Stereochemistry vi PPT

Mr. Shivshankar Purushottam More
M.Sc.B.ed.NET,SET
Assistant Professor
Department of Chemistry
Late Ku.Durga K.Banmeru Science
College, Lonar Dist.Buldana 443302
E-Mail-shivshankarmore087@gmail.com

Introduction
 Stereochemistry is the branch of chemistry that deals with the study of
three dimension structure of molecule.
 One of the most powerful tools that a chemist can make of to determine
the course of a chemical reaction.
Isomerism:-
Compound which have same molecular formula but differ in their physical
and chemical properties are called isomer and the phenomenon called as
isomerism
Types of isomerism:- Isomerism may be define as structural and
stereoisomerism
Structural isomerism:-The isomerism in which compound have the
same molecular formula but differ from each other by having different
structural is called structural or constitutional isomerism

Structural Isomerism
Chain Isomerism Positional Isomerism
Same molecular formula but different if the molecule have same molecular
Their chain called as Chain Isomerism formula but differ in the attachment of
the same functional group
Functional Isomerism e.g.
Same molecular formula but
different their functional group Propyl alcohol iso-propyl alcohol
e.g.
Ethyl alcohol Dimethyl ether
C
H3
CH2
CH2
CH3
n-butane iso-butane
C
H3
CH3
CH3
C
H3
CH2
CH2
CH2
CH3
n-pantane
C
H3
CH3
CH3
CH3
neo-pentane
C
H3
CH
CH3
CH2
CH3
iso-pentane
C
H3
CH2
CH2
OH
C
H3
CH3
OH
C
H3
CH2
OH C
H3 O CH3

Stereoisomerism
There are two types of stereoisomerism:-
1) Geometrical Isomerism
2) Optical isomerism
Optical Isomerism
Element of symmetry :- According to group,
theory symmetry properties of a given molecule
tell us whether or not molecule is super
imposable with its mirror image.
A molecule that has a plane of symmetry,
A center of symmetry or an alternative axis

Term of the Optical Isomerism
Plane Polarized Light:- The ordinary light is passed through in Nicol’s prism then the
plane pillarized light is rotate in only one direction called as plane polarized light (PPL)
Optical Activity:- The property of compound to rotate plane of plane polarized light in
clockwise or anticlockwise direction is called as optical activity.
Dextro-rotatory:-The optical isomers which rotate the plane polarized light is clockwise
direction are called as dextro-rotatory compound and denoted by ‘d’ or (+) (Latin dextro
means Left hand side)
Plane Polarized Light Dextro-Rotatory Light

 Laevo-Rototary:-The optical isomer which rotate the plane polarized light in anti-clockwise
direction or to left hand side direction called as laevo-rotatory compound.it is denoted as ‘l’
or (-) (Latin laevo means left hand side)
.
The optical active compound is due to the presence of
asymmetric carbon atom in compound. The compound which
shows this property is called as optically active compound
Plane polarized light Laevo-Rotatroy light

 Asymmetric Carbon Atom :- When the four valences of carbon atom are fulfilled by
different atoms or groups then that atom is called as asymmetric carbon atom and it is
denoted by star trick or asterisk

Asymmetric and chiral Molecule
The term asymmetric means absence of any symmetry. The molecule is
asymmetric if is has no element of symmetry(Plane of symmetry, Centre of
symmetry, and Axis of symmetry)
The term of Chiral(mans Greek Chier means hand) is used for those objects
which have right handed and left handed forms .i.e the molecule has”
handedness” and the property of handedness is as chirality. An object which
is not superimposable upon its mirror image is chiral.
All asymmetric molecule are chiral. However, Some molecule having
simple axis of symmetry and no other element of symmetry are also chiral, but
these are not strictly asymmetric. Hence the chiral molecule may molecule
may not necessarily be asymmetric.

Superimposable and non Superimposable image

Element of Symmetry
Plane of Symmetry:-It is a common element and is
defined as a plane of symmetry that bisects a
molecule in such a way that the two halves of
the molecule are mirror reflection of each other.
This is commonly found in achiral molecule.
A classical example is that of meso tartaric acid.
A lower half of the molecule is a mirror reflection
of the other half and the two halves are mirror
images of each other. The two are super impossible
and thus the meso tartaric acid is optically inactive.
Therefore, a molecule that has a plane of symmetry
can not be choral.

Centre of Symmetry
In an achiral molecule is the center of symmetry .
A center of symmetry is a point in the center of the
molecule or an object such that any straight line
drawn through this point, It encounter equivalence
parts of the molecule on either side of the center.
Any face card in a deck of playing cards has a center
of symmetry A staggered conformation of ethene also
has a center of symmetry as shown below.

Axis of Symmetry
A compound is said to have a simple n-fold axis
of symmetry if on rotation through 360 degree/n
fold around that axis leads to an arrangement
which is indistinguishable form the origin.
e.g 1-2,dichoroethane has simple axis of symmetry
that passes through the midpoint of the molecule
and is perpendicular to the plane of symmetry

n-fold axis alternative of symmetry
A molecule is said to have an alternative if an arrangement identical to
the original is restored when i) a molecule is rotated through 180 degree
about an axis through the molecule and ii) rotated molecule is reflected
in a mirror that is perpendicular to the axis of rotation identical with
original obtained
e.g. 1,3 dichloro-2,4difluro cyclobutene is rotated through 180degree
about the axis, an arrangement superimposable on original is obtained.
The multiply 360 degree/180=2 thus this compound has two fold
alternative axis of symmetry.
Example

Enantiomers and Enantiomorphs
The optical isomers which have equal and opposite optical rotation are
called as enantiomers.
Enantiomers have non-superimposable mirror image relationship.
Enantiomers possess same physical and chemical properties and differ only
in direction of rotation of plane polarized light

Diastereomers or Diastereomorphs
The stereoisomers that are not mirror images of each other are called as diastereome
enantiomers they are diastereomers
e.g different structure of Tartaric acid
I II III IV

A compound I and II are non-superimposable mirror image, they are
enantiomers and III and IV are also enantiomers. But compound I & II are
not mirror image of each other hence they are called as diastereoisomers
and II and IV are also diastereomers.
Diastereomer have different physical and chemical properties like melting
point ,boiling point, solubility, density, viscosity and refractive indices etc.
Diastereoselective:- Is the preference for the formation of one or more than one
diastereomer over the other in an organic reaction.
I II III IV

Configuration
The particle arrangement of atoms and group in the space characterizing a particular
stereoisomer is called configuration. Configuration can be changed only by breaking
and making of bonds.
Configuration of Enantiomers lactic acid

Absolute Configuration:- The actual arrangement of atom is a space, of
optical active compound is called its absolute configuration.
Relative Configuration:- The configuration relationship between two optically
active compound can be determining by converting on into other by reaction.
The alcohol and chloride shows that whether two compound i.e product and
reactant have similar or opposite configuration is called as relative configuration
of different compound.
C
H3
C CH2
C
H3 OH
CH3
+ Cl
H
CH3
C CH2
CH3
H OH+ O
H2
(-)-2- Methyl 1-Butanol (+)2-Methyl 1-Chloro Butane

D and L Configuration
D and L system was used to specify the configuration at the asymmetric carbon atom. In this system
the configuration of an enantiomers is related to standard glyceraldehyde. This two form the
glyceraldehyde were arbitrarily
Assign the absolute configuration. If the configuration at asymmetric carbon Atom of compound can be
related to D(+)- glyceraldehyde, it belong D-series and it can be related to L(-)-it belong to L- series
D-Form L-Form

Example of R and S Configuration
Priorities:- 4>3>2>1
Priorities:- -I>-Br>-Cl>-F
Priorities:- -OH>-CHO>-CH2OH>-H
Priorities:- -NH2>-COOH>-CH3>-H

Racemization
The process of converting optically active compound (+)or (-) into racemic mixture (-
+) is called racemization. The (+) and (-) form of most of the compound are capable of
racemization under the influence the of heat, light or chemical reagent.
Resolution
The process of separation of racemic mixture into its two pure enantiomers is known
as resolution. Resolution involve the separation of d-form and l-form of racemic mixture.
Resolution is possible by physical, Chemical and biochemical processes.

Method of resolution of optical isomers
Chemical Method:-1) the racemic mixture is treatted with an optically active
reagent. Which can be easily react with the enantiomorphs in the mixture so
that a pair of diastereoisomers is obtained.
2) Diastereoisomers are easily separated by some suitable physical method like
fractional distillation, fractional crystallization, chromatography.
3) Each of the diastereoisomers is treated with suitable reagent to generate the
enantiomorphs separately.
4) The enantiomorphs are then isolated and purified.
(D acid + L acid) +2Dbase (D acid .D base) + (L acid. D base)
Racemic mixture Optically active Diasterioisomeric salts

The process can be simplified as below:-

B) Geometrical Isomerism
The molecule possess same molecular formula and same Structural formula having double bond and there
is only difference of the arrangement of atoms or groups about the double in the space are called as
geometrical isomerism and the phenomenon is called as geometrical isomerism
e.g Maleic acid and Fumaric acid (C4H4O4) are geometrical isomers of each other.
Cis and Trans isomer 1,2 dibromo ethane
Cis and trans Isomer of Melic acid and Fumaric acid

Cis-Trans Nomenclature
The cis and trans nomenclature cannot be used to designate the geometrical
isomer. This method is used for the identification of position atom or groups of
atom in a plane. When same group on same side across the C=C,it is cis isomer.
When same group on different side it is trans isomer.
e.g. Cis and trans isomer of 1,2 dichloro ethene

E-Z Nomenclature
To designate the geometrical isomer E-Z nomenclature system is used. Following method is adopted for
E-Z nomenclature.( E- german, enitegen means different, Z- Zusamen means together)
Rule :-1)

Example of E-Z nomenclature
1) 3)
2)
4)
C
H3
H
C C
Cl
Br
(E)
C
H3
H
C C
Br
Cl
(Z)
C
H3 CH2
C C
CH2 CH3
CH3
H
Z- 3-methylhex-3-ene
C
H3 CH2
C C
CH3
CH2
H CH3
E-3-methylhex-3-ene
C
H3 CH2
C C
CH3
C
H3
H
E-3-methylpent-2-ene(trans)
C
H3 CH2
C
C
H3
C
CH3
H
Z--methylpent-ene(cis)

Method of structure Determination
There is no absolute method for the determination of the configuration
of cis-trans isomer. However following are the relative method useful to
determine the configuration.
1) Melting Point :- The cis and trans isomer have different melting points
and boiling point e. g cis-1,2 dibromo ethylene melts at.-53 degree, while
the trans isomer at-6 degree. A trans isomer has generally high M.P. than
corresponding cis isomer
2) Dipole Moment:- The dipole moment of the isomer are quite different.
The cis isomer, in general has a high dipole moment e.g cis-1,2-
dichloroethylene has dipole moment 1.89D while trans compound has zero
dipole moment. In the cis isomer the individual bond moment are active
while in trans the vector sum is zero.
H
C C
Cl
Cl
H
H
C C
Cl Cl
H
Trans dipole moment is zero & Cis dipole moment is 1.89D

In case of isomer where one substituent is electron releasing and the other electron withdrawing,
the bond moment are fully additive in trans isomer. Thus the trans isomer in such case has a high
dipole moment than the corresponding cis isomer. Thus in 1-Chloroprene we have
3) Solubility:- The poor packing leads to weaker force of attraction between the molecule in the
crystal lattice of the cis isomer. These weaker forces of attraction can be easily broken by the
dielectric constant of the solvent and hence the cis isomers have greater solubility than trans
isomers. Following are the relative method useful to determine the configuration.
Cl
H
C
H3 H C
H3
H
Cl H
Trans isomer 1.97 DM and cis isomer 1.7DM

 Chemical method:- The formation of a cycle molecule from an open chain molecule takes please
easily only when the reacting group are close to each other. This fact is useful in assigning
configuration to cis-tans isomers in which the doubly bonded carbon atom carry group that are
capable of reacting with each other. The configuration of molecule and fumaric acids is
determining on this basis.
Malic acid readily forms cyclic anhydride whereas fumaric acid does not.
Obviously melic acid must be cis form and fumaric acid trams form.

C) Conformational Isomerism
The isomer in which position of atom of atoms of group of atoms is change by only
rotation across C-C without making and breaking the bond this is known as confirmer
isomer and phenomenon is called as conformational isomerism.
Bayer’s Strain Theory (Stability of cycloalkenes)
Adolf Von Baeyer was honored with a Nobel Prize for the discovery of strain theory
in 1905
The normal tetrahedral bond
Angel is 109 degree but
Cyclopropane is 60 degree
The deviation can be
calculated as
 1/2(1090.28’-600)=24044’

e.g. In Cyclobutane 1/2(1090.28’-900)=9044’
Sr. No Cycloalkane
Deviation from normal tetrahedral angel
(Angel strain)
)01 Cyclopropane +24044’
02 Cyclobutane +9044’
03 Cyclopentane +44’
04 Cyclohexane -5016’
05 Cycloheptane -9033’
06 Cyclooctane -12046
Angel strain in cycloalkene

Conformations
When the molecule is saturated or aclicyclic,free rotation about carbon carbon single bond or C-C-C atom or other
atom can easily occur without breaking original bond and so different arrangement or the atoms are possible which can
be converted into one another. This is called as conformations and different arrangement are known as conformational
isomer conformers or rotational isomer.
Newman projection Formula:-

Sawhorse Projection Formula
 A sawhorse projection is a view of a molecule down a particular carbon-carbon bond.
 Groups connected to both the front and back carbons are drawn using sticks at 120° angles.
 To convert the wedge-dash structure to a sawhorse projection, we must look at the molecule
from one end.
 Assume that we view from the left hand side. Then the three bonds nearest us form an
inverted "Y".
 We draw a sawhorse template with a long line from lower left to upper right.

Conformation of Ethane
The molecular formula
of ethane is
C2H6 .Ethane
possesses two
conformation in
three dimension
i.e eclipsed form
of ethane and
staggered form of
ethane as shown

 In eclipsed from the hydrogen the atoms of one carbon atom exactly
present in front of each other in a same plane or we can say that in
eclipsed from the electron clouds are closed to each other and hence
there is repulsion between these electron clouds(hydrogen atom ) so this
increase energy of molecule and stability decrease.
 It was not observed in staggered form because the electron clouds are
not in front of each other so staggered form is more stable than that of
eclipsed form.
.

Conformation of n-Butane
The molecular formula of n-butane is C4H10.There are main two conformations possible for n-butane that is
eclipsed and staggered conformation.
N-Butane possesses two eclipsed arrangement in the molecule. In one case methyl group of one carbon
atom comes exactly in front of another methyl group of another carbon atom this arrangement is called as
fully eclipsed molecule.
H H
CH3
H H
CH3
H H
CH3
C
H3
H
H
H H
CH3
H H
CH3
H H
CH3
H
H
CH3
Fully Eclipsed Partially Eclipsed Anti or Skew Gauche

 Butane is an alkane with the presence of C-C bonds. Normally, when we rotate
the molecule of butane at the axis of the C-C bond, it shows
different conformation isomerism. Generally, Butane has four conformation
isomers which are fully eclipsed, gauche, eclipsed, and anti butane
conformational isomers. Lets us discuss these isomers below.
• When we look at the chemical structure of butane, we can see that it has two
substituents which are methyl groups attached to the two end carbon atoms. The
methyl group is comparatively larger than hydrogen atoms. However, we can also
see that everything on the front side is eclipsing with the structure in the
backside. One methyl group is present in front of the other methyl group, and the
other hydrogen atoms are kept at the back of other hydrogen atoms. In short, the
structure is completely eclipsing or they are fully eclipsed.

• Now, if we rotate the front methyl group by 60° or when the dihedral angle is
60° then there is a formation of gauche or staggered conformation. It basically
has the presence of identical groups at 60 degrees from one another. This type of
conformation is more stable as there is a little steric hindrance between the same
molecules.
• If we make the dihedral angle as 120° then this form of isomer is also known as
eclipsed conformation. However, this eclipsed form is quite different from the
previous fully eclipsed state. The basic difference is that in this the methyl group
is eclipsing the hydrogen atom and not the methyl group.
• On rotating the isomer at an angle of 180° the two methyl groups are found to
lie exactly opposite to each other. This conformation is known as staggered
conformation or anti conformation.

Ranking Of Conformation Isomers
The ranking of the conformation isomers is given below taking into account the
energy levels from lowest to highest. It is given as;
1)Anti 2) Gauche 3)Eclipsed 4) Fully eclipsed

Newman Projection Structure n-Butane

Conformation of Cyclohexane
The molecular formula of cyclohexane is C6H12.Cyclohexane possesses two conformation viz.
Chair form or Z form and boat form or C form
These structure for cyclohexane were staggered in 1890 by Hermann Sachse as shown below.
Boat Form
Chair Form

 In both form there are two types of bond present in C-H atom.
 Six bonds are radiate in the plane of ring and the hydrogen attached to these bonds are called
as equatorial hydrogen and these bond are called as equatorial bond.
 These are six carbon-Hydrogen bond present in the molecule
 These six bond are parallel to the symmetry of ring and these bonds are called as axial bond
and hydrogen atom attached to these bond are called are axial hydrogen.

Flag-pole interaction
 Chair form of cyclohexane Is more stable than boat form.
 In boat from C1,C2,C3 and C4 present in same plane while remaining two carbon atom
present above below plane and hence four axial and four equatorial hydrogen on C1 to
C4 in boat from cause more steric hinderance(repulsion) between these two hydrogen
i.e 1to 4 is called as bowsprit(bs) flagpole(fp).
 This is also cause more bowsprit flagpole of hydrogen and hence the stability of boat
from decrease this is not occur in chair form so chair form is more stable that boat
form of cyclohexane.
 The chair form and one of the twist form correspond to the half chair form of
cyclohexane.
 This conformation having considerable angle strain and torsion strain has about 10.7-
11.5 Kcal more energy than the chair conformation.

Energy Level Diagram For cyclohexane Ring Flip

Stereochemistry vi PPT

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