
~6
0.83.6
CH3
CH3
CH3
H3C
CH3H3C CH3
CH3 CH3CH3 CH3
CH3
I II III IV VIV
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II. Conformations of Cycloalkanes
A. Stabilities of cycloalkanesDHcomb
per CH2
Total
ring strain
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> C12
small
normal
medium
large
angle strain and
torsional strain
minimal strain
transannular
steric strain
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II. Conformations of Cycloalkanes
A. Stabilities of cycloalkanes
HH
HH
H Hpoor overlap = bond angle strain
(i.e., 109.5sp3in 60 triangle)
plus ,
H
H
H
H
H
H
all Hs eclipsed =
torsional strain
Chem3D
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II. Conformations of Cycloalkanes
A. Stabilities of cycloalkanes
H
H
H
H
H
H
H
HH
H
H
H
HH
H
H
planar, 90but all eclipsed
puckered, 88slightly more angle strain,
but less eclipsing strain
Chem3D
planar, 108
but all eclipsed
envelope
relieves eclipsing
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II. Conformations of Cycloalkanes
B. Conformations in cyclohexane
1. chair and boat conformations
H
HH
H
H
H
HH
H
HH
HH
H
H
H
H
H
HH
HH
H
H
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 kcalring strain ~ 7 kcal
DG~ 7 kcal
Chem3D
skewed boat ~ 1.5 kcal
more stable than boat
(0.01% at room temp.)
flagpole interaction
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II. Conformations of Cycloalkanes
B. Conformations in cyclohexane
2. equatorial and axial positions
H
HH
H
H
H
HH
H
HH
H
H
HH
H
H
H
HH
H
HH
H
axialpositions equatorialpositions
3. chair-chair interconversion
H
HH
H
H
H
HH
H
HH
H
H
H H
H
H
H
H
HH
HH
H
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
C. Substituted cyclohexanes
CH3
CH3
H
H
H
CH3
H
1,3-diaxial
repulsions
equatorial
(95%)
no steric strain
(anti)
axial
(5%)
steric repulsions
(gauche)
DG~ 1.8 kcal
(or 0.9 kcal
per CH3-H
repulsion)
Chem3D
Ray
http://web.uccs.edu/danderso/chem331/pp/cyclohexconf.ppthttp://web.uccs.edu/danderso/chem331/pp/cyclohexconf.ppt8/10/2019 11.Konformasi Alkana Dan Sikloalkana
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II. Conformations of Cycloalkanes
C. Substituted cyclohexanes
H
H
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
CH3
CH3
CH3
CH3trans- cis-1,4-dimethylcyclohexane
stereoisomers
*configurational conformational
(cannot convert from (can be converted fromone to another without to another by rotation
breaking bonds) about a bond)
*geometric isomers
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II. Conformations of Cycloalkanes
D. Disubstituted cyclohexanes
CH3
CH3
CH3CH3
H
H
CH3 H
CH3
H
CH3
CH3
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
CH3
CH3
H3C CH3
CH3
CH3
H
H
H
CH3
CH3
H
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
CH3
CH3
CH3CH3
H
H
H CH3
CH3
H
1gaucheinteraction
= 0.9 kcal
4 1,3-diaxial repulsions
= 4 x 0.9 = 3.6 kcal
DG~ 2.7 kcal
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II. Conformations of Cycloalkanes
D. Disubstituted cyclohexanes
CH3
CH3
CH3H3C
H CH3CH3
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:
CH3
CH3
tBu
CH3
DG~ 3.7 kcal
1.8 kcal5.5 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.
CheckAnswer
CH3
CH3CH3CH3
CH(CH3)2
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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.
CH3
CH3CH3
CH3CH3
CH3CH3
CH(CH3)2
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.
CH3
CH(CH3)2
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III. Polycyclic Rings
OHdecalin borneol adamantane prismane
bicyclic tricyclic tetracyclic
Bicycloalkanes:
bicyclo[x.y.z]alkane (x y z)
numbering starts at a bridgehead,proceeds around the largest bridge first,
then around successively smaller
bridges
C
C
Cz
Cy
Cx
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III. Polycyclic Rings
bicyclo[4.0]decane
bicyclo[2.2.1.]heptane
bicyclo[4.1.0]heptane
bicyclo[3.2.1]octane
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IV. Heterocyclic Compounds
O
O
O
O
N
H
NH
ethylene oxide
oxiraneoxacyclopropane
oxetane
oxacyclobutane
tetrahydrofuran
oxacyclopentane
tetrahydropyran
oxacyclohexane
pyrrolidine
azacyclopentane
piperidine
azacyclohexane
O O
furan pyran