Sect 4'3: Conformations of Cycloalkanes

1
Sect 4.3 Conformations of Cycloalkanes
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Issues in small rings
H
Small rings have two problems
C
ANGLE STRAIN
H
A carbon with four bonds requires a tetrahedral
hybrid with 109o angles.
If that carbon is inside a small ring, the
desired 109o angle cannot be achieved.
TORSIONAL STRAIN
If the rings are planar, there are many eclipsed
hydrogen atoms, representing torsional strain.
BOTH FACTORS RAISE THE ENERGY OF THE SYSTEM
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Cyclopropane
Cyclopropane has angle strain and torsional
strain
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Planar cyclobutane
(actual molecule is not planar)
internal angle 90o ANGLE STRAIN
notice that all hydrogens are eclipsed
TORSIONAL STRAIN
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CYCLOBUTANE - PUCKERED RING
Actual molecule is puckered as shown - it folds
in the middle about 25o from planar
Still has angle strain
PUCKERING
relieves eclipsing which relieves TORSIONAL
STRAIN
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CYCLOPENTANE
essentially no ANGLE STRAIN
planar pentagon
108o
puckering (envelope) relieves eclipsing and,
therefore, TORSIONAL STRAIN
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PUCKERING RELIEVES TORSIONAL STRAIN
IN CYCLOPENTANE
3.0 kcal/mol
ENERGY
planar
0 kcal/mol
envelope conformation
planar cyclopentane would have all hydrogens
eclipsed TORSIONAL STRAIN
The pucker moves around the ring.
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METHYLCYCLOPENTANE
equatorial methyl
axial methyl
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EQUATORIAL METHYLCYCLOPENTANE HAS LOWER ENERGY
THAN AXIAL
PLANAR
3.4 kcal/mol
axial
ENERGY
0.9 kcal/mol
0 kcal/mol
equatorial
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Molecules viewed with Chime

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Sect 4.4 Evaluation of Angle Strain
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Sect 4.5 Cyclohexane
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PLANAR CYCLOHEXANE
(doesnt exist)
If cyclohexane were planar all of the
hydrogens would be eclipsed, resulting in
torsional strain.
There would also be angle strain - a hexagon
has 120o internal angles.
Cyclohexane is not planar ...
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Cyclohexane is puckered (chair conformation)
The perfect molecule! No angle strain and no
torsional strain.
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COMPLETE STAGGERING ABOUT ALL BONDS
axial
equatorial
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Cyclohexane(boat conformation)
bowsprit-flagpole interaction
Torsional strain along side of boat and steric
strain at top, but there is no angle strain.
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THE CHAIR CONFORMATION HAS LOWER ENERGY THAN THE
BOAT
ENERGY
29 kJ/mol
boat
0 kJ/mol
chair
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Cyclohexane(axial bonds)
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Cyclohexane(equatorial bonds)
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Cyclohexane(showing both axial andequatorial
bonds
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CHAIR CONFORMATION
axial
equatorial
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NITROGEN AND OXYGEN HETEROATOMS CAN REPLACE CARBON
All of these atoms have sp3 hybridization.
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RING INVERSION
starting ring
axial red
equatorial blue
axial blue
equatorial red
axial and equatorial hydrogens are interchanged
when the ring is inverted
inverted ring
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THE CHAIR IS A RIGID CONFORMATION
IT DOES NOT FLEX
THE BOAT IS A FLEXIBLE CONFORMATION
IT CAN FLEX TO RELIEVE TORSIONAL AND STERIC
STRAIN
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THE BOAT IS A FLEXIBLE CONFORMATION
IT WILL TWIST OR FLEX
twisting relieves the eclipsing at the
bottom of the ring ...
H
H
and the bowsprit-flagpole hydrogens move apart
TWIST BOAT
SKEW BOAT
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THE TWIST BOAT HAS LOWER ENERGY
THAN THE BOAT
ENERGY
6 kJ/mol
boat
0 kJ/mol
twist boat
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DONT FORGET
VIRTUAL MOLECULAR MODEL SET
Click on START at the bottom left of the computer
on University computers. Click on PROGRAMS Click
on Netscape Communicator (4.7) to launch Netscape
Navigator. Using Google, type in the address for
the Dept. of Chemistry, WWU http//www.chem.wwu
Select course materials, select WWU virtual
molecular model set You may need the free
program, Chime, to run this program. Note
Internet Explorer and Netscape 7.1 wont work!