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