2. Energy profile of the interconversion of
cyclohexane conformations
- Chair conformation: In this conformation, all
bonds are in staggered form, make molecule most
stable puckered conformation.
- Twist boat or skew boat conformation: This
conformation exists in between chair and boat
conformation and more stable than boat but less
stable than chair conformation.
- Half chair conformation: This is highly unstable
and strained conformation of Cyclohexane.
- Boat conformation: This is less stable
conformation than chair form by 6.8 kcal/mole is
due to flagpole interaction of hydrogen atoms.
3. Baeyer postulated
that any deviation
of bond angles
from the normal
tetrahedral value
would impose a
condition of
internal strain on
the ring.
4. The bond angle is of 1090 28'
(or 109.50) for carbon atom in
tetrahedral geometry
(methane molecule).
Baeyer proposed “any
deviation of bond angle from
ideal bond angle value
(109.50) will produce a strain
in molecule. Higher the
deviation lesser the instability.
5. These assumptions are helpful
to understand instability of
cycloalkane ring systems.
- The ring of cyclopropane is
triangle. All the three angles
are of 600 in place of 109.50
(normal bond angle for carbon
atom) to adjust them into
triangle ring system.
- In same manner, cyclobutane
is square and the bond angles
are of 900 in place of 109.50
(normal bond angle for carbon
atom) to adjust them into
square ring system.
6. As a result of this, the strain is
more in cyclopropane as
compare to cyclobutane. It will
make cyclopropane less stable
than cyclobutane. So,
cyclopropane easily undergoes
ring opening reaction as compare
to cyclobutane.
- According to Baeyer, the
relative order of stability for
some common cycloalkanes is as
under.
7. Limitation of Baeyer’s strain theory
Successfulness of Baeyer’s angle strain theory
- Baeyer rightly proposed that cyclopropane and
cyclobutane are flat molecule and having angle of
60˚ and 90˚ those are much deviated from the
ideal tetrahedral value of 109.50 hence these
ring systems are unstable and easily undergo
ring opening reactions. There is much angle
strain in cyclopropane and cyclobutane.
- Baeyer also proposed that Cyclopentane is not
need to be planar but it is planar as in that
condition the angle is much near to ideal
tetrahedral angle.
8. Orbital Picture of Angle Strain
- As we known that the bond formation effective
atomic orbital overlap is required.
- For carbon atom, in a molecule if it is sp3
hybridized then the bond angle is 109.50 and
orbital overlap is maximum.
- This is the reason for highly stable compounds.
- For an open chain compound, when carbon is
connected to other two carbon atoms, it is sp3
hybridized and utilize these hybrid orbitals to form
the bonds.
- In cyclopropane, the bond angle is 600 instead of
109.50.
- The carbon Atom don’t use sp3 hybridized orbital
hybridized to form the bond
- Hence, the bond is weaker as compare to usual
carbon-carbon bond
9. Coulson-Moffitt Model (Bent bond and Banana bond)
- Bent bond One of the first bent bond theories for
cyclopropane was the so-called Coulson-Moffitt
Model
- A bent bond, also known as a banana bond, is a
type of covalent chemical bond with a geometry
somewhat reminiscent of a banana.
- The term itself is a general representation of
electron density or configuration resembling a
similar "bent" structure within small ring molecules,
such as cyclopropane (C3H6) or as a representation
of double or triple bonds within a compound that is
an alternative to the sigma and pi bond model.
10. Sachse Mohr’s theory (Theory of strainless
rings)
- Baeyer suggested that both large and small
polymethylene rings should be strained, but
Hermann Sachse, soon pointed out that large
rings need not be strained, because the
carbons need not be coplanar.
- Sachse Mohr’s theory proposed that higher
member ring can become free from strain if
all the ring carbons are not forced into one
plane. They exhibit in two non-planar
puckered conformations both of which are
completely free from strain. These are called
the Chair Form and Boat Form
11. The chair conformation is the
most stable conformation of
cyclohexane.
- 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.
12. Strain means deformation of bond length and bond
angle from their ideal values under the influence of
internal force.
- This gives rise to an instability within a structure
with higher internal energy.
- Strain is term used for the energy associated with
a system due to its geometry.
- When a bond length or bond angle is distorted,
potential energy is stored in the molecule. this
potential energy that is stored in the molecules is
referred to as the strain or the strain energy.
- Type of Strain: (i) Torsional Strain or Eclipsed
Strain (ii) Angle Strain (iii) Steric Strain or Van-der
Walls Strain
13. Torsional strain or eclipsing strain is the increase
in potential energy of a molecule due to repulsion
between electrons in bonds that do not share an
atom
Steric strain can also be referred to as steric
hindrance, which is related to Van Der Waals
repulsion, or the strain that occurs as the distance
between two atoms or substituents decreases. Steric
strain occurs when multiple substituents are close
together, and cannot be separated by rotation.
Angle Strain: Strain caused when the actual bond
angles in the atoms of the molecule differ from the
ideal bond angles of their geometry.
