Description
UNIT- V Pharmaceutical Organic Chemistry- II
CYCLO ALKANES BP-301T
S.I.T.M (Department of Pharmacy)
Cyclo alkanes:
Stability – Baeyer’s strain theory, Limitation of Baeyer’s strain theory, Coulson and Moffitt
modification, Sachse Mohr’s theory ( Theory of strainless rings), reactions of cyclopropane and
cyclobutane only.
Introduction:
The carbon atoms which are attached to one another to form ring are called cyclic compounds.
Saturated hydrocarbon in which the carbon atoms are joined by single covalent bonds to form a ring
known as Cycloalkanes or Cycloparaffins.
Hydrocarbons with three or more C atoms in a ring are called Cycloalkanes.
Cyclic aliphatic hydrocarbons may be homocyclic or hetrocyclic.
Homocyclic – In which the ring are made up of only carbon atoms.
Hetrocyclic – In which the ring contains at least one hetro atoms.
General Formula- CnH2n.
Nomenclature:
Cycloalkanes have general formula CnH2n or (CH2)n where n = 3,4,5,6……….etc.
In these compounds a number of methylene (CH2) units are joined together to form a ring, they are
also called as polymethylenes.
a) Common name :
The name of cycloalkanes is obtained by prefixing the number of carbon atoms in the ring
before the word methylene.
The prefix tri, tetra, penta etc. are used to present three, four or five carbon atoms in the
ring.
b) IUPAC System:
In this system saturated alicyclic compounds are named by prefix cyclo- to the name of
corresponding open chain alkaines having the same number of carbon atoms.
Cyclo alkanes are represented by simple geometric figures.
METHODS OF PREPARATION
1. Freund’s method : When 1,3 or 1,4 dihalogen derivatives of alkanes react with metal (sodium or
zinc), then cycloalkanes are obtained by dehalogenation of dihalogen derivatives of alkanes.
2. Dieckmann condensation :
When esters of dicarboxylic acid like adipic or pimelic acid is react with sodium ethoxide,
they give cyclic β – keto esters.
Which on hydrolysis and decarboxylation form cyclopentanone.
The cyclopentanone then undergo clemmenson reduction to give cyclo alkanes.
This method is mostly suitable for preparation of five or six membered rings.
3. [2 + 2] cyclo addition reaction : It is an addition reaction between the two unsaturated molecules
to form a cyclic product. In these reaction new sigma bonds are formed by the loss of π- bonds of
reactant.
4. Diels – Alder reaction :
It is also called [4 + 2] cycloaddition reaction.
This reaction involve the addition of an alkene (i.e. 2 π electron) which act as dienophile and a
conjugated diene (i.e. 4 π electron) to form six – membered system.
This reaction takes place easily under thermal conditions.
5. By reduction of aromatic compounds : Catalytic reduction of benzene under pressure in the
presence of nickel (catalyst) yields cyclohexane.
6. Reduction of cyclic carbonyl compounds :
a) Clemmensen’s reduction : Cyclic ketones when reduced in the presence of Zn amalgam and
conc. HCl gets converted to cycloalkanes.
b) Wolff- Kishner reduction : Cyclic ketones on heating with hydrazine yield hydrazone which
when treated with KOH in glycol form cycloalkanes.
PROPERTIES OF CYCLOALKANES
Physical Properties :
The first two member (i.e. Cyclopropane and cyclobutane) are colourless gases, having
pleasent odour;
The higher members (i.e. up to cycloheptane) are liquids; and more higher member are
solids.
They are insoluble in water, and soluble in ethanol and ether.
Cycloalkanes are lighter than water and non- polar in nature, hence they are insoluble in
water but soluble in non- polar solvents like acetone, CCl4 etc.
Their boiling point, melting point, and density increases proportionally with molecular
weight.
Chemical Properties :
(Reaction of Cyclopropane and Cyclobutane Only):- Some of important reactions of cyclopropane and
cyclobutene are following.
1. Free Radical Substitution Reaction:- Cycloalkanes undergo free radical substitution reaction
when they treated with halogen at 525- 575K in the dark or at room temperature in the presence of
light.
2. Addition Reaction :
(i) Addition of Halogens: Cycloalkanes undergo addition reaction and yield open chain products.
(ii) Addition of Halogen Acid:
(iii) Addition of Hydrogen :
(iv) Photohalogenation: Reaction between cycloalkanes and halogens in the presence of light yields
the corresponding halocycloalkanes.
RELATIVE STABILITIES OF CYCLOALKANES
BAYER’S STRAIN THEORY
The difference in the relative stabilities of cycloalkanes was initially explained in terms of Bayer’s strain
theory.
1. In cycloalkanes each carbon atom is sp3 hybrid and bonded to other four carbon atoms. The angle
between any two bonds should be tetrahedral (109.5⸰).
2. Due to cyclic nature, cycloalkanes are planar, resulting the bond angles between adjacent carbon
atoms in the ring are not longer equal to the normal tetrahedral angles i.e. 109.5⸰. The angle
deviation varies with size of the ring.
3. Any distortion or deviation from the normal tetrahedral angle causes a strain in the ring and produce
instability to resultant molecule.
This strain is known as angle strain.
Angle strain = ½ [109.5⸰ – Bond angle in the planar ring]
4. The greater the deviation from the normal angle, greater will be angle strain and hence the reactivity
of cycloalkane.
5. The more the stability of ring system, the more easily it is formed, or in other words the ease of
formation of cycloalkanes is directly related to stability of the ring.
It is evident that cyclopropane has maximum angle strain so it is less stable.
And cycloheptane has least angle strain, therefore it is most stable of all the cycloalkanes.
The positive value of angle strain indicates that the bond angle are compressed from normal
tetrahedral angle.
Negative values imply expansion of bond angles from tetrahedral angles.
But the magnitude of angle strain determines relative reactivity of different cycloalkanes not the
sign of bond angle.
Evidences in favour of Baeyer’s Strain Theory:-
The angle strain decreases the relative stability and ease of formation increases.
This can be explained on the basis of heat of combustion per methylene group in each cycloalkane.
Cyclopropane and cyclobutene possess more heat of combustion (i.e. 697Kj / mol and 686 Kj / mol
respectively) per CH2 group than corresponding alkanes.
Therefore, they are less stable and have tendency to undergo addition reaction through ring fission.
Bayer’s strain theory explained the reactivity of ethylenic bond. A double bond considered as a two
membered ring.
The angle strain in case of double bond should be higher than for cyclic ring. Therefore, alkenes are
more reactive than cycloalkanes.
Limitation of Baeyer’s Strain Theory :-
Bayer’s Strain Theory has some limitations as it failed to explain the following facts:
1. This theory provides planar model of cycloalkanes.
2. This theory states that stability of cyclopentane is more than cyclohexane; but stability of
cyclohexane and other higher cycloalkanes is found to be high.
3. According to this theory carbon – carbon double bond forms with much difficulty; however it form
easily.
COULSON AND MOFFITT’S MODIFICATION
Cyclopropane has a triangular planar structure with internal bond angle between carbon – carbon
bonds are 60⁰.
The sp3 hybrid orbitals of carbon atoms in cyclopropane can not undergo complete overlap with
each other because this requires a considerable deviation from the tetrahedral angle of 109⁰- 5⁰.
Due to this there is a considerable amount of ring strain in cyclopropane.
In cyclopropane re- hybridization occurs and bonding between the carbon centers exist in term of
‘bent’ bonds wherein carbon carbon bonds are bent outwards so inter orbital angle is 105⁰ which
reduce the level of bond strain.
So it is intermediate between σ and π bonding.
These bonds are also called as banana bonds.
The C-C bonds have more p- character than normal whereas C-H bonds have S-character.
Thus it can be seen that ring strain weakens the C-C bonds of cyclopropane ring.
Therefore cyclopropane is much more reactive than alkanes or other higher ring systems.
SACHSE – MOHR THEORY OF STRAINLESS RINGS
As earlier stated, that Baeyer’s theory could explain only the reactivity/ stability of rings containing
five or less carbon atom.
But failed to explain the stability of rings containing six or more carbon atoms.
Hence, Sachse and Mohr proposed this theory to explain extra stability of rings with six or more
carbon atoms.
According to this theory, the carbon atoms of five member and smaller rings lie in a single plane
but in six membered and larger rings the carbon atoms lie in different plane.
These rings are nonplanar or puckered rings.
In these puckered rings the normal tetrahedral angle remains unaltered and therefore angle strain is
negligible.
Hence these rings are called as strain less rings.
Sachse and Mohr propose the existence of two types of multiplanar or puckered strain less rings
known as Boat and Chair forms for cyclohexane.
Both these forms are free from angle strain and can be interconverted just by rotation around single
bond.
Bigger rings are difficult to synthesize there is no single strain. For the formation of ring the
terminal ends of the chain must come closer.
So that the proper interaction between the groups present at two ends of chain.
But in large sized rings, the terminal end of open chain may also intersect with the terminal end of
the other chain, such intermolecular coupling leads to the formation of different products.
That why bigger rings are difficult to synthesize.
If the reaction is carried under dilute conditions, then collisions between the two different chains are
minimized and intermolecular ring closure becomes more feasible than intramolecular ring closure.
