1) Cyclohexane is the most stable cycloalkane due to its chair conformation, which avoids angle and torsional strain through staggered bonding. 2) Bayer strain theory states that angle strain increases with deviation from the ideal tetrahedral bond angle of 109.5 degrees. Cyclopropane has the most strain due to its 60 degree bond angles. 3) In cyclohexane's chair conformation, four carbons lie in a plane while two are above and below, allowing all bonds to be staggered and avoid strain. Monosubstituted cyclohexanes prefer the equatorial position to avoid 1,3-diaxial interactions.

1. BAYER STRAIN THEORY & CONFORMATIONAL
ANALYSIS OF CYCLOHEXANE
Dr. L. Sakthikumar M.Sc., M.Phil.,Ph.d
Assistant professor in Chemistry
Saiva Bhanu Kshatriya College
Aruppukottai – 626101
Tamilnadu
E-mail : lsakthisbk@gmail.com
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2. BAYER STRAIN THEORY
 The theory proposed by Adolf von Bayer in 1885,
popularly known as Bayer Strain theory, suggested
that the angle strain in cycloalkanes increases
proportionally as the deviation from regular
tetrahedron geometry increases.
 Cyclohexane ring is most stable among the other
cycloalkanes because it does not have any bond
angle strain. The cyclopentane ring with an
internal angle 108° would be next stable ring than
cyclohexane (as it has minimum deviation from a
regular tetrahedral angle of 109.5°).
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3. INTERNAL BOND ANGLES
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5.  One can compare the stabilities of cycloalkanes by
using their heats of combustion data. Minimum
value of heats of combustion suggests maximum
stability.
 According to this data, cyclo propane has a
maximum strain 115 KJ/mole and is least stable
while cyclohexane has the least strain 0 KJ/mole
and is most stable.
 Molecules show strain arising out of non-ideal
geometry. The internal energy of the molecule is
dependent on energy factors. These are
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6. TOTAL STRAIN ENERGY = 1 + 2 + 3 + 4
 1 = stretching and compression of bonds (bond strain)
 2 = Bond angle distortion (Baeyer strain, classical strain)
 3 = Tortional strain (eclipsing strain)
 4 = steric strain (vander waals strain)
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7.  Steric strain: Any type of repulsion between two closely
spaced non-bonded atoms or group of atoms is termed
as steric strain. The repulsions between electrons of
these closely spaced atoms imparts a destabilising
effect and is the major cause of the overall strain in a
molecule. It can be classified into various types of
strains as described below.
 Angle strain: If the angle between a pair of adjacent
bonds on a carbon atom is less than the tetrahedral
angle (i.e., 109.5o) then there is a destabilisation due to
the bond pair - bond pair repulsion.
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8.  Van der Waals strain: This type of strain arises
when the electron clouds of a pair of bulky
groups are too close to each other. It leads to
an increase in energy of the system due to the
electrostatic repulsion of the electron clouds.
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9.  Torsional strain: When a molecule is able to rotate around a sigma bond, the other
three bonds on each carbon holding the single bond move relative to each other
resulting in different levels of torsional strain. The relative movement of these bonds
can be better understood in terms of dihedral angle.

 A dihedral angle (θ) is the angle between the two intersecting planes formed by the
bonds on adjacent carbon atoms. The maximum strain is observed when these
bonds are closest to each other, i.e. in an eclipsing position (θ= 0°).
 Hence this strain is also known as eclipsing strain. With the rotation around the
sigma bond, the dihedral angle changes and consequently the repulsion between
the bonded pair of electrons (torsional strain) also changes. The strain is minimum
at θ= 60° (staggered conformation).
 Ring strain: Generally, a ring or cyclic structure is less stable than an equivalent
acyclic structure, primarily due to angle and torsional strain. The extra energy is
released when the ring is opened is known as ring strain.
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10. CONFORMATIONAL ANALYSIS OF SMALL RINGS
CYCLOPROPANE
 The three carbons in cyclopropane should lie in a plane as any three
points describe a plane. All the C-C bond lengths in propane are
same which suggests that all the three carbons must lie at the
corners of an equilateral triangle.
 In this geometry, the deviation from the regular tetrahedral angle is
the maximum (from normal tetrahedral angle of 109.5° to 60°) and
hence there is extensive strain in the cyclopropane ring.
 This is evident from the large heat of combustion value per methyl
group. Infact the C-C bonds in cyclopropane are bent in order to form
an equilateral triangle.
 Further strain is imparted by eclipsing interactions as all the C-H
bonds in cyclopropane are eclipsed and C-C bond rotation to relieve
this strain is not possible due to the rigidity of the ring. Hence
cyclopropane is considerably strained molecule with a ring strain of
115 KJ/mole and readily undergoes reactions involving ring opening.
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12. CONFORMATIONAL ANALYSIS OF CYCLOBUTANE
 In cyclobutane, the ring adopts a puckered or wing-
shaped conformation. Though the distortion from the
planar structure reduces the torsional strain (eclipsed
bonds move slightly away), it increases the angle strain
(angle decreases as the ring puckers).
 A balance between the two is struck when the ring
puckers to about 34° (i.e. when one methylene moves
34° up or below the rest of the three carbon forming a
plane). T
 The planar form of cyclobutane (where all the bonds are
eclipsed) is just 1.4 kcal/mole more in energy than the
puckered conformation.
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14. CONFORMATIONAL ANALYSIS OF CYCLOPENTANE
 Unlike what was predicted by Baeyer, cyclopentane is not entirely
strain free even though in a planar conformation the C-C-C bond
angles are close to 109.5°.
 The eclipsed bonds in the planar cyclobutane impart considerable
torsional strain forcing it to attain a more stable puckered
conformation.
 The heat of combustion data indicates the total strain in the ring. On
distorting the planarity, cyclopentane ring has an angle ring strain of
27 KJ/mole partly relieves the torsional strain but increases the
angle strain (similar to cyclobutane).
 The minimum energy conformation is thus adopted by balancing the
two opposing effects.
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16. CONFORMATIONAL ANALYSIS OF CYCLOHEXANE
 As predicted by heats of combustion, cyclohexane is strain free ring.
It does contain neither angle strain nor torsional strain in its chair
conformation.
 This is because it can pucker in such a way that all of the bonds are
perfectly staggered, and in this conformation all of the bonds are
109°.
 In the chair conformation, four carbons lie in a plane while two lie
above and below this plane. There is no angle ring strain in the
cyclohexane ring.
 There are two types of C-H bonds in cyclohexane. The vertical bonds
parallel to the axis are called axial bonds.
 The near horizontal bonds radiating away from the equator are called
equatorial bonds. They are slanting up and slanting down
alternatively.
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18.  Cyclohexane theoretically can exist in two different chair
conformations as shown in figure and the two forms can interconvert
easily through ring flipping as shown in the figure Ring flipping is
nothing but a rotation of single bonds in the molecule.
 An interesting point to note in this interconversion is that all the axial
bonds in ring 1 (chair) become equatorial bonds in ring 2 (inverted
chair) and vice versa.
 There are other infinite conformations of cyclohexane of varying
energies besides chair conformation due to the rotation around
single bonds.
 Some of them with special names are boat, twist boat and half chair
conformations (ones representing the peaks and valleys in the
energy diagram of cyclohexane conformations).
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20. ENERGY PROFILE DIAGRAM
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21.  From the figure we can see that chair conformation has the minimum energy out of
all the possible conformations and is therefore called as the stable and ground state
conformation of cyclohexane.
 The boat conformation is 30 kJ/mol higher in energy than the chair conformation.
 This may largely be attributed to torsional strain among the four pairs of hydrogens
and to some extent to the flagpole interactions between the hydrogens on the
diagonally opposite carbons.
 In twist boat conformation, the planarity of the four carbons (as in boat) is distorted
or twisted and hence the name ‘twists boat’. By twisting the planar carbon
somewhat relieved.
 The twist boat conformation is 23 kJ/mol higher energy than chair conformation.
Likewise the half chair conformation is 45.2 KJ/mole higher energy (represented by
the highest energy peaks) than chair form.
 The Table informations summarize the amount and types of strain in various
conformations of cyclohexane.
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23. CONFORMATIONAL ANALYSIS OF
MONO SUBSTITUTED CYCLOHEXANE
 Though all the six carbons in cyclohexane are equivalent, there are two
types of bonds on each carbon - namely axial and equatorial where the
substituent can be placed.
 Let us take methyl group as the substituent for conformational analysis of
mono substituted cyclohexane.
 The two possibilities we have are: either methyl group occupies axial
position or it occupies equatorial position.
 The two possibilities we have are: either methyl group occupies axial
position or it occupies equatorial position.
 The two can interchange through ring flipping as shown in figure.
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25.  If we draw Newman projection for methylcyclohexane,
we will see that the axial methylcyclohexane experiences
gauche interactions between the methyl group and ring
methylene (figure) while equatorial methylcyclohexane
has no such interactions and hence has somewhat
lesser energy than the axial methylcyclohexane.
 Besides gauche interactions, axial methyl group
experience steric repulsions from the nearby axial
hydrogens which further increases the energy of axial
methylcyclohexane.
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26.  These steric interactions between the substituent in axial position at C-1
and the axial hydrogens at C-3 are called 1,3-diaxial interactions. When the
methyl group is at equatorial position, there are no significant gauche
interactions as methyl group is anti to ring C-C bond.
 There are no diaxial interactions as well and hence equatorial conformation
becomes more stable than the axial.
 The equatorial conformation is favored in the equilibrium because the axial
isomer has about 7.5 kJ/mole of steric strain.
 This observation can be generalized to all the substituents, if present at the
equatorial position will always be more stable than at the axial position.
 The conformation of methylcyclohexane with an equatorial methyl group is
more stable than the conformation with an axial methyl group by 7.6 kJ/
mole.
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28. METHOD OF DRAWING CHAIR CONFORMATION
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