Rdp lecture note_on_cycloalkane B.SC. second year students of Tribhuvan university students.

cycloalkanes.
Introduction,Nomenclature,Isomeris
m,Synthesis, properties and stability
of different ring compounds.
R.D.Pandey, Lecturer of
chemistry,SXC,Maitighar,KTM,Nepal

Chemists class rings as small,normal, medium, and large depending on their size.
•small, n = 3 or 4
•normal, n = 5, 6, or 7
•medium, n = 8–about 14
•large, n > about 14
This is because these different classes all have different properties and synthetic
routes to making them

Cycloalkanes have molecular formula CnH2n and contain carbon atoms
arranged in a ring.In organic chemistry, many common molecules are
cyclic.
Cycloalkanes are simply alkanes that are cyclic.
Just add cyclo-before their name. (The opposite of cyclic is acyclic).
Notice the general formula for
cycloalkanes is CnH2n

Ball model of common cycloalanes.

Nomenclature of Cycloalkanes
This is the same as for alkanes, although two extra rules apply.
Rule A: Decide whether the cyclic or acyclic portion contains more carbons.
This determines the base name. (Alkyl substituted cycloalkane or cycloalkane
substituted alkane).
Rule B: Carbons are numbered to give the lowest numbers for substituted
carbons .
Numbering starts at the most substituted carbon, and goes around in order to
give the lowest numbers.
Examples:
CH3
methylcyclopentane
H3C CH3
H3C CH3
(2,3-dimethylbutan-2-yl)cyclohexane

Cycloalkanes are named by using similar rules, but the prefix cyclo-immediately
precedes the name of the parent

Name and number the substituents. No number is needed to indicate the
location of a single substituent.
For rings with more than one substituent, begin numbering at one substituent and
proceed around the ring to give the second substituent the lowest number.

With two different substituents, number the ring to assign the lower number to
the substituents alphabetically.
Note the special case of an alkane composed of both a ring and a long chain. If the
number of carbons in the ring is greater than or equal to the number of carbons in the
longest chain, the compound is named as a cycloalkane.

When a single ring system is attached to a single chain with a greater number of
carbon
atoms, or when more than one ring system is attached to a single chain, then it is
appropriate
to name the compounds as cycloalkylalkanes. For example.
CH3
1-Cyclobutylpentane
1,3-Dicyclohexylpropane

When there are more acyclic than cyclic carbons, the cyclic part becomes a
cycloalkyl substituent.

Nomenclature of Bicyclic compounds
• Bicyclic Alkanes
• When two or more rings are joined, the molecule
is said to be polycyclic.
• (A molecule with two joined rings is bicyclic, etc.)
• There are 3 ways that rings can be joined:
• Fused
• Bridged
• Spirocyclic

• A)Fused Rings :
Fused rings share two adjacent carbon atoms and the
bond between them. (These are the most common).
• B)Bridged Rings:
These share two non-adjacent carbon atoms (the
bridgehead carbons) and one or more carbon atoms
between them.
• C) Spirocyclic Compounds :
The two rings share only one carbon atom. (These are
comparatively rare).

Nomenclature of Bicyclic Alkanes
• Rule 1:
The name is based on the number of carbons in the ring
systems.
• Rule 2:
This name is prefixed by bicyclo- (or spiro-), and square
brackets with three (or two) numbers.
• Rule 3:
For fused and bridged compounds: count the carbon
bridges around the shared atoms, and arrange the
three numbers in decreasing order. (Spirocyclic systems
only have two numbers, but the same rule applies).

Some organic compounds are identified using common names that do not follow
the IUPAC system of nomenclature. Many of these names were given long ago
before the IUPAC system was adopted, and are still widely used. Additionally,
some names are descriptive of shape and structure, like those below:

Geometric Isomerism In Cycloalkanes
Open chain alkanes undergo free rotation about their C-C bonds.
Alkenes, with double bonds, cannot undergo free rotation.
Cycloalkanes also cannot undergo free rotation.
Substituted cycloalkanes can also give rise to cis and trans isomers. The two
adjacent carbon must have two different groups to show G.I.

Preparation of cycloalkane
Diel’s Alder reaction:The addition of conjugated diene to another unsaturated molecule
generally called Dienophile ,gives adduct. This reaction is called Diel’s Alder reaction.
Diel’s Alder reaction is an example of pericyclic reaction, a reaction that takes place in
one step by a cyclic shift of electrons . It is an example of (4+2) cycloaddition reaction.
CH2
CH2
+ CH2
CH3
 CH3
Diene(4
An electron withdrawing group bonded to one of the SP2 carbon increases the
reactivity of dienophile. Such group withdraw electron from the double bond making
the Diel’s Alder reaction easier to initiate by pumping a partial positive charge on a
carbon of dienophile .
CH2
+
CH2
CH2
CHO

R.D.Pandey, Lecturer of CHO

Dienes are activated by electron donating groups such as –OCH3 ,-CH3 ,-N(CH3)2 etc.
CH2
CH2
+
CH2
COCH 3

COCH 3
Bicyclic product s obtained if the dienophile is cyclic. This reaction was awarded
nobel prize in chemistry in 1950. Only diene with s-cis configuration undergoes
this type of addition.
CH2
CH2
CH2
H2C
S-cis configuration.
S-trans configuration.

From esters of dicarboxylic acid.
Dieckmann’s reaction: Esters of dicarboxylic acid s when heated with sodium
followed by hydrolysis cyclic ketone is formed which on Clemmensen reduction gives
cycloalanes . It s an intramolecular Claisen condensation reaction.
The addition of base to 1,6-diester causes the diester to undergo an intramolecular
condensation ,thereby forming a five membered ring β- ketoester.
COOEt
COOEt
Na,, -C2 H5 OH
CO
H2O/H+ ,-C2H5OH
COOEt
2-carboethoxy pentanone
CO
COOH
, -CO2
COZn/Hg, Conc.HCl,redn
cyclopentanone
2-carboxypentanone

From malonic ester
Perkin’s method: This method is suitable for the synthesis of cycloalkane upto six
membered ring and suggested by Perkin. When terminal dibromide is treated with
malonic ester in presence of sodium ethoxide, a cycloalkane-1,1-dicarboxylic ester
is formed which upon hydrolysis followed by heating gives the corresponding
cycloalkane.
CH2
(CH2)n
+ Na CH
Br
+
COOEt
COOEt
.. -
CH2
COOEt
COOEt
(CH2)n-2
Br
CH2
COOEt
COOEt
.. -
(CH2)n-2
Br
Br
CH2
(CH2)n-2
COOEt
COOEt
CH2
(CH2)n-2
CH2
COOH
(CH2)n-2 COOH
CH2
CH-COOH
CH2
(CH2)n-2
CH2
CH2
C2H5ONa
-NaBr
H2O/H+,heat
-C2H5OH
Heat
-CO2
heat
-CO2

From calcium,barium or thorium salt
of dicarboxylic acid
• Wislicicenus method: On heating Ca or Ba salt
of dicarboxylic acids, cyclic ketone is formed
which can be further reduced to cycloalkane
by Clemmensen reduction using Zn-Hg/
conc.HCl.
CH2
CH2
CH2
CH2
COO
COO
Ca
calcium adipate
heat
Distillation
CH2
CH2
CH2
CH2
C=O +CaCO3
Zn/Hg
Conc.HCl
CH2 CH2
CH2 CH2
CH2

From α,ω-dihalide
Freund’s method: When α,ω-dihalogen compounds are heated with Zn or Na metal in
presence of EtOH, cycloalkane is formed.3,4,5,6 membered rings are prepared by this
method .When two X-atoms are further than 1,6- position ,they do not form ring
compounds but undergo the Wurtz reaction.
Cl
Cl
+
EtOH,
Zn +
ZnCl2

Physical Properties
• Cyclopropane and cyclobutane are gases and higher members are liquid.
• They are insoluble in water but soluble in organic solvent.
• Their m.p. and b.p. increases as their molecular wt. increases.
• Note that, compared with the corresponding straight-chain alkanes , the
cycloalkanes have higher boiling and melting points as well as higher densities.
These differences are due in large part to increased London interactions of the
relatively more rigid and more symmetric cyclic systems.
• In comparing lower cycloalkanes possessing an odd number of carbons with those
having an even number, we find a pronounced alternation in their melting points.
This phenomenon has been ascribed to differences in crystal packing forces
between the two series
• They are nonpolar, relatively inert compounds with boiling points and melting
points that depend on their molecular weights. The cycloalkanes are held in a
more compact cyclic shape, so their physical properties are similar to those of the
compact, branched alkanes.

Chemical properties.
• The cycloalkanes resemble alkanes in chemical
properties.
• They do not react with acid ,bases, oxidising
agent and reducing agent.

Q.
Discuss the stability of cycloalkane in
terms of Bayer’s strain theory.
• In order to explain the stability of cycloalkane ,Adolf
Von Bayer(1889) proposed a theory known as Bayer’s
strain theory. The main facts of this theory are as
follows;
• i) the carbon is tetrahedral. The carbon atom lie at the
center of the tetrahedron and it’s four valences are
directed towards the four corners of tetrahedron. The
bond angle is 1090 28’ .
• The deviation of bond from normal tetrahedron causes
a strain in the molecule which is the cause of reactivity.
Example the deviation of bond in alkene and alkyne
makes them reactive.

• Iii)Cycloalkanes are planar molecules i.e. all the
carbon atom forming ring lie in the same plane.
Thus cyclopropane is the equilateral triangle
,cyclobutane is the square and cyclopentanes and
onwards are polygons.
• Iv) If these molecules are
Considered in the above mentioned shapes ,
then there is large deviation from the normal angle
(109028’) .This strain is called “angle strain”.

• The greater the deviation of bond from 1090
28’ , the greater will be the angle strain.
Consequently the more reactive will be the
molecule. He calculated the deviation of bond
by using the formula-
d = ½(1090 28’ –α ) ,where, d is the
deviation in bond angle from
• The deviation of
tetrahedral angle and α is the
observed angle in cycloalkane.
bond in cyclopropane = ½(109028’-600 )=240 44’
.Like wise in cyclobutane,cyclopentane and
cyclohexane are respectively 9044’,0044’,-5016’

• ½ means deviation is equally shared between two
bonds. +ve value for deviation shows the bond
angle has been compressed and –ve value for the
deviation show the bond angle has been
expanded .
• From the above calculation, it is obvious that the
deviation from the normal tetrahedral angle is
maximum in cyclopropane causing maximum
strain in the molecule(Bayer’s most unstable
compound).Hence it is the most reactive.

• The deviation in cyclobutane is less than
cyclopropane causing less strain in the
molecule. The deviation of bond in
cyclopentanes is least .It is the most stable
compound(Bayer’s most stable).The deviation
in cyclohexane and cycloheptane would go on
increasing .Thus they will be less stable
compound .Thus the synthesis of large ring
would be very difficult because the deviation
of bond would go on increasing.

• V) The more stable a ring system, more easily
it can be synthesized.
• If Bayer’s strain theory is accepted ,stability of
cyclohexane and other higher members
should be lower than cyclopentane.But
experimentally, cyclohexane and higher
members have been found as stable as
cyclopentane ring, why? This is opposed by
heat of combustion data of cycloalkane.

Factors affecting stability of cyclo-alkane
Besides torsional strain and steric strain, the conformations of cycloalkanes are also
affected by angle strain.
Keep in mind the three different types of strain in organic molecules:
• Torsional strain (pitzer strain): strain caused by eclipsing interactions.
This is caused by the repulsion of the bonding electrons of one substituent with
bonding electrons of another substituent on the adjacent atom.
• Steric strain (Vanderwaals strain): strain produced when atoms are forced too close
to each other.
This is caused by atoms or groups approaching each other too closely i.e. nearer than
vander waal’s radi.
• Angle strain(Baeyer’s strain): strain produced when bond angles deviate from
109.5° (for sp3 hybridized atoms).
Cyclic compounds twist and bend in order to achieve a final structure which minimize
the above three kinds of strain that can destabilize a cyclic compound.

Ring Strain in Cycloalkanes
If a cycloalkane requires bond angles different to 109.5° then the sp3
orbitals cannot overlap as efficiently as possible.
This gives rise to angle strain (Bayer strain).
Consider planar cyclobutane:
Along with the angle strain, there is also eclipsing of the hydrogens – torsional strain.
The combination of angle and torsional strains is called Ring Strain.
Calculation of Ring Strain
This is calculated through heats of combustion.
Already seen that alkanes can be combusted – so can cycloalkanes. R.D.Pandey, Lecturer of

The energy released is the heat of combustion, and this value can be
converted in useful information
Note: Cyclopropane and cyclobutane are R.D.Pandey, highly Lecturer strained
of
Cyclopentane and cycloheptane have chemistry,low SXC,ring Maitighar,strain KTM,.cyclohexane Nepal
has no ring strain !

Cyclopropane
Cyclopropane is the most strained cycloalkane . This is due to two effects: Angle
Strain and Torsional Effects
Bonding Overlap is reduced because the enforced 60° bond angle leads to poor
overlap of the sp3 orbitals.
The three membered ring has to be planar, and all the C-H’s are eclipsed.
The angle strain is larger than the torsional effects for cyclopropane

• The carbon atoms of alkanes are sp3 hybridized ⇒ the bond angle is 109.5°
• The internal angle of cyclopropane is 60° and departs from the ideal value
by a very large amount — by 49.5°.
• Angle strain: the potential energy rise resulted from compression of the
internal angle of a cycloalkane from normal sp3-hybridized carbon angle.
• The sp3 orbitals of the carbon atoms cannot overlap as effectively as they
do in alkane (where perfect end-on overlap is possible).
• The C—C bonds of cyclopropane are “bent” ⇒ orbital overlap is less
effectively (the orbitals used for these bonds are not purely sp3, they
contain more p character) ⇒ the C—C bonds of cyclopropane are weaker
⇒ cyclopropane has greater potential energy.

Cyclobutane
Cyclobutane is neither planar, nor a perfect square. A planar geometry would
force all the C-H bonds into eclipsing positions. Cyclobutane actually adopts a
slightly puckered conformation like butterfly, with bond angles of 88°. This
increases angle strain but reduces torsional strain.

Cyclopentane
Cyclopentane has little torsional strain and angle strain.Cyclopentane is not planar
either, since this also would require all C-H’s to be eclipsing. The molecule adopts a
puckered ‘envelope’ conformation, which reduces the torsional strain. The internal
angles are 108° ⇒ very little angle strain if it was planar ⇒considerably torsional
strain. Cyclopentane is flexible and shifts rapidly form one conformation to another.

Stability of cyclohexane
Cyclohexane : Sacche-Mohr concept of strainless ring:
Cyclohexane is by far the most common cycloalkane in nature and also in organic
chemistry.
Zero ring strain implies the bond angles must be 109.5° (no angle strain) and
also no eclipsing interactions between the C-H bonds (no torsional strain).
Boat and Chair Conformations
Cyclohexane adopts a puckered structure. The most stable conformation for
cyclohexane is called the chair conformation .
In the chair
conformation, all
the bond angles
are 109.5° and all
the C-H bonds are
staggered. (Zero
ring strain)

Axial And Equatorial Positions
If we look at an instantaneous snapshot of cyclohexane in a chair conformation, there
are 2 types of C-H bond. Six of the C-H bonds point straight up and down (axial
bonds).
Six of the C-H bonds point out from the ring (equatorial bonds).
Notice the alternating pattern of the positions.

Boat Conformation
Cyclohexane can also exist in another conformation called the boat. The boat is just
a chair with the footrest flipped up.
This also has bond angles of 109.5° and thus avoids any angle strain, but there is
torsional strain.
The two hydrogens at the ends of the boat are in close contact, causing torsional
strain. These flagpole hydrogens are eclipsed.
To avoid these unfavorable interactions, the boat conformation skews slightly, giving
a twist boat conformation .

The chair is the lowest energy conformation, although since the energy barrier to ring flip is
fairly small, there will always be some other conformations present.
The half chair is the point of highest energy, and is not a stable conformation

Conformations of Monosubstituted Cyclohexanes
A substituent on a cyclohexyl ring can occupy either an axial or equatorial position.
Consider methylcyclohexane: The chair conformation with the methyl axial can interconvert
via a boat conformation into a chair conformation with the methyl equatorial.
The energy barrier for this is low, and this interconversion takes place rapidly at room
temperature, although the conformation of lower energy predominates.
It is found that the methyl equatorial conformation is 1.7kcal/mol lower in energy than the
methyl axial conformation.
Both chair conformations are lower in energy than the boat.

When the methyl group is axial, it is gauche to C3 and C5.
When the methyl is equatorial, it is anti to C3.
So axial methylcyclohexane has 2 gauche
interactions (2 x 0.9kcal)
Equatorial methylcyclohexane has no
gauche interactions.
Predict that EQUATORIAL is favored by
1.8kcal. (Good agreement).

This gauche interaction is also known as a 1,3 diaxial interaction. The axial
substituents on C1 and C3 are close in space and their electron clouds repel one
another.
Generally a larger substituent gives rise to a larger difference in energy between
the axial and equatorial conformations.

Industrial source.
• Petroleum from certain areas is rich in
cycloalkane(in particular California), known to
petroleum industry as naphthenes( mainly
contains cylohexanes,
methylcyclohexane,methylcyclopentane, and 1,2-
dimethylcyclopentane.
• Addition of hydrogen to aromatic compounds
yields cyclic aliphatic compounds specially
cyclohexane derivatives.For example,
hydrogenation of benzene yields pure
cyclohexane.

Industrial source.

Rdp lecture note_on_cycloalkane B.SC. second year students of Tribhuvan university students.

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