coulson and moffit's modification

1. Coulson and Moffitt’s
modification

2. C. A. Coulson and W, A. Moffitt (of Oxford
University) proposed bent bonds between
carbon atoms of cyclopropane rings based on
the basis of quantum mechanical calculations.
Bent bonds between carbon atoms of
cyclopropane rings; this idea is supported by
electron density maps based on X-ray studies.

3. Bent bonds Theory
C. A. Coulson A. Moffitt

4. Molecular orbital theory
• Stability of cyclohexane in terms of
Molecular orbital theory.
• Covalent bond between two atoms is
formed by overlap of orbitals of the
atoms.
• The greater the extent of overlap the
stronger is the bond formed.

5. • The atomic orbitals overlap to the
maximum extent if they overlap the
stronger is the bond formed.
• The atomic orbital overlap to maximum
extent if they overlap along this axes.
• Axes of SP3 orbital are at 109 28 to each
other C-C bond and C-C-C have their
maximum strength.

6. • In cyclopropane –C-C-C bond angle is 60
• In cyclobutane –C-C-C- bond angle is 90
• In higher cyclohexane –C-C-C bond angle
is 109 28
• In cyclopropane the overlap of SP3
orbitals of carbon is less than the overlap
of SP3 orbitals of carbon in alkane.

7. • Carbon uses sp2 orbitals for carbon-hydrogen
bonds (which are short and strong).
• The high p character of these carbon-carbon
bonds, and their location largely outside the
ring.
• Ring-opening is due to the weakness of the
carbon-carbon bonds,

8. Cyclopropane

11. sp3 orbitals overlap of cyclopropane
and propane

12. • Overlap of SP3 orbitals of carbon in cyclo pentane
or cyclohexane is maximum because in these case
it is possible for the SP3 orbital to overlap along
their axes. The bond angle being approximately
equal to 109 28
• It implies cyclo propane are weaker than cyclo
butane. It undergoes ring opening readily and
drastic condition required to bring clevage in
cyclo butane.
• The higher cyclo alkanes behave like alkanes.

13. Ring-opening is due to the weakness of the
carbon-carbon bonds, but the way in which it
happens reflects the unusual nature of the
bonds; all this stains ultimately from the
geometry of the rings and angle strain.

14. Factors affecting stability of
conformations
• Any atom tends to have bond angles that
match those of its bonding orbitals:
tetrahedral (109.5) for SP3-hybridized carbon,
for example. Any deviations from the
"normal" bond angles are accompanied by
angle strain.

15. Any pair of tetrahedral carbons attached to each
other tend to have their bonds staggered. That is to
say, any ethane-like portion of a molecule tends,
like ethane, to take up a staggered conformation.
Any deviations from the staggered arrangement are
accompanied by torsional strain

16. Nonbonded interaction-steric strain
• Non-bonded atoms (or groups) that just touch
each other that is, that are about as far apart as
the sum of their van der Waals radii attract each
other. If brought any closer together, they repel
each other: such crowding together is
accompanied by van der Waals strain (steric
strain).
• These non-bonded interactions can be either
repulsive or attractive,and the result can be
either destabilization or stabilization of the
conformation.

17. Difference in Torsional and Steric
Strain

18. All these factors, working together or opposing each
other, determine the net stability of a conformation.
To figure out what the most stable conformation of a
particular molecule should be, one ideally should
consider all possible combinations of bond angles, angles
of rotation, and even bond lengths, and see which
combination results in the lowest energy content.
A start in this direction feasible only by use of computers
has been made, most notably by ProfessorJames F.
Hendrickson (of Brandeis University).