Cyclohexane Conformational Analysis
Conformational Analysis of
Cyclohexane
Conformational Analysis of
Cycloalkanes - II
A three-dimensional model of the chair form
of cyclohexane with all H atoms drawn
•
Three C atoms pucker up and three C atoms pucker
down, alternating around the ring.
•
Each C in has two different kinds of hydrogens:
(1)
axial hydrogens
are located above and below the
ring (along a perpendicular axis);
(2)
equatorial hydrogens
are located in the plane of
the ring (around the equator).
How to Draw a
Chair Conformation
all opposite bonds
are parrallel
Axial bonds
and
Equatorial
bonds
Rings can Flip from one Chair
Conformation to Another
Flipping Chair Conformations
•
All axial bonds become equatorial
•
All equatorial bonds become axial
•
All “up” bonds stay up
•
All “down” bonds stay down
•
An important conformational change in cyclohexane involves
“
ring-flipping.
” Ring-flipping is a two-step process.
•
As a result of a ring flip, the up carbons become down
carbons, and the down carbons become up carbons.
•
Axial and equatorial H atoms are also interconverted during a
ring-flip. Axial H atoms become equatorial H atoms, and
equatorial H atoms become axial H atoms.
Ring-flipping interconverts axial and equatorial
hydrogens in cyclohexane
•
The chair forms of cyclohexane are 7 kcal/mol
more stable than the boat forms.
•
The
boat conformation
is destabilized by torsional
strain because the hydrogens on the four carbon
atoms in the plane are eclipsed.
•
Additionally, there is steric strain because two
hydrogens at either end of the boat, the “
flag
pole
” hydrogens, are forced close to each other.
Two views of
the boat
conformation
of cyclohexane
The chair conformation
has no ring
strain.
All bond angles are 109.5
o
and all C-H
bonds are perfectly staggered
Chair Conformation
Cyclohexane
The
boat conformation
is less stable because
of flagpole interactions and tortional strain
along the bottom of the boat
Boat Conformation
The chair and boat conformers are
interconvertible at ordinary temperature and
cannot be isolated independently
The
twist conformation
is intermediate in
stability between the boat and the chair
conformation
At room temp, the ratio of chair to boat
conformation at equilibrium is 1000:1
Energy Profile for the Chair-Chair Interconversion of
Cyclohexane
Conformational analysis of cyclohexane involves studying its chair form, axial and equatorial bonds, ring-flipping process, and stability comparison between chair and boat forms. The ring-flipping process interconverts axial and equatorial hydrogens, resulting in a more stable chair conformation. Visual representations aid in grasping the concept of cyclohexane's structural dynamics.
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Presentation Transcript
Conformational Analysis of Cycloalkanes - II Conformational Analysis of Cyclohexane
A three-dimensional model of the chair form of cyclohexane with all H atoms drawn
Three C atoms pucker up and three C atoms pucker down, alternating around the ring. Each C in has two different kinds of hydrogens: (1) axial hydrogens are located above and below the ring (along a perpendicular axis); (2) equatorial hydrogens are located in the plane of the ring (around the equator).
How to Draw a Chair Conformation all opposite bonds are parrallel
Rings can Flip from one Chair Conformation to Another
Flipping Chair Conformations All axial bonds become equatorial All equatorial bonds become axial All up bonds stay up All down bonds stay down
An important conformational change in cyclohexane involves ring-flipping. Ring-flipping is a two-step process. As a result of a ring flip, the up carbons become down carbons, and the down carbons become up carbons. Axial and equatorial H atoms are also interconverted during a ring-flip. Axial H atoms become equatorial H atoms, and equatorial H atoms become axial H atoms.
Ring-flipping interconverts axial and equatorial hydrogens in cyclohexane The chair forms of cyclohexane are 7 kcal/mol more stable than the boat forms.
The boat conformation is destabilized by torsional strain because the hydrogens on the four carbon atoms in the plane are eclipsed. Additionally, there is steric strain because two hydrogens at either end of the boat, the flag pole hydrogens, are forced close to each other. Two views of the boat conformation of cyclohexane
The chair conformation has no ring strain. All bond angles are 109.5o and all C-H bonds are perfectly staggered
The boat conformation is less stable because of flagpole interactions and tortional strain along the bottom of the boat
The chair and boat conformers are interconvertible at ordinary temperature and cannot be isolated independently
The twist conformation is intermediate in stability between the boat and the chair conformation At room temp, the ratio of chair to boat conformation at equilibrium is 1000:1
Energy Profile for the Chair-Chair Interconversion of Cyclohexane half-chair half-chair boat twist-boat chair twist-boat chair
