UNIT V

Cylcoalkanes
S.Y.B.Pharm. Sem III
BP301-Pharmaceutical Organic Chemistry-II
Amey Deshpande
Cylcoalkanes
• Stabilities –
• Baeyer’s strain theory,
• limitation of Baeyer’s strain theory,
• Coulson and Moffitt’s modification,
• Sachse Mohr’s theory (Theory of strainless rings),
• Reactions of cyclopropane and cyclobutane only

• Additional – Introduction to cycloalkanes

2
 Introduction to cycloalkanes
• What are cycloalkanes?
• Cycloalkanes are also called cycloparaffins.
• They are Alicyclic Compounds i.e. they are
both aliphatic and cyclic compounds, and
their properties are almost similar to
alkanes.
• Cycloalkanes are saturated hydrocarbons,
that means all the carbons present do not
contain any pi bond, all their valency are
satisfied with sigma bonds.
• Cycloalkanes have General Formula CnH2n
with 1st member Cyclopropane (Formula:
C3H6).
3
STABILITY OF CYCLO-ALKANES
• A stable organic compound means it is very less reactive.
• By the term stability we get an idea about chemical reactivity of a
compound.
• To describe stability of cycloalkanes some very famous theoretical
concepts are there viz.
1. Bayer Strain Theory
2. Sachse Mohr Concept
3. Coulson and Moffitt’s modification

4
STABILITY OF CYCLOALKANES
Baeyer’s Strain Theory
• Baeyer’s theory is based upon some assumptions that are helpful to
understand instability of cycloalkane ring systems.
1. All ring systems are planar. Deviation from normal tetrahedral angles
results in to instable cycloalkanes.
2. The large ring systems involve negative strain hence do not exists.
3. The bond angles in cyclohexane and higher cycloalkanes
(cycloheptane, cyclooctane, cyclononane, etc.) are not larger than
109.5⁰ because the carbon rings of those compounds are not planar
(flat).

5
STABILITY OF CYCLOALKANES
Baeyer’s Strain Theory
• Adolf Baeyer Proposed a theory to explain the relative stability of the
first few cycloalkanes on the fact that the normal angle between any
pair of bonds of carbon atom is 109⁰ 28' (or 109.5⁰) in tetrahedral
geometry (methane molecule).
• Baeyer postulated that any deviation of bond angles from the normal
tetrahedral value would impose a condition of internal strain on the
ring. This is called Angle Strain, which determined the stability of the
ring.
• Baeyer proposed that the optimum overlap of atomic orbitals is
achieved for bond angel of 109.5⁰ (for carbon atom, in a molecule if it is
sp3 hybridized and orbital overlap is maximum).
6
STABILITY OF CYCLOALKANES
Baeyer’s Strain Theory
• In short, it is ideal bond angle for alkane compounds that produces maximum bond strength
and stable molecule.
• For an open chain compound (propane), when carbon is connected to other two carbon
atoms, it is sp3 hybridized and utilize these hybrid orbitals to form the bonds (strong sigma
bonds).
• In cyclopropane, the carbon atom don’t use these hybrid orbitals to form the bonds hence, the
bond (bent bond) is weaker as compare to usual carbon-carbon bond. This is described as
Angle Strain.

7
STABILITY OF CYCLOALKANES
Baeyer’s Strain Theory
• If bond angles deviate from the ideal then the ring produce strain.
Higher strain produces higher instability, increased reactivity and
increases heat of combustion.
• In simple higher the deviation lesser the instability.
• Baeyer observed different bond angles for different cycloalkanes and
also observed some different properties and stability.
• On this basis, he proposed angle strain theory. The theory explains
reactivity and stability of cycloalkanes.

8
STABILITY OF CYCLOALKANES
Baeyer’s Strain Theory

9
STABILITY OF CYCLOALKANES
Baeyer’s Strain Theory
• The ring of cyclopropane is triangle. All the three angles
are of 600 in place of 109.50 (normal bond angle for
carbon atom) this implies that the normal tetrahedral
angle between two bonds is compressed to 600 and that
each of the two bonds involved is pulled in by 24.75o
• The value 24.75o then represents the angle strain or the
deviation through which each bond bends from the
normal tetrahedral direction.
• In same manner, cyclobutane is square and the bond
angles are of 90o in place of 109.5o (normal bond angle for
carbon atom) to adjust them into square ring system
(angle strain 9.75o)
10
STABILITY OF CYCLOALKANES
Baeyer’s Strain Theory
• The deviation for cyclopropane and cyclobutane ring systems then normal
tetrahedral angle will produce strain in ring. The ring strain will make them
unstable as compare to molecules having tetrahedral bond angle.
• According to Baeyer cyclopropane should be highly strained molecule and
most unstable compare to cyclobutane. The triangle ring therefore, be
expected to open up on slightest provocation and thus releasing the strain
within it. This is actually observed –cyclopropane undergoes ring opening
reactions with Br2, HBr and H2 to give open chain addition products.
• Cyclopentane (angle strain 0.75o ) consider to be under least strain and
should be most stable. Thus it is not surprising that it does not undergo
ring-opening reactions.
11
STABILITY OF CYCLOALKANES
Baeyer’s Strain Theory
• The angle strain in cyclohexane is higher than that in case of cyclopentane. This
strain increases continuously with increase in the number of carbon atoms in the
ring.
• According to the theory, cyclohexane and the higher cycloalkanes should become
increasingly unstable and hence more reactive.
• Contrary to this prediction, cyclohexane and the higher members are found to be
quite stable, they do not undergo addition reaction instead they react by
substitution reaction. Thus the theory satisfactorily accounts for the first three
cycloalkanes, but it is not valid for cyclohexane and higher members.
• According to Baeyer, the relative order of stability for some common cycloalkanes
is as under
Cyclopentane > Cyclobutane > Cyclopropane
12
STABILITY OF CYCLOALKANES
Baeyer’s Strain Theory
• Limitations of Baeyer’s angle strain theory
• Baeyer was not able to explain the effect of angle strain in larger ring
systems.
• According to Baeyer Cyclopentane 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 ring systems are not planar but wrinkled (puckered) to eliminate
angle strain.

13