Chemistry
Session
Stereochemistry – 1
Session Objectives
1.
Structural isomerism: chain, positional,
functional, ring-chain, isomerism, metamerism
, tautomerism Homologous series
2.
Stereoisomerism: conformational, optical and
geometrical isomerism
Stereoisomerism
Isomers
same molecular formula
Constitutional Isomers
Different nature/sequence
of bonds
Conformational Isomers
Differ by rotation about a
single bond
Enantiomers
Non-superposable mirror
images
Stereoisomers
Different arrangement of
groups in space
Configurational Isomers
Interconversion requires breaking
bonds
Diastereoisomers
Not mirror images
Structural isomerism
Chain Isomerism: When two or more compounds have similar
molecular formula but different carbon skelton.
Functional Isomerism:
Having the same molecular formula but different functional groups.
For example, CH3 CH2 OH &
CH3OCH3
Positional Isomerism: When two or more compounds differ in the
position of substituent atom and group on the carbon skelton.
Metamerism: Arises due to unequal distribution of alkyl groups on
either side of the functional group in the molecule.
CH3 CH2 O CH2 CH3
Ethoxyethane
CH3 O CH2 CH2 CH3
1 - Methoxypropane
Conformational
Carbon atoms connected through  bonds in alkanes can undergo
rotation leading to conformational isomers.
Conformations of ethane
H
H
H
H
H
Staggered
H
H H
H H
H
H
Eclipsed
Newmann and Sawhorse projection
CH3
HH
HH
H
H
Newman projection
H
H3C
CH3
H
H
H
CH3
CH3
3
H
H
CH3
Eclipsed
H
H
H CH
H
CH3
Staggered
Sawhorse projection
Interconversions
Fischer
Sawhorse
H
CO2H
H
OH
H
OH
CO2H

H 3C
OH
CO2H
OH
CO2H
Interconversions
H
Br
H
CH3
CH3
Br
CH3

H
Br
Br
H
CH3
Interconversions
CHO
B
H
H
H
H
Cl
Br
Cl
F
CH2OH
H
H
Cl
CH2OH
Br
CHO
CHO
CH2OH
Interconversions
COH
2
CO2H
H
OH
CH3

H
HO
CH3
Interconversions
OH
OH
Br
Ph
CH3

Ph
Br
CH3
Conformational Isomers
Results from free rotation about C-C single bond.
Conformers differ in energy through
i.
Torsional strain: through repulsive interaction between bonding
electrons in adjacent -bonds.
ii. 'Steric strain’ or ‘van der Waals’ strain – through space interaction
between filled orbitals on groups attached to adjacent atoms.
Ethane
H
H H
H H
H
H
H
H
H
‘sawhorse’
‘Newman’
‘staggered’
H
H
H
H
H
H
H
H
H
HH
H
‘sawhorse’
HH
‘Newman’
‘eclipsed’
Conformations of Cycloalkanes
H
H H
H
H
H
Chair form
H
H
H H
H
H
H
H
H
H
H
H
H
H
H
H
Boat form
H
H
Geometrical isomerism
H
H
C
C
C
H
25
C
H
2
5
Cis-isomer
C
H
3
H
C
C
H
3
C
C
H
25
Trans-isomer
E-Z configuration
When all the groups attached to the carbon carbon double bond are
different, cis trans nomenclature is not possible.
We give the priority to the groups on each carbon atom on the bases
of atomic number.
Now, we see whether the high priority groups are at same side or not.
For example:
1
1
Cl
Br
C
H
2
C
1
2
CH3
Br
C
CH3
2
Higher priority substituents
on same side. Z-isomer.
C
H
Cl
2
1
Higher priority substituents
on opposite side. E-isomer.
Geometrical isomerism
Number of geometrical isomers =
2x x = No. of double bonds
For compounds with two different terminal groups
Example
H3C –CH= CH–CH= CH–CH= CH–C2H5
No. of geometrical isomers =23=8
Geometrical isomerism
Number of geometrical isomers =2x 1  2y 1
For two identical terminal groups in alkene
y=
x
For even no. of double bonds
2
y=
x +1
For odd no. of double bonds
2
2,4-hexadiene
3 geometrical isomers
Optical isomerism
Required asymmetric carbon atom called ‘achiral’.
Optical isomerism
Same molecular formula but different behaviour with the plane
polarised light.
1. Presence of chiral C–atom
(for single asymmetric centre)
For more than one asymmetric centres –
1.
Non-superimposable mirror images
Enantiomers (d,l–pair)
Note: Superimposable mirror images
Meso compounds
Optical isomerism
2. No plane or centre of symmetry
CH3
CO2H
H
OH
H
Br
H
OH
Br
H
CO2H
Plane of symmetry (Meso)
CH3
No plane of symmetry
(Enantiomer)
Optical isomerism
CH3
O
H
||
NH
C
C
C
C
||
H
O
NH
CH3
Centre of symmetry results optical inactivity
Optical isomerism
1.
When the molecule is asymmetrical:
Number of enantiomers = 2m
Number of meso isomers = 0
2.
For symmetrical molecule with even number of asymmetric
centres :
Number of enantiomers = 2m–1
Number of
m 
 1
meso isomers=2 2 
Optical isomerism
3.
For symmetrical molecule and
odd number of asymmetric centres
Number of
 m1
 2 
m1


enantiomers = 2
 2
m1
Number of meso isomers = 2 2
Thank you