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Paper II : CH-212 • Section – I (Organic Chemistry) • Chapter 1: Stereoisomerism • Chapter 2: Organic reaction Mechanism • Section – II (Inorganic Chemistry) • Chapter 3: General Principles of Metallurgy • Chapter 4: Metallurgy of Aluminium
(Electrometallurgy):
• Chapter 5: Metallurgy of Iron and Steel (Pyrometallurgy)
• Chapter 6: Corrosion and Passivity
STEREOISOMERISM
• Stereochemistry: The study of three dimensional structures of molecuels is called
as stereochemistry.
• Stereo Isomers: Molecules with same molecular formula, same structure but
different arrangement of atoms or groups in
space are called as stereoisomers.
• Optical Isomers: The stereo isomers, which rotate the plane of plane polarized light in
equal and opposite direction are called as
optical isomers.
• Configurational Isomers: Stereo isomers which can not be inter converted by rotation of
bonds are called as configurational isomers.
S-2-bromobutane R-2-bromobutane
• Conformational Isomers: Stereo isomers, which can be inter converted by rotation of
bonds are conformational isomers.
• E.g. Eclipsed and Staggered form of ethane
• Enantiomers: The stereo isomers, which are non-superimposable mirror images of each
other are called as enantiomers.
• Geometrical Isomers: Cis and Trans isomers formed due to restricted rotation of double
bonds are called as geometrical isomers.
• E.g. cis-2-butene and trans-2-butene
• Diastereomers: Stereo isomers, which are not mirror images of each other are called as
diastereomers.
• Meso Compound: A compound which is optically inactive despite the presence of
chiral carbon atoms is called as meso
compound.
• Chiral Carbon or Asymmetric Carbon: Carbon attached to four different atoms or groups is called
as chiral carbon.
Erythro and Threo Isomers
• In the Fischer projection if like atoms or groups are on the same side of the bond joining the chiral
carbons, the isomer is called as erythro isomer.
• In the Fischer projection if like atoms or groups are on the opposite side of the bond joining the chiral
carbons, the isomer is called as threo isomer.
• Absolute Configuration: It is the description of spatial arrangement of the atoms of a chiral
molecule in terms of R and S.
D and L configuration
• D or L configuration is assigned in the Fischer form of an asymmetric molecule, on the basis of
arrangement of groups at the lower most
asymmetric C atom.
• Optical Activity: The property of rotating the plane of plane polarized light in clockwise or anti
clockwise direction is called as Optical Activity.
• Optically Active Compound: The compound which rotates the plane of plane polarized light in
clockwise or anti clockwise direction is called as
Optically Active Compound.
• Plane Polarized Light: The light vibrating in single plane is called as plane polarized light.
• Dextro Rotatory Compound: Compound, which rotates the plane of plane polarized light in clockwise direction is called as dextro rotatory compound. E.g. d-2-bromobutane.
• Leavo Rotatory Compound: Compound, which rotates the plane of plane polarized light in anti clockwise direction is called as leavo rotatory compound. E.g. l-2-bromobutane
• Racemic Mixture: A racemic mixture is a mixture of two enantiomers in equal proportions. E.g. mixture
of d and l lactic acid. It is designated as (dl) lactic
acid.
• Specific Rotation: Rotation of plane polarised light shown by optically active compound when its
concentration is 1g/ml and path length of
polarimeter is one dm.
Making of Cyclic Compound
• Baeyer’s Strain Theory • The atoms present in any cycloalkane molecule lie
in same plane and thus all cycloalkanes are planar.
• Angle strain is the increase in potential energy of a molecule due to bond angles deviating from the
ideal values. More angle strain makes the ring less
stable. The larger rings are difficult to make because
they are highly unstable.
• Angle Strain: The deviation from normal bond angle is called as angle strain.
Angle strain = ½ (109.5 – internal angle)
Relation Between Angle Strain and Heat of
Combustion • As the angle strain increases, the heat of
combustion per CH2 group also increases.
• In cyclohexane, heat of combustion per CH2 group is equal to open chain alkanes because
chair form of cyclohexane has no angle strain.
Molecule Int. Angle Angle Strain
Heat of
Combustion per
CH2
Cyclopropane 60 24.75◦ 166.6
Cyclobutane 90 9.75 164
Cyclopentane 108 0.75◦ 158.7
Cyclohexane 120 5.25◦ 157.4
• Draw a k of Baeyer’s Strain Theory • He assumed that all the rings are planar. But rings with
more than five carbon atoms are puckered in structure and get rid of angle strain. Therefore, Baeyer’s Angle Strain theory works for smaller rings with upto four carbon atoms.
• For smaller rings like cyclopropane and cyclobutane, heat of combustion per CH2 group is high and they validate the strain theory in smaller rings. But in larger rings, the heat of combustion is not very high therefore they are not unstable.
Cyclohexane in Newmann
Chair Conformation
Cyclohexane in Newmann
Boat Conformation
Conformations of Cyclohexane
Nearly 0Kcal/mole Nearly 0Kcal/mole
11 Kcal/mole
7.1 Kcal/mole
5.5 Kcal/mole
11 Kcal/mole
• Chair Conformation: All the hydrogen atoms are staggered. There is no torsional interaction and zero
angle strain in the molecule in chair form. Therefore
it is the most stable conformation of cyclohexane.
• Half Chair: In this conformation there are large number of pairs of eclipsing hydrogen atoms
causing torsional interaction. In addition angle
strain makes the molecule more unstable and the
energy of the molecule in this form is 11 Kcal/mole.
This is the least stable conformation.
• Boat Conformation: In this conformation there are four pairs of torsional interaction in the molecule. In addition hydrogen atoms on C1 and C4 are in flag pole position and the distance to accommodate them is 1.8A. However the sum of van der waals radii of two hydrogen atoms is 2.5 A. Since they are very close they repel each other and energy of molecule in this form is 7.1 Kcal/mole.
• Twisted Boat: To decrease the flag pole interaction the C1 and C4 are take away from each other and the shape of boat is slightly twisted. This form is called as twisted boat conformation. The energy is 5.5Kcal/mole in this conformation.
Mono Substituted Cyclohexane
• Methylcyclohexane is a mixture of two conformational isomers. These isomers have different energies and one form is more stable than the other.
• van der Waals repulsions occur between one of the methyl hydrogens and the two axial hydrogens on
the same face of the ring and it is called as 1,3-
diaxial interactions. The energy of the molecule in
this arrangement increases by 1.8 kcal/mole.
• Hence equatorial form of methylcyclohexane is more stable than axial form of methyl cyclohexane.
Other Substituents
F 0.63, Cl 1.80, Br 1.59 and I 1.80
Disubstituted Cyclohexane • Cis-1,2-dimethylcyclohexane
• Trans-1,2-dimethylcyclohexane
• Cis-1,3-dimethylcyclohexane
• Trans-1,3-dimethylcyclohexane
• Trans-1,3-dimethylcyclohexane
Cis-1,4-dimethylcyclohexane
• Trans-1,4-dimethylcyclohexane
Locking of Conformation
• In tert-butylcyclohexane, the size of the group is very large. When this group is present in axial position it results in very
strong 1,3-diaxial interaction and after ring flipping the
equatorial conformation of tert-butylcyclohexane is formed
which is more stable due to no repulsion. Hence equatorial
conformationof tertiary butyl group is favored over the
axial conformation by about 20 kJ/mol (about 5 kcal/mol).
A sample of tert-butylcyclohexane exists in practically
equatorial form and it is called as locking of conformation.
Factors Affecting Stability of
Conformational Isomers
• Angle Strain: Any deviation from the normal bond angle is called as angle strain.
Angle Strain = Normal angle – Observed angle • Because of angle strain, the energy of the
molecule increase and it becomes less stable.
• Torsional Strain: Any pair of tetrahedral carbon attached to each other have their
bonds staggered. Any deviation from
staggered arrangement leads to torsional
strain. When the bonds on adjacent carbon
atoms are not staggered their electronic
clouds repel each other and the molecule
becomes less stable in this conformation.
• van der Waal’s Strain (Steric Strain): Non bonded atoms can not be placed at a distance
less than the sum of their van der Waal radii.
If they come closer than this distance, they
start repelling each other.
R and S Configuration to Molecules
with Two Asymmetric Carbon Atoms
First Exchange Second Exchange
First Exchange Second Exchange