Smita Asthana @ St.Ann's
1
1. Flying-Wedge or Wedge-Dash projection
The Flying-Wedge projection is the most common three-dimensional
representation of a three dimensional molecule on a two dimensional surface
(paper). This kind of representation is usually done for molecules containing
chiral centre. In this representation, the ordinary lines represent bonds in the
plane of the paper. A solid Wedge (
plane of the paper and a dashed wedge (
) represents a bond above the
) or a broken line (
)
represents a bond below the plane of the paper.
Methane
Smita Asthana @ St.Ann's
2
The Flying-Wedge projection formula of (R)- Lactic acid , for example, can
be shown as follows…..
H3C
bonds in the plane
of the paper
bond bellow the plane of
the paper
OH
H3C
H
bond above the plane of
the paper
(R) - Lactic acid
Smita Asthana @ St.Ann's
3
2. FISCHER PROJECTION FORMULAE
The carbon chain is projected vertically, the horizontal bonds attached to a carbon are
considered to be above the plane of the paper and towards the viewer and the vertical
bonds are considered to be below the plane of the paper and at the back of viewer.
•In Fischer Formula, if two like groups are on the same side, the molecule is called
‘Erythro” and if two like groups are on opposite side it is called ‘threo’ .
Smita Asthana @ St.Ann's
4
3. SAWHORSE FORMULAE
In this representation, the molecule is viewed slightly from above and form the right
and then projected on the paper. The bond between the two carbon atoms is drawn
diagonally and of a relatively greater length for the sake of clarity. The lower left hand
carbon is taken as the front carbon and the upper right hand carbon as the back carbon .
•All parallel bonds in sawhorse formula are Eclipsed and all anti parallel bonds are
opposite or trans/anti to each other. The sawhorse presentation of Eclipsed and
staggered conformations of Ethane are as follow.
Smita Asthana @ St.Ann's
5
Smita Asthana @ St.Ann's
6
4. NEWMAN PROJETION FORMULAE
Newman devised a very simple method of projecting three dimensional formula on
paper which are known as Newman projections.
•In these Formulae the molecule is viewed from the front. The carbon atom nearer to
the eye is represented by a point and the three atoms or groups are shown attached to
it by three lines at an angle of 1200 to each other.
•In Newman’s formula all parallel bonds are Eclipsed and all anti- parallel (or) opposite
bonds are
Smita Asthana @ St.Ann's
7
•Newman projections for Eclipsed and staggered conformation of Ethane are
Smita Asthana @ St.Ann's
8
Smita Asthana @ St.Ann's
9
ISOMERISM
Isomers are molecules that have the same molecular
formula, but have a different arrangement of the atoms in
space.
StereoisomersIn stereoisomerism, the atoms making up the isomers are joined up in the
same order, but still manage to have a different spatial arrangement.
Smita Asthana @ St.Ann's
10
Smita Asthana @ St.Ann's
11
Conformational isomerism
Conformations of Acyclic Alkanes
Smita Asthana @ St.Ann's
12
• Different conformations differ in their energy content. Energy is maximum when
two bond pairs are closest to each other. This is the eclipsed conformation, the
C—H bonds on one carbon are directly aligned with the C—H bonds on the
adjacent carbon. This is the most unstable form.
• In the staggered conformation, the C—H bonds on one carbon bisect the H—C—
H bond angle on the adjacent carbon. The energies are minimum as the bond
pairs are as far as possible. This is most stable form.
Smita Asthana @ St.Ann's
13
• Rotating the atoms on one carbon by 60° converts an eclipsed
conformation into a staggered conformation, and vice versa.
• The angle that separates a bond on one atom from a bond on an
adjacent atom is called a dihedral angle. For ethane in the
staggered conformation, the dihedral angle for the C—H bonds is
60°. For eclipsed ethane, it is 0°.
Smita Asthana @ St.Ann's
14
Cnformational Analysis of Ethane
•Conformations are different spatial
arrangements of a molecule that are
generated by rotation about single
bonds.
Smita Asthana @ St.Ann's
15
Ethane’s Conformations
• The most stable conformation of ethane has all six C–
H bonds away from each other (staggered).
• The least stable conformation has all six C–H bonds
as close as possible (eclipsed) in a Newman
projection.
Smita Asthana @ St.Ann's
16
Ethane’s Conformations
• The barrier to rotation between conformations is small (12
kJ/mol; 2.9 kcal/mol)
• The eclipsed conformers are 12 kJ/mol higher in energy than
the staggered conformers – energy due to torsional strain
Smita Asthana @ St.Ann's
Dihedral angle
17
Ethane
• eclipsed
conformation
• staggered
conformation
Smita Asthana @ St.Ann's
18
Conformational Analysis of
Butane
Smita Asthana @ St.Ann's
19
Butane’s Conformations
• anti conformation is the most stable conformation
of butane .It has two methyl groups 180° away from each
other.
Smita Asthana @ St.Ann's
20
Butane’s Conformations
• Rotation around the C2–C3 gives eclipsed conformation, the
methyl groups are too close.
• 16kJ/mol is due to steric and torsional strain.
Smita Asthana @ St.Ann's
21
Butane’s Conformations
• gauche conformation is the
staggered conformation with methyl
groups 60° apart.
• Although it has no eclipsing interactions,
it is 3.8 kJ/mol higher in energy than the
anti conformation.
•
This is due to steric strain.
Smita Asthana @ St.Ann's
22
Butane’s Conformations
Smita Asthana @ St.Ann's
23
Smita Asthana @ St.Ann's
24
Chirality
The most common (but not the only) cause of chirality in organic
molecules is a tetrahedral atom, most commonly carbon, bonded to
four different groups
A carbon with four different groups bonded to it is called a chiral
center
all chiral centers are stereocenters, but not all stereocenters are
chiral centers
Enantiomers: stereoisomers that are nonsuperposable mirror images
refers to the relationship between pairs of objects
Smita Asthana @ St.Ann's
25
monochromatic light source
Optical Rotation and Polarimetry
optical polarizer - only allows "horizontal" light to pass through
optical polarizer - only allows "vertical" light to pass through
Chiral molecules will rotate polarized light:
randomly oriented light
chiral material
detector
monochromatic light source
optical polarizer - only allows "vertical" light to pass through
Smita Asthana @ St.Ann's
optical polarizer - only allows "horizontal" light to pass through
26
Optical Rotation and Polarimetry
randomly oriented light
detector
chiral material
maximum signal
monochromatic light source
optical polarizer
optical polarizer - only allows "vertical" light to pass through
The amount (in degrees) that a chiral material will rotate light is called the optical rotation. Different chiral
molecules will have optical rotations that vary in direction and size of the optical rotation. Enantiomers will
always have equal optical rotations but in opposite directions.
The optical purity of a substance can be measured by comparing the optical rotation of the sample to the
known optical rotation of a single entantiomer of that compound. Optical purity is usually reported in percent
entantiomeric excess (%ee).
sample rotation
%ee =
single enantiomer rotation
X 100
Enantiomeric excess is the % of the sample that is non-racemic. For example, 80% ee means that there is
90% of one enantiomer and 10% of the other.Smita Asthana @ St.Ann's
27
RACEMISATION
i) By the action of heat
ii) By treatment with chemical reagents
iii) Autoracemisation
RESOLUTION OF A RACEMIC MIXTURE
The separation of a racemic mixture into its two enantiomers is called resolution.
1. Mechanical method – Physical Difference -Pasteur separated the crystals of sodium
ammonium tartarate Na(NH4) C4H4O62H2O (Racemate). Since this method brings
experimental difficulties and has only limited application, it is only of historical
importance now.
2. Biochemical method –Penicilium glaucum (a mould) when grown in a dilute solution
of a racemate, it attacks the dextro from leaving behind the laevo form. Thus, the
laevo form remains practically unaffected. Although, it is a slow method yet it is of
wide application. Biochemcal method of separation has some disadvantages.
i) One form is always destroyed
ii) As dilute solutions re used, the amount of the second isomer left behind is very
small.
iii) It is difficult to select a micro-organism
which
Smita Asthana @
St.Ann'sattacks only one of the enantiomers
28
3. Chemical method (Resolution by salt formation)- separation of racemic
mixture of lactic acid. The racemic mixture of lactic acid is treated with 1brucine ( a base) when salts, called diastereomers are obtained.
dl lactic acid + I –brucine  I-brucine-d-lactate+I-brucine I-lactate.
The two lactates (diastereomers) are then separated by fractional
crystallisation and then each is separately treated with HCI to get the two
enantiomers of lactic acid.
I-brucine-d-lactate+HCI d-lactic acid +I-brucine. HCI
I-brucine-I-lactate+HCI  I-lactic acid +I-brucine. HCI
Smita Asthana @ St.Ann's
29
4. Kinetic method – Diffrence in rate of reaction - resolution of racemic
mandelic acid; I-menthol (an alcohol) reacts faster with d-mandelic acid than
with I-mandelic acid to form ester. Clearly, when, dI-mandelic acid (a racemic
mixture) is treated with limited quantity of I-menthol, the product formed is rich
in d-ester then I-ester.
5. Selective adsorption- One of the enantiomers is selectively adsorbed on the
surface of the adsorbent. The solution collected at the bottom of the
adsorbent column is richer in the other enantiomer. This results in the
separation of the racemic mixture.
Smita Asthana @ St.Ann's
30
Examples of Enantiomers
•
•
•
Molecules that have one carbon with 4 different substituents have a non-superimposable mirror image
Enantiomers = non-superimposable mirror image stereoisomers
Build molecular models to see this
Smita Asthana @ St.Ann's
31
Types of Stereoisomers
Stereo isomers contains same molecular formula, same bonding sequence, different spatial
orientation.
• Two types of stereoisomers:
– Enantiomers (Mirror image)
• two compounds that are nonsuperimposable mirror
images of each other
– Diastereomers (Non-Mirror Image)
• Two stereoisomers that are not mirror images of each
other
• Geometric isomers (cis-trans isomers) are one type of
diastereomer. Smita Asthana @ St.Ann's
32
Geometric (cis / trans) isomerism
In one, the two chlorine atoms are locked on opposite sides of the double bond. This is known as the
trans isomer. (trans : from latin meaning "across" - as in transatlantic).
In the other, the two chlorine atoms are locked on the same side of the double bond. This is know as
the cis isomer. (cis : from latin meaning "on this side")
Smita Asthana @ St.Ann's
33
E-Z NOTATION
The simple convention of denoting the geometrical isomers by cis/trans descriptors is
not sufficient when there are more than two different substituents on a double bond.
To differentiate the stereochemistry in them, a new system of nomenclature known as
the E-Z notation method is to be adopted.
According to this method, if the groups with higher priorities are present on the
opposite sides of the double bond, that isomer is denoted by E.
Where E = Entgegen ( the German word for 'opposite')
However, if the groups with higher priorities are on the same side of the double bond,
that isomer is denoted by Z.
Where Z = Zusammen (the German word for 'together')
The letters E and Z are represented within parentheses and are separated from the rest
of the name with a hyphen.
Smita Asthana @ St.Ann's
34
E,Z Configuration
uses priority rules
higher priority groups - same side, Z
higher priority - opposite sides, E
higher
higher
C
lower
higher
lower
C
C
lower
C
lower
higher
E
Z
Smita Asthana @ St.Ann's
35
Step by step procedure to determine the E-Z configuration
The following procedure is to be adopted to denote the geometrical isomers by E & Z
descriptors.
1. First determine the higher priority group on each end of the double bond.
2. If the higher priority groups are on the opposite sides of double bond, the isomer is
denoted by the descriptor, E.
3. Otherwise if they are on the same side of double bond, the Z descriptor must be
used.
Smita Asthana @ St.Ann's
36
Cahn–Ingold–Prelog priority rules
Assignment of priorities
R/S and E/Z descriptors are assigned by using a system for ranking priority
of the groups attached to each stereocenter.
1. Compare the atomic number (Z) of the atoms directly attached to the
stereocenter; the group having the atom of higher atomic number
receives higher priority.
2. If there is a tie, we must consider the atoms at distance 2 from the
stereocenter—as a list is made for each group of the atoms bonded to the
one directly attached to the stereocenter. Each list is arranged in order of
decreasing atomic number. Then the lists are compared atom by atom; at the
earliest difference, the group containing the atom of higher atomic number
receives higher priority.
3 A double or triple bond atom is considered as 2 or 3 of each atom.
Smita Asthana @ St.Ann's
37
Smita Asthana @ St.Ann's
38