aliphatic cyclic compounds, alicyclic compounds, cyclic compounds, cycloalkanes, nomenclature, preparations and reaction, reactions of cycloalkanes, addition reactions of cyclopropane and cyclobutane, Baeyer's strain theory, angle strain, their heat of combustion and stabilities, Sachse and Mohr prediction, Pitzer's strain theory, torsional strain, cyclopropane, cyclobutane, cyclopentane, cyclohexane, chair form and boat form of cyclohexane, axial and equatorial hydrogen atoms,

1. CYCLIC ALIPHATIC
COMPOUNDS
CYCLOALKANES

2. Cyclic aliphatic compounds:
Nomenclature, industrial source, preparation,
reactions, reactivity of cyclopropane and cyclobutane
by comparing with alkanes, stability of cycloalkanes–
Baeyer’s strain theory, Sachse and Mohr prediction
and Pitzer’s strain theory, factors affecting stability of
conformations, conformational structure of
cyclobutane, cyclopentane and cyclohexane,
equatorial and axial bonds. 5 hrs
SYLLABUS

3. CYCLOALKANES
Cycloalkanes or cycloparaffins are saturated
hydrocarbons in which the carbon atoms are joined by
single covalent bonds to form a ring.They are also
called alicyclic compounds.
Chemists have divided all organic compounds into two
broad classes: aliphatic(originated from Greek word
‘aleiphar’ meaning fat, as some of its important
compounds are present in the fat.) compounds and
aromatic(originated from Greek word aroma meaning
sweet-smelling.) compounds.

4. The prefix ali-is added because they resemble with
aliphatic compounds.
The general formula of unsubstituted cycloalkanes is
CnH2n . The first member of the homologous series
of cycloalkanes is cyclopropane, C3H6.
The cyclic nature of some of cycloalkanes confers
very special properties on them.

5. NOMENCLATURE OF ALIPHATIC COMPOUNDS
The IUPAC rules for naming cycloalkanes are as
follows:
i. The name of an unsubstituted cycloalkanes
is obtained by adding prefix, cyclo- before the
name of normal alkanes having same no. of
carbon atoms as in the ring.

6. EXAMPLES:
Cyclopropane
(Common:Trimethylene) Cyclobutane
(Tetramethylene)
A traingle
A square
A hexagon
A pentagon
Cyclopentane
(Pentamethylene)
Cyclohexane
(Hexamethylene)
ii. One or more substituents on the ring are named
and their positions are indicated by numbers. The ring
is numbered in such a way that carbon bearing
substituents should be minimum.

7. EXAMPLES:
CH
CH3 CH3
Isopropyl cyclohexane
Cl
Chlorocyclohexane
NO2 CH3
C2H5
Nitrocyclohexane 1-Ethyl-3-methyl
cyclohexane
1
2
3
Alphabetical rule
O
Cl CH3
1 2
3
4
4-Chloro-4-methyl
cyclohexanone
CH3 CH3
Cyclopentanone
1, 1-Dimethyl cyclopentane

8. OH
CH3
1
2
2-Methyl cyclohexanol
CH3
Cl
1
2
1-Chloro-2-methyl
cyclohexane
Cl
CH3
1-Chloro-2-methyl cyclopentane
1
2
CH3
C2H5
Cl
4-Chloro-2-ethyl-1-methyl
cyclohexane(not 1-chloro-3-
ethyl-4-methyl cyclohexane)
(sum of locants rule)
CH2CH2CH2CH2CH3
1-cyclobutyl pentane
1 2 3 4 5
iii.When a single ring system is attached at a single chain
with a greater no. of carbon atoms or more than one ring
system is attached to a single chain, then it is named as
cycloalkyl alkanes. Examples:
1 3
2
1, 3-Dicyclohexylpropane

9. SOME MORE EXAMPLES:
Cl
CH3
CH3
1
2
3
4
1-Chloro-2,4-dimethyl cyclohexane
Sum of locants rule
OH
C(CH3)3
1 2
3
3-(1,1-dimethyl ethyl) cyclohexanol

10. INDUSTRIAL SOURCES
Ptroleum from certain areas(in particular California, US state)
is rich in cycloalkanes known as naphthenes in petroleum
industry. Among these are cyclohexane, methylcyclohexane,
methylcyclopentane and 1,2-dimethylcyclopentane.
By the elimination of hydrogen from aliphatic compounds
yields aromatic compounds by the reforming or aromatization
process. Similarly, addition of hydrogen to aromatic
compounds yields cyclic aliphatic compounds, specifically
cyclohexane derivatives.

11. An important example of this is the hydrogenation of benzene
to yield pure cyclohexane.
+ 3H2
Ni, 150-2500C
25 atm
Cyclohexane.
Benzene
+ 3H2
Ni, 150-2500C
150 atm
OH
Cyclohexanol (aliphatic)
Phenol (aromatic)
(Hydrogenation of substituted benzene)
From cyclohexanol , many other cyclic compounds containing a six-membered ring can be made.

12. PREPARATIONS OF CYCLOALKANES
1. FROM DIHALIDES
(BY COUPLING OF TWO ALKYL GROUPS THAT ARE PART OF THE SAME MOLECULE)
i. The one of alkyl groups of the dihalide is converted into an organometallic compound.
This particular method works only for preparation of cyclopropane.
Example:
CH2 X CH2-ZnX
CH2
CH2 X CH2-X
ii. Terminal dihalides treated with sod. Or Zn form cycloalkanes. This reaction is an
extension of Wurtz reaction & is useful for the preparation of 3- to 6-membered rings.
CH2
Zn, NaI -ZnX2
cyclopropane
1,3-Dihalopropane
Example:

13. When the Ca or Ba salts of dicarboxylic acids are heated, cyclic ketones are formed. The cyclic
ketones can be readily converted into the corresponding cycloalkanes by Clemmenssen
reduction. Example
2. FROM CALCIUM SALTS OF DICARBOXYLIC ACIDS
CH2CH2-COO-
Ca2+ ∆ Zn/Hg
CH2CH2-COO- Conc.HCl
-CaCO3 Cyclopentanone
Cyclopentane
Calcium adipate
CH2-Cl
CH2 + 2Na ∆ + 2NaCl
CH2Cl
Cyclopropane

14. 3. FROM ESTERS OF DICARBOXYLIC ACIDS (Dieckmann reaction).
Esters of dicarboxylic acids when treated with Na undergo intramolecular acetoacetic ester
condensation and a β – ketoester is formed.
β – ketoester on hydrolysis followed by decarboxylation give cyclic ketones which on
Clemmenssen reduction give corresponding cycloalkanes. Example:
CH2-CH2-CO OC2H5 Na CH2 CH2
C=O
CH2-CH H -C2H5OH CH2 CH
COOC2H5 COOC2H5
β-Ketoester
Diethyl adipate
α
β

15. CH2-CH2 ∆ CH2 CH2
CO C=O
CH2-CH H2O/H+ CH2 CH
COOC2H5 COOH
β-Ketoester β-Ketoacid
(Hydrolysis)
∆/-CO2
(Decarboxylation)
Cyclopentanone
Zn-Hg/conc.HCl
Cyclopentane
(Reduction)

16. 4. FROM ALKENES(Simmons Smith reaction, a cycloaddition reactions)
The most important route to rings of many different sizes is through the important class of
reactions called cycloadditions: reactions in which molecules are called together to form
rings.
When alkenes are treated with methylene iodide (CH2I2) in presence of a zinc-copper
couple, cyclopropane derivatives are formed. Examples:
Zn-Cu couple
CH3-CH=CH2 + CH2I2 CH3-CH-CH2
Diiodomethane Ether(stirred)
(Methylene iodide) CH2
Methyl cyclopropane
Zn-Cu couple
CH3-CH=CH -CH3 + CH2I2 CH3-CH-CH – CH3
Diiodomethane Ether(stirred)
(Methylene iodide) CH2
1,2-Dimethyl cyclopropane
CH2I2 + Zn(Cu) ICH2ZnI
Carbene like species called a carbenoid
The carbenoid then brings about the stereospecific addition of a CH2 group directly to the
double bond.

17. light
CH3-CH=CH -CH3 + CH2 N2 CH3-CH-CH – CH3
2-Butene Diazomethane + N2
CH2
1,2-Dimethyl cyclopropane
Methylene is formed by the photolysis of either diazomethane, CH2N2 or ketene, CH2=C=O.
CH2=N⁺=N̅ : CH2 + N2
Diazomethane Methylene
(Very poisonous yellow gas)
UV light
CH2=C=O
UV light or ∆
:
CH2
+ CO
Ketene Mthylene
(Carbene)

18. CHEMICAL PROPERTIES OF CYCLOALKANES
Clcloalkanes resemble alkanes in their chemical behavior. However, cyclopropane & cyclobutane
are exceptions. With certain reagents, they undergo ring opening and give addition products.
(A) SUBSTITUTION REACTIONS
Like alkanes, cycloalkanes undergo chiefly free radical substitution. For example: substitution
with Cl2 & Br2.
+ Cl2
UV light
Cl
Cyclopropane Chlorocyclopropane
+ Cl2
UV light
Cl
+ HCl
+ HCl
Cyclohexane Chlorocyclohexane

19. Cyclopentane
+ Br2
3000C
+ HBr
Br
Bromocyclopentane
(B) RING OPENING REACTIONS
(REACTIONS OF SMALL RING COMPOUNDS.CYCLOPROPANE & CYCLOBUTANE )
Besides the free-radical substitution reactions, cyclopropane and cyclobutane undergo certain
reactions of a quite different type: addition. The addition reactions destroy the cyclopropane and
cyclobutane ring system & yield open – chain products.
EXAMPLES:
i) Addition of Cl2 & Br2.
Cyclopropane reacts with chlorine and bromine in dark to form addition products. CCl4 is
used as a solvent.
Cyclopropane
+ Br2
CCl4/Dark
CH2 – CH2 – CH2
Br Br
1,3-Dibromopropane

20. + Cl2
FeCl3 CH2 – CH2 – CH2
Cl Cl
1,3-Dichloropropane
NOTE: Cyclobutane & higher members do not give this reactions.
ii) Addition of HBr & HI.
Cyclopropane reacts with conc. HBr & HI to yield 1-Bromopropane & 1-Iodopropane
respectively.
+ HBr
Conc.
CH2 – CH2 – CH2 - Br
1-Bromopropane
NOTE: Cyclobutane & higher members donot give this reactions.

21. iii) Addition of H2.
Cyclopropane & cyclobutane reacts with hydrogen in presence of Ni catalyst to give
propane & n-butane respectively.
(NOTE: Higher temperature is required for cyclobutane)
+ H2
Ni, 800C CH3CH2CH3
Propane
+ H2
Ni, 2000C
CH3CH2CH2CH3
n-Butane

22. iv) Oxidation.
Cycloalkanes undergo oxidation with hot alkaline KMnO4 to form dicarboxylic acids
Cyclohexane
+ 5[O]
KMnO4/OH
̅
∆
CH2CH2COOH
CH2CH2COOH
Adipic acid
+ H2O
Ni, H2, 800C
Cl2, FeCl3
Conc. H2SO4
H2O
CH2 – CH2 – CH2
Cl Cl
1,3-Dichloropropane
CH3CH2CH3
Propane
CH2 – CH2 – CH2
H OH
n-Propyl alcohol

23. Cyclobutane doesnot undergo most of the ring opening
reactions of cyclopropane; it is hydrogenated, but only
under more vigorous conditions than those required for
cyclopropane. Thus cyclobutane undergoes addition less
readily than cyclopropane & with some exceptions,
cyclopropane less readily than an alkene. The remarkable
thing is that these cycloalkanes undergo addition at all.

24. BAEYER’S STRAIN THEORY
(Stability of cycloalkanes)
This theory was proposed by a German chemist Adolf von Baeyer of the University of Munich in
1885 to explain the relative stability of the first few cycloalkanes. His theory is based on the
normal angle between any pair of bonds of a carbon atom is 109028´, known as normal
tetrahedral bond angle.
The part of his theory that deals with the ring-opening tendencies of cyclopropane & cyclobutane
is generally accepted today & other part of his theory is based on false assumptions and have
been discarded.
BAEYER’S ARGUMENTS (ASSUMPTIONS) ARE:
1). In general, the normal bond angle between any of bonds of a carbon atom is 109028´,
known as normal tetrahedral bond angle (of an SP3 hybridized carbon atom).
He postulated that any deviation of bond angles from the normal tetrahedral value
will impose an internal strain on the ring.

25. Adolf von Baeyer of the University of
Munich,Germany(1835-1917)

26. 2). He also assumed that all cycloalkanes are planar.
He calculated angle between any pair of bonds of a carbon atom in various
cycloalkanes assuming that all cycloalkanes are planar. He also calculated the
angles through which each of the bond is deflected from the normal direction. This
is called Angle Strain.
Angle strain is the factor which determines the stability of the cycloalkanes.
Formula to calculate angle strain(α) is,
α = ½ (109028´ - bond angle of cycloalkane)
Cycloalkanes Bond angle Angle strain, α
Cyclopropane(n=3) 600 24044´,1/2(109028´-600)
Cyclobutane(n=4) 900 9044´,1/2(109028´-900)
Cycopentane(n=5) 1080 0044´,1/2(109028´-1080)
Cyclohexane(n=6) 1200 -5016´,1/2(109028´-1200)
Cycloheptane(n=7) 128034´ -9033´1/2(109028´-128034´)
Cyclooctane(n=8) 1350 -12046´,1/2(109028´-1350)
Cyclononane(n=9) 1800 -35016´,1/2(109028´-1800)

27. The ring cyclopropane is a triangle with three angles of 600 & the ring
of cyclobutane is a square with four angle of 900. In these
cycloalkanes, a pair of bonds does not assume the tetrahedral angle but
is compressed to 600 or 900 to fit the geometry of the corresponding
ring.
Since in cyclopropane, the three carbon atoms occupy the corners of
an equilateral triangle, cyclopropane has C – C – C bond angle of 600.
It shows that the normal tetrahedral angle of 109028´ between the pair
of bonds is compressed to 600.Each of the two bonds in an equilateral
triangle is compressed by 1/2(109028´-600)= 24044´. This value
represents the angle strain exhibited by the cycloalkane ring or
deviation per bond from normal tetrahedral direction.
600
109028´
24044´ 24044´
(α)
(α)
The angle strain for other
cycloalkanes can be calculated in the
same way. Whether the angle
strain(α) is – or +, its magnitude
indicates the extent of strain in the
ring.

28. The value of angle strain is maximum in the case of cyclopropane.
Thus according to the Baeyer strain theory, cyclopropane should be
highly strained molecule & consequently most unstable. Thus it is
expected for cyclopropane to open – up on slight effort & thus
releasing the strain within it. This is actually so. It undergoes ring
opening reactions with Br2, HBr & H2(Ni) to give open chain
addition products.
The value of angle strain in case of cyclobutane is less than that in
case of cyclopropane. Thus it is expected that cyclobutane is more
stable than cyclopropane. This is actually so. As expected,
cyclobutane undergoes ring opening reactions but only under more
drastic(vigorous) conditions.

29. The angle strain value is minimum in case of cyclopentane. Thus
it is expected that cyclopentane should be under least strain and
should be most stable. It is actually so. Cyclopentane doesnot
undergo ring-opening reactions.
The angle strain value in case of cyclohexane is higher than that
in case of cyclopentane. The angle strain increases continuously
with the increase of corbon atoms in the ring as shown in the
table above (slide no.26). According to Baeyer strain theory, it
can be precticted that cyclohexane and higher members should
become increasingly unstable i.e., the order of instability of
cycloalkanes higher than cyclopentane should be:
cyclononane> cyclooctane >cycloheptane >cyclohexane.
Hence Baeyer considered that rings smaller or larger
than cyclopentane or cyclohexane are unstable.

30. But it is actually not so. Contrary to this prediction, cyclohexane and
higher members are found to be quite stable as they do not give ring
opening reactions but give substitution reactions. Thus they resemble
open-chain alkanes in reactivity.
Thus, Baeyer strain theory can satisfactorily explain the exceptional
reactivity of cyclopropane, cyclobutane & cyclopentane only but
not cyclohexane and higher members. In other words, this theory is
valid for cyclopropane, cyclobutane & cyclopentane only but not
valid for cyclohexane and higher members.

31. Sachse and Mohr prediction
In order to explain the stability of cyclohexane and higher members,
Sachse & Mohr(1918) has proposed that rings can become free from
angle strain if all the carbon atoms in the ring are not forced into one
plane as was assumed in Baeyer strain theory. If the geometry of
cyclohexane and higher members are not planar & are assumed a
‘folded’ or ‘puckered’ structure, the normal tetrahedral angles of
109028´ are retained. As a result, the molecules of cyclohexane &
higher members get relieved from strain.

32. For example, cyclohexane can exist in two non-planar puckered
conformations both of which are completely free from strain. These
two puckered conformations are CHAIR FORM & BOAT FORM
because of their shape as shown below:
Chair form Boat form
Such non-planar , ‘puckered’ rings are also possible for higher
cycloalkanes in which ring carbons can have normal tetrahedral
angles.
The chair form of cyclohexane is more stable than boat form. In
ordinary conditions, cyclohexane molecules mostly exist in chair
form.

33. On examination of chair form, it is found that the hydrogen atoms
can be of two categories. Six of the twelve C - H bonds point
straight up or down generally perpendicular to the average plane of
the ring. These six hydrogen atoms, by analogy with the earth, are
called Axial Hydrogens. Rest six hydrogen atoms lie slightly above
or slightly below the plane of the ring. These hydrogen atoms, again
by analogy with the equator of the earth, are called Equatorial
hydrogens as shown below:
a
a
a
a
a
a
e
e
e
e
e
e
He=Equatorial hydrogens
Ha=Axial hydrogens

34. RED= AXIAL Hydrogens
BLUE=EQUATORIAL Hydrogens

35. PITZER’S STRAIN THEORY
Any strain resulting from torsion is called Pitzer strain or eclipsing
strain. In other words torsional strain is also called Pitzer strain.
Strain caused by the close approach of atoms or groups separated
by three covalent bonds is called TORSIONAL STRAIN.
In the molecule W-X-Y-Z, atoms W and Z may experience
torsional strain in a particular conformation (such as an eclipsed
conformation).

36. Let us see the eclipse conformation of 1,2-dichloroethane :
Sawhorse
projections:
Newman
projections:
Eclipsed conformation
More strain
Anti-staggered
conformation
Less strain

37. When the chlorine atoms of 1,2-dichloroethane are aligned
(an eclipsed conformation), the chlorine atoms experience
torsional strain. The eclipsed hydrogen atoms also experience
torsional strain (but less than the chlorine atoms because hydrogen
has a smaller atomic radius than chlorine). This torsional strain is
relieved when carbon-carbon bond rotation changes
the molecule into a staggered conformation (such as the anti-
staggered conformation shown here).

38. STABILITIES OF CYCLOALKANES
HEATS OF COMBUSTION AND RELATIVE STABILITIES OF
CYCLOALKANES
The heat of combustion is the quantity of heat evolved when one
mole of a compound is completely burned to CO2 and H2O.
Let us see whether the heats of combustion of various cycloalkanes
support or not support Baeyer’s proposal that rings smaller or
larger than cyclopentane or cyclohexane are unstable.
For open -chain alkanes, the heat of combustion per methylene
group (-CH2- group ) is very close to 157.4Kcal/mol.
Lists of heats of combustion measured for some of cycloalkanes are
given below:

39. Ring
size
Heat of combustion per
CH2, Kcal/mol.
Ring
size
Heat of combustion per
CH2, Kcal/mol.
3 166.6 10 158.6
4 164.0 11 158.4
5 158.7 12 157.6
6 157.4 (Most stable) 13 157.8
7 158.3 14 157.4
8 158.6 15 157.5
9 158.8 17 157.2
Open-chain 157.4
TABLE:

40. We notice that the heat of combustion per –CH2- group in
cyclpropane is 9 kcal and in cyclobutane 7 Kcal higher than the
open-chain value of 157.4.
Whatever the compound in which it occurs, a –CH2- group
produces the same products on complete combustion: CO2 &
H2O.
-CH2- + 3/2 O2 CO2 + H2O + heat
The value of heat of combustion per –CH2- should be the same
whatever the compound in which it occurs. But it is not so, as
shown in table above.
The values of heat of combustion per –CH2- evolved by
cyclopropane and cyclobutane are higher than open chain
compound, it means that they contain more energy per –CH2-
group.

41. The values of heat of combustion per –CH2- evolved by
cyclopropane and cyclobutane are higher than open chain
compound, it means that they contain more energy per –CH2-
group. This result is in agreement with Baeyer’s strain theory
that cyclopropane & cyclobutane are less stable than open chain
compounds.
It is reasonable to suppose that the tendency to undergo ring –
opening reactions for these cycloalkanes is related to this
instability that is, the ring opening reactions of cyclopropane and
cyclobutane occur due to less stability than open chain
compounds.
According to Baeyer’s strain theory, rings larger than
cyclopentane & cyclohexane also should be unstable due to
higher angle strain & hence also should give high heats of
combustion. This instability should increase steadily with
increase of size of ring as angle strain increases in same order.

42. But, the heats of combustion of rings higher than 4-membered are
not found to increase as expected on the basis of Baeyer’s strain
theory and not deviate much from the open-chain value of 157.4.
But, the one of the biggest deviation occurs in Baeyer’s ‘most
stable’ compound, cyclopentane: 1.3 Kcal per –CH2- group,
5×1.3= 6.5 Kcal per molecule
The seven member to eleven member rings have about the same
value as cyclopentane.
The rings containing twelve or more carbon atoms have
indistinguishable heat of combustion from open chain value.

43. Contrary to Baeyer’s theory, none of these rings is appreciably
less stable than open chain compounds and larger ones are
completely free of strain. Furthermore, once they have been
synthesized, these large ring cycloalkanes have little tendency to
undergo ring-opening reactions characteristic of cyclopropane
and cyclobutane.
Then what is wrong with Baeyer’s theory that it does not apply
to the rings larger than four members?
Answer. Due to false assumption that all rings of cycloalkanes
are planar or flat. Actually rings larger than four member are not
planar but they are puckered so that they are free from angle
strain.

44. A three-membered ring must be planar since ring of three point
difines a plane.
A four-membered ring need not be planar, but puckering of the
ring would cause more angle strain.
A five – membered also need not be planar, but puckering of the
ring would increase angle strain.
All rings larger than these (3-, 4- and 5-membered) are puckered.
If large rings are stable, why are they difficult to synthesize?
Answer – if a compound can not be synthesized, it does not
necessarily mean that the compound is unstable. The synthesis of
stable compound may be difficult due to other factors.

45. The closing of ring occurs only when the two ends of the chain
come close enough to each other for the formation of bond. This
opportunity to come two ends close enough to each other
decreases with increase the chain length. Under these conditions
the end of one chain is more likely to encounter the end of
another chain to form an entire different product.
CH2Y
CH2Y
CH2 – CH2
Fig. Ring closure

46. CH2Y
CH2Y
CH2Y
CH2Y
CH2Y
CH2Y
CH2 - CH2
CH2-CH2
CH2Y
CH2Y
Fig. chain lengthening

47. This fact is taken into consideration to make large rings.
Reactions are carried out in very dilute solutions so that
collisions between two different chains are unlikely and ring-
closing reaction, although slow, in principle is more favorable.
Five- and six-membered rings are most commonly encountered
in organic synthesis, because they are large enough to be free of
angle strain and small enough that ring closure is likely.

48. FACTORS AFFECTING STABILITY OF CONFORMATIONS
Any atom likes to have bond angles according to bonding
orbitals: tetrahedral(109028’) for sp3-hybridized carbon, for
example.
Thus any deviations from the staggered arrangement are
accompanied by TORSIONAL STRAIN.
Thus any deviations from the “normal” bond angles as required
by the geometry of bonding orbitals are accompanied by
ANGLE STRAIN.
Any two of tetrahedral carbon atoms attached to each other like
to have their bonds staggered.

49. Any atoms or groups that are not bonded to each other can
interact in several ways, depending on their size, polarity and
their closeness . These non-bonded interactions can be either
repulsive or attractive, and as a result destabilization or
stabilization of the conformation occurs.
Any two non-bonded atoms or groups if just touch each other or
in other words, that are about as far as the sum of their van der
Waals radii- attract each other.
Thus, if these two non-bonded atoms or groups come any closer
than van der Waals radii, they repel each other. Such crowding
together is accompanied by van der Waals strain(STERIC STRAIN)

50. Non-bonded atoms (or groups) like to take positions which give
most favorable dipole-dipole interactions: that is, positions that
minimize dipole-dipole repulsions or maximize dipole –dipole
attractions.(A particular powerful attraction occurs due to the
special kind of dipole-dipole interaction called the hydrogen
bond.)
The net stability of a conformation is determined with the help
of all these factors(mentioned above), working together or
opposing each other.

51. Energy contents of all possible combinations of bond angle,
angles of rotation, and even bond lengths are calculated and the
combination which gives the lowest energy content is considered.
Such calculations have become quite feasible through the use of
computers.
Not only calculations but also experimental measurements show
that the final result is a compromise, and that few molecules have
the idealized conformations that we assign them and, for
convenience, usually work with.
For example, there is no tetrahedral carbon compound-except
with four same substituents- has exactly tetrahedral bond angles:
a molecule acquires a certain amount of angle strain to relieve
van der Waals strain or dipole – dipole interaction.

52. In the Gauche conformer of n-butane, the dihedral angle between
methyl groups is not 600, but larger than 600: in Gauche
conformations, the two methyl groups are closer than the sum of
their van der Waals radii causing steric strain, so the molecule
accepts some torsional strain to relieve van der Waals (steric)
strain between two methyl groups.

53. Remaining of syllabus:
conformational structure of cyclobutane, cyclopentane and
cyclohexane, equatorial and axial bonds.
CONFORMATIONS OF CYCLOALKANES
1. CYCLOHEXANE
The cyclohexane is the most important of the cycloalkanes.
Let us examine different conformations of cyclohexane.
The six-membered rings are, in general, more stable than
five-membered ones as evidenced by its “normal” heat of
combustion (i.e., 157.4Kcal/mol per CH2).

54. There is considerable evidence that the “chair” form is the
most stable conformation of cyclohexane.
In this non-planar chair form, the carbon-carbon bond
angles are all 109028’ and therefore free of angle. The chair
conformation is also free from of torsional strain. In chair
conformation, the atoms are seen to be perfectly
staggered(as shown in fig. below):
Fig. A Newman projection of the chair conformation of cyclohexane

55. Moreover, the hydrogen atoms at opposite carbon atoms of
the ring of cyclohexane are maximally separated.
Fig. Illustration of large separation between hydrogen atoms
at opposite corners of the ring (designed C3 & C6 ) when
the ring is in the chair conformation.
3
6

56. The chair conformation is thus not only free of angle strain but
also free of torsional strain. It lies at an energy minimum and is
therefore a conformational isomer. The chair form is the most
stable conformation of cyclohexane, and, indeed, of nearly every
derivative of cyclohexane.
By partial rotations about the carbon-carbon bonds of the ring (or
by flipping the “right” end of the molecule up ) , the chair
conformation gives another shape called the “boat” conformation.
The boat conformation, like chair conformation, is free of angle
strain.

57. In addition, two of the hydrogen atoms on C3 & C6 are close
enough to each other to cause van der Waals repulsion. This
effect is called the “flagpole” interaction of the boat conformation
(as shown in fig. below):
But the boat conformation, unlike chair conformation, is not free
of torsional strain because in boat conformation, the atoms are
seen to be eclipsed.
This effect is called the “flagpole” interaction of the boat
conformation (as shown in fig. below):

60. Torsional strain & flagpole interactions cause the boat to increase
energy than the chair conformation.
It has been calculated that the boat conformation is less stable by
7.1Kcal/mol than the chair conformation.
The boat conformation is not a conformer but a transition state
because it is believed that it does not lie at an energy minimum,
but at an energy maximum.
Now, what are those two conformers that lie on either
side of the boat conformation?

61. Although the chair conformation is more stable, it is much more
rigid than the boat form.
In other words, the boat conformation is quite flexible.
By flexing the boat conformation, a new conformation is obtained,
known as the “twist” conformation.
By flexing the boat conformation, it can relieve some of its
torsional strain & at the same time, reduce the flagpole interactions
as well.
Thus, the twist conformation is more stable than the boat
conformation i.e., the twist conformation has a lower energy than
the boat conformation.

62. But, the twist conformation is, still, less stable than the chair
conformation since the stability gained by flexing is insufficient.
The twist boat conformation is also a conformer as it lies at an
energy minimum.
Between the two different conformers, chair and twist boat, there
is another transition state known as half-chair conformation which
, with angle strain & torsional strain, lies about 11Kcal above the
chair form.

63. In the half –chair conformation, the carbon atoms of one end of the
ring have become coplanar.
The overall relationship are summerized in fig. below:

64. 11Kcal
1.6Kcal
5.5Kcal

65. As energy barriers between the chair, boat and twist conformations
of cyclohexane are low, it is not possible to separate them at room
temperature.
At room temperature, the molecules have enough energies to cause
approx, 1 million interconversions to occur per second.
And more than 99% of the molecules are estimated to be in a chair
conformation at any given moment due to its greater stability.
At equilibrium between the chair and twist-boat forms, the more
stable chair form is favored by the ratio 10,000:1 at room
temperature.(from M&B)

66. Conformationally, the chair form of cyclohexane is the perfect
specimen of a cycloalkane whereas the planar cyclopentane must
be the the poorest.
In planar cyclopentane, there is exact bond eclipsing between
every pair of carbons, as a result there is much torsional strain and
less angle strain. To partially relieve this torsional strain,
cyclopentane takes on a slight puckered conformation, even at the
cost of a little angle strain.
The internal angle of a regular pentagon are 1080, a value very
close to the normal tetrahedral bond angle of 109028’. Therefore, if
cyclopentane molecules were planar, they would have very little
angle strain.
2. CYCLOPENTANE

67. But, planarity would cause considerable torsional strain because all
ten hydrogen atoms would be eclipsed. Consequently,
cyclopentane assumes a slight bent conformation in which one or
two of the atoms of the ring are out of the plane of the others.
Hence, with little torsional strain and angle strain,
cyclopentane is almost as stable as cyclohexane.

70. 2. CYCLOBUTANE
In planar cyclobutane, there is exact bond eclipsing between every
pair of carbons, as a result there is much torsional strain and less
angle strain. To partially relieve this torsional strain, cyclobutane
takes on a slight puckered(or folded) conformation, even at the
cost of a little angle strain.
The internal angle of a square are 900, thus planar cyclobutane has
angle strain, lower than cyclopropane but higher than
cyclopentane.
In non-planar cyclobutane, the internal angles are 880-deviation by
more than 210 from the normal bond angle.

71. Fig. The “folded” or “bent” conformation of cyclobutane

73. Now, the question one may ask is: What is the stable
conformation of a cyclohexane derivative in which one
hydrogen atom has been replaced by an alkyl
substituent?
Let us consider methyl cyclohexane as an example of cycloalkane.
Since the methyl group is the largest substituent on the ring, and
hence subject to crowding, we must focus our attention on methyl
in estimating stabilities of various conformations of the methyl
cyclohexane.
Methyl cyclohexane has two possible chair conformations (as
shown below) .
These two conformations are interconvertible the partial rotations
that constitute a ring flip.

74. Fig. conformations of methyl cyclohexane

75. Fig. 1,3-diaxial interaction between the two axial hydrogen atoms & the axial
methyl group in the axial conformation of methyl cyclohexane are shown
with dashed arrows. Less crowing occurs in the equatorial conformation.
Axial –CH3 more crowded than equatorial –CH3
1
2
3
5
4

76. Two conformations show that when
equatorial –CH3 is axial, it is so close to the
two axial hydrogen atoms on the same side of
the molecule that the van der Waals forces
between them are repulsive. This type of
steric strain, because it arises from an
interaction between axial groups on carbon
atoms 1 and 3 (or 5) is called a 1,3-diaxial
interaction.

77. Studies indicate that the conformation with the methyl group
equatorial is more stable than the conformation with methyl group
axial by about 1.8 Kcal mol-1.
Thus in the equilibrium mixture, the conformation with the methyl
group in equatorial position is the predominant one.
Calculations show that it constitute about 95% of the equilibrium
mixture at room temperature.
Similar studies with other substituents show that there is generally
less repulsive interaction when the groups are equatorial rather than
axial.

78. As we know that conformation with the methyl group equatorial is
more stable than the conformation with methyl group axial by about
1.8 Kcal mol-1, this value is due to the 1,3-diaxial interaction of
one -CH3 group and two hydrogen atoms. On this basis, we assign a
value of 0.9 Kcal /mol. to each 1,3-diaxial methyl-hydrogen
interaction. Consequently, we can account well for the energy
differences between conformations of a variety of cyclohexanes
containing more than one methyl groups.

79. (Both calculations and experimental measurements show that the
final result is a compromise, and few molecules have the idealzed
conformations that we assign them and for convenience, usually
work with. For example, probably no tetravalent carbon
compound-except one with four identical substituents-has exactly
tetrahedral bond angles: a molecule accepts a certain amount of
angle strain to relieve van der Waals strain or dipole-dipole
interaction. In the gauche conformer of n-butane, the dihedral
angle between the methyl group is not 600 , but almost certainly
larger: the molecule accepts some torsional strain to ease van der
Waals strain (steric strain) between the methyl groups:)

80. CH3
CH3
H H
H
H
Anti-conformation
CH3
CH3
H
H
H
H
H
H
H
H
CH3
CH3
Gauche
confor-
mations
(I)
(II)
(III)
Staggered conformations of butane: