6. STRAIN
Deformation of a bond
length and bond angles from
their ideal values under the
influence of internal forces.
7. • It gives rise to instability within in a
structure with high internal energy.
• When a bond angle or length is distorted
the potential energy is stored.
• The stored potential energy in the
molecule is referred as strain or strain
energy.
9. Torsional Strain
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.
10. Angle strain
Angle strain is the increase in
potential energy of a molecule due
to bond angles deviating from the
ideal values.
11. Steric strain
Steric strain (van der Waals strain): An increase
in molecular potential energy (strain) caused
when atoms or groups separated by at least four
covalent bonds are forced closer than their van
der Waals radii allow.
12. Adolf Von Baeyer
(of the University of Munich)
In 1885
proposed a
theory to
explain the
relative
stability of
cycloalkanes.
13. His theory based on the fact
that the normal angle between
any pair of bonds of a carbon
atom is 109◦C 28’.
14.
15. Baeyer postulated that any
deviation of bond angles from
the normal tetrahedral value
would impose a condition of
internal strain on the ring.
16. • He assumed that all cycloalkanes are planar.
• He calculated the angles through which each
of the valence bond was deflected from the
normal direction in the formation of the
various ring .
• This is called angle strain.
• Angle strain determines the stability of the
ring.
19. Cyclopropane
• Three carbon atoms occupy the
corners of an equilateral triangle.
• Cyclopropane has C-C-C bond angle
of 60◦C
• Normal tetrahedral angle is 109◦ 28’
between any two bond is
compressed to 60◦.
22. Cyclopropane
• Each of the bond is pulled in by 24 ◦
44’
• Whether the angle strain is positive
or negative, its magnitude determines
the extent of strain in the ring.
• Angle of strain is more in
cyclopropane.
23.
24. • According to Baeyer strain theory , cyclo
propane should be highly strained molecule
and consequently most unstable.
• It is expected to open up on slightest
provocation and thus releasing the strain
within it.
• Thus cyclo propane undergoes ring opening
reaction.
27. Cyclopentane
• Angle of strain is minimum in cyclo
pentane.
• It is under least strain and more stable.
• It does not undergo ring opening
reaction.
• The angle of regular pentagan 108 are
very close to the tetrahedral angle 109 5
.Hence cyclopentane is virtually free of
angle strain.
28. Baeyer proposed [incorrectly] there should
be a certain amount of strain in
cyclohexane , further he suggested
[incorrectly]that as one proceed to
cycloheptane, cyclooctane, the deviation of
the bond angle is progressively larger and
the molecules would become progressively
more strained.
29. • Baeyer considered the rings smaller or larger
than cyclopentane and cyclohexane were
unstable. Because of instability that the 3 and
4 membered ring underwent ring opening
reaction.
• It was because of this instability greater
difficulty was encountered in the synthesis of
larger ring.
31. Heat Combustion
• The quantity of heat evolved when one
mole of compound is burned to CO2 and
water.
• Heat of combustion support of Baeyer’s
proposal that rings smaller or larger than
cyclopentane and cyclohexane are
unstable.
32. Ring Size Heat of Combustion
3 166.6
4 164
5 158.7
6 157.4
7 158.3
14 157.2
Open chain compound 157.4
33. • Cyclohexane angle of strain increases with
increases in the number of carbon atom in the
ring.
• According to baeyer’s theory cyclohexane and
higher alkane should be unstable and reactive.
• Contrary to this prediction, cyclohexane and
the higher members are found to be quite
stable.
34. • They donot undergo ring opening reactions.
• Instead they resemble open chain alkanes in
reactivity i.e it undergoes substitution.
• But bayers strain theory satisfactorily account
for cyclo propane, cyclobutane and
cyclopentane. But it is not valid for
cyclohexane and higher member.
35. Limitation of Baeyer’s strain theory
• Bayer was not able to explain the effect of angle
strain in larger ring systems.
• According to Baeyer cyclo pentane should be
much stable than cyclohexane but practically it is
reversed.
• Larger ring systems are not possible according to
Baeyer as they have negative strain but they exist
and much stable
• Larger rings are not planar but puckered to
eliminate angle strain.
