Isomerism

isomerism
Group 4:
Hoang Ngo Linh Chi BTWE08005
Cai Anh Duong BTIU08155
Nguyen Ngoc Minh Chau BTWE08004
Nguyen Tran Thuy Linh BTUN08089
Pham Thi Truc Linh BTIU08087
Tran Thi Anh Thuy. BTIU08114
VIET NAM NATIONAL UNIVERSITY HO CHI MINH CITY
INTERNATIONAL UNIVERSITY
SCHOOL OF BIOTECHNOLOGY
Course: Organic Chemistry
Lecturer: Hoang Le Son Ph.D
OUTLINE:
Definition
Isomer.
Constitutional isomers.
Stereochemistry
Stereoisomers.
Symmetry
Classification
Stereoisomers:
Chirality
Chiral and achiral
Stereogenic center
planes of symmetry
Number of stereogenic center
Enantiomers
Examples
Properties
Nomenclature: R-S convention
Priority rules
Cyclic Compounds
Diastereomers
Examples
Properties
Compare between enantiomers and diastereomers
Plane-Polarized Light

Ordinary light
Plane-polarized light
Rotation of Polarized Light
dextrorotatory.
levorotatory
Optically active
Optical purity
Resolution
Racemic mixture
Resolution
Fischer projections

References
Definition:
Isomer
Constitutional isomers
Stereoisomers
Stereochemistry
Configurational isomers
Symmetry
1. Isomer:
Same molecular formula.
Different compounds.
2. Constitutional isomers:
Same molecular formulas
Differ in connectivity.
3. Stereoisomers:

The same connectivity, same molecular formula, but are arranged differently in space.
Diastereomer and Enantiomer

4. Stereochemistry:
Study of the relative spatial arrangement of atoms within molecules.
An important branch is the study of chiral molecules.
6. Symmetry:
II. Classification :
Stereoisomers
Chirality or handedness

Chiral molecules
Chiral is used to describe an object that is non-superposable on its mirror image
Mirror image
Mirror image is a reflection in reverse
Mirror image may or may not be superimposable
A human hand is chiral object
Achiral molecule
Molecule that are superposable on their mirror image are achiral
Example
Stereogenic center
A carbon atom with four different atoms or group of atoms bonded to it : chiral carbon atom
In this case: stereogenic center is chiral center
Any location where an interchange of groups leads to a stereoisomers is stereogenic center
Plane of symmetry: is an imaginary plane that divide a molecule in half, such that one half is the mirror image of other half
A center of symmetry: is an imaginary point that any straight line extends to point on opposites surfaces at equal distances
How Many Stereoisomers Are Possible?
maximum number of stereoisomers
= 2n,

where n = number of stereocenters
(sterogenic carbons)
sometimes fewer
than this number
will exist
Enantiomers

Stereoisomers.
Non-superimposable mirror images of each other.
Always Chiral molecules.
Have no plane of symmetry.

Condition:
occur only with compounds whose molecules are chiral.

Examples:
Lactic acid


Properties:

Physical properties:
same physical properties : melting point, boiling point, color, hardness, density.
EXCEPT : they rotate the plane-polarized light by equal amounts but in opposite directions.

Enantiomers

Chemical properties:
have identical chemical properties.

Enantiomers

Enantiomers


Optical activity:
One enantiomer will rotate the light clockwise and the other will rotate the light counterclockwise by an equal amount.
This ability of enantiomers to rotate light is called optical activity.
Enantiomers

Naming: R-S convention

Priority rules (Cahn, Ingold, Prelog)
The higher the atomic number, the higher the priority


















Increasing Priority
Enantiomers


e) Naming: R-S convention

If priority cannot be assigned on the basis of the atoms bonded to the stereocenter  look to the next set of atoms.






Enantiomers


e) Naming: R-S convention

Atoms participating in a double or triple bond are considered to be bonded to an equivalent number of similar atoms by single bonds

Enantiomers


e) Naming: R-S convention

Atoms participating in a double or triple bond are considered to be bonded to an equivalent number of similar atoms by single bonds

Enantiomers

e) Naming: R-S convention
Locate the stereocenter
Assign a priority to each substituent from 1 (highest) to 4 (lowest)
Orient the molecule so that the group of lowest priority (4) is directed away from you
Read the three groups projecting toward you in order from highest (1) to lowest priority (3)
If reading is clockwise, configuration is R (from the Latin rectus). If it is counterclockwise, configuration is S (from the Latin sinister).
D-L System
Commonly used by biochemistry.
The D-L system is most frequently used for sugars and amino acids.
D-L System
D-L System
Diastereomers :

Stereoisomers.
Not mirror images.
Non-superimposable
Have plane of symmetry
Cis-trans isomers

Condition

Stereoiosmers that are not mirror images
Stereoiosmers that are not enantiomers







Cis-2 butene
Trans-2 butene
Examples

The cis versions: means “ same side”
The trans versions: means “opposide side”
41
enantiomers 1
enantiomers 2
diastereomers
S
R
R
S
S
S
R
R
mirror
2-Bromo-3-chlorobutane
42
meso
enantiomers
diastereomers
S
R
S
S
R
R
2,3-Dichlorobutane
Diastereomers :

Properties:
they have different physical properties 
they have different chemical properties
Example
 
Meso compound 
are achiral by virtue of a symmetry plane, but contain a stereogenic center.
Plane polarized light
Ordinary light: light vibrating in all planes perpendicular to its direction of propagation

Plane polarized light: is the vector sum of left and right circularly polarized light
Plane-Polarized Light Beam
50
.
unpolarized
beam
wavelength
l
polarized beam
Sine waves
are not aligned
in the same
plane.
NOT PLANE-POLARIZED
END
VIEW
SIDE
VIEW
Plane Polarized Light
Compounds that are optically active contain molecules that are chiral.
Polarimeter: a device for measuring the extent of rotation of plane polarized light
51
Polarimeter
52

sample molecule
Na vapor lamp
polarizer
analyzer
Optical Activity
Optical activity describes the phenomenon by which chiral molecules are observed to rotate polarized light in either a clockwise or counterclockwise direction.
Observed rotation: the number of degrees, , through which a compound rotates the plane of polarized light
Dextrorotatory (+): rotation of the plane of polarized light to the right
Levorotatory (-): rotation of the plane of polarized light to the left
53
Recrystallized tartaric acid
Two different kinds of crystals that were mirror images.
Each type of crystal rotated light in opposite directions
55
(+)-tartaric acid
(-)-tartaric acid
meso -tartaric acid
Tartaric Acid
Optical Activity
specific rotation:
Calculated in this way is a physical property of an optically active substance
56
a = observed rotation
c = concentration ( g/mL )
l = length of cell ( dm )
D = yellow light from sodium lamp
T = temperature ( Celsius )
Optical Activity
For a pair of enantiomers, the value of the specific rotation of each is the same, but opposite in sign
57
Optical Purity
Optical purity: a way of describing the composition of a mixture of enantiomers



Enantiomeric excess: the difference between the percentage of two enantiomers in a mixture


optical purity is numerically equal to enantiomeric excess, but is experimentally determined
60
Racemic mixture
Contains (R) and (S) enantiomers with equal amount.
optically inactive
Is often designated as being (+_)
Ex : the racemix mixture of (R)-2-butanol and (S)-2-butanol :
( +- )2-butanol
Racemic mixture
Resolution
the separation of a racemic mixture into its enantiomers
Methods :
Pasteur method
Resolution by enzymes
Chromatography using chiral media