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Organic Chemistry 4e Carey | |||||
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Organic Chemistry 4e Carey | |||||
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Cycloalkanes |
Chapter 3: Conformations of Alkanes and Cycloalkanes |
Commonly encountered in natural products, such as steroids. Cyclopentanes and cyclohexanes are the most common. Other than cyclopropane these are non-planar or "puckered" systems. This puckering relieves some of the ring strain by lowering angle and torsional strains.
Cyclohexane: C6H12
| The most stable
conformation of cyclohexane is the chair form shown to the right.
The C-C-C bonds are very close to 109.5o, so it is almost free
of angle strain. It is also a staggered conformation and so is free of
torsional strain. You can rotate the molecule in the image to show this
just like a Newman projection.
The chair is the most stable conformation of cyclohexane. |
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| A second, much less
stable conformer is the boat conformation. This too is almost
free of angle strain, but in contrast has torsional strain associated
with eclipsed bonds at four of the C atoms. You can rotate the molecule
in the image to show this just like a Newman projection.
In addition, a steric interaction of the H atoms inside the bow and the stern, known as the flagpole interaction also destabilizes the boat. Click here to highlight flagpole hydrogen atoms Click here to reset colors |
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| A third conformation
is produced by twisting the boat to give the twistorskew-boat
conformation. The twist relieves some of the torsional strain
of the boat and moves the flagpole H further apart reducing the steric
strain. Consequently the twist boat is slightly more stable than the boat.
Click here to highlight flagpole hydrogen atoms Click here to reset colors |
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| Conformational rotation (also known as ring-flipping) of cyclohexane interconverts two chair forms. This proceeds from chair to twist boat to boat to twist boat to chair. Watch the animation carefully. |
In chair cyclohexane there
are two types of positions, axial and equatorial.
The axial positions point perpendicular to the plane of the ring,
whereas the equatorial positions are around the plane of the ring.
These can be highlighted in the diagram below.
| Click here to highlight an equatorial hydrogen atom;
Click here to highlight all equatorial hydrogen atoms; Click here to highlight an axial hydrogen atoms Click here to highlight all axial hydrogen atoms Click here to highlight the carbon skeleton Click here to reset colors |
OTHER CYCLOALKANES
Other than cyclopropane
(which must be planar), cycloalkanes are also "puckered" to relieve some of
the ring strain. The structures of some of the smaller cycloalkanes are shown
below with the planar structures for contrast. In each case, manipulate the
images to look for the deviation from planarity and the effect this has on the
eclipsing interactions of adjacent H atoms and C-C bonds. In order to be able
to compare the strain in each member of the cycloalkane series, the heat of
combustion per methylene (i.e. -CH2-) is also given.
CYCLOPROPANE DHc / CH2 = 166.6 kcal/mol |
CYCLOBUTANE DHc / CH2 = 162.7 kcal/mol |
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CYCLOPENTANE DHc / CH2 = 157.3 kcal/mol |
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CYCLOHEXANE DHc / CH2 = 156.1 kcal/mol |
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