1. Isomers Are different compounds with the same molecular formula
Not mirror images
Mirror images
All stereocentres have
opposite configuration
Alkenes
Cyclic? see 
≥ 2 stereocentres
Only some have opposite configuration

2. Constitutional isomers
Constitutional (STRUCTURAL ISOMER) isomers
Are compounds have the same molecular formula, but their atoms are bonded in a different orders
Thus they have the SAME MOLECULAR FORMULA but DIFFERENT STRUCTURAL FORMULA
Character of structural isomer
Same molecular formula
Structural isomers have different physical properties
Different arrangement of atoms
Or different position of functional group
Or containing different functional groups
Constitutional isomers

3. Examples of Constitutional Isomers
1-Butene
2-Butene

4. pentane
2-methylbutane
2,2-dimethylpropane

5. The C6H14 Isomers hexane 2-Methylpentane 3-Methylpentane
CH3CH2CH2CH2CH2CH3
hexane
(CH3)2CHCH2CH2CH3
2-Methylpentane
(CH3CH2)2CHCH3
3-Methylpentane
(CH3)2CHCH(CH3)2
2,3-Dimethylbutane
(CH3)3CCH2CH3
2,2-Dimethylbutane 9

6. II-Stereoisomers: Optical isomers (chiral molecules) A- Enantiomeres
Stereoisomers are isomers that have the same molecular and structural formula but differ only in the arrangements of atoms in space
Types of Stereoisomers are:
Optical isomers (chiral molecules)
A- Enantiomeres
B- Diastereomeres
2. Cis/trans isomer ( achiral molecules)
II-Stereoisomers: 6

7. Geometric Isomerism (cis/trans)
If two compounds have the same molecular and structural formula, but differ in the spatial arrangement of different atoms or groups about a ring or about a double bond
Example of Ring Cis-Trans Isomers
Cis: groups on same side of ring
Trans: groups on opposite side of ring
trans-1,2-dibromocyclohexane
cis-1,2-dibromocyclohexane
If two identical groups occur on the same side of the double bond the compound is cis
If they are on opposite sides the compound is trans
Example of Alkene Cis-Trans Isomers
cis-1,2-dibromoethene
trans-1,2-dibromoethene

8. Chirality is a property present in an object that cannot be superimposed on its mirror image.
Chiral objects or chiral molecules do not have a plane of symmetry, i.e. they are asymmetric.
Chirality is typically seen in molecules that have a chiral or asymmetric carbon atom.

9. Optical isomers
Enantiomers are optically active, non-superimposable mirror image molecules
A molecule that has a non-superimposable mirror image is chiral.
Objects (and molecules) that are superposable on their mirror images are achiral

10. Asymmetric Carbon
Many chiral compounds contain an asymmetric carbon atom –
Chiral or asymmetric carbons have four different atoms or four different group attached to it.
It is a tetrahedral carbon atom

11. Optical activity
Enantiomers are optically active; that is, they rotate plane-polarized light.
One isomer of an enantiomeric pair rotates polarized light to the left (counterclockwise). The other isomer rotates polarized light to the right (clockwise). The degree of rotation is the same but in opposite directions.
Rotation of polarized light to the right is indicated by a (+) dextrorotatory
Rotation of polarized light to the left is indiated by a (-) levorotatory .

12. rotated polarized light
Optical Isomers
polarizing filter
plane polarized light
optically active sample in solution
Polarimeter: a device for measuring the extent of rotation of plane-polarized light
Dextrorotatory (d) = right rotation
Levorotatory (l) = left rotation
rotated polarized light

13. RACEMIC MIXTURE = equal amounts of two enantiomers; These mixtures are optically inactive.
Because two enantiomers rotate plane-polarized light to an equal extent but in opposite directions, the rotations cancel, and no rotation is observed.
RESOLUTION: Separation of a racemic mixture into its optically active forms (pair of enantiomers)

14. DIASTEREOMERS (non-enantiomeric stereoisomers )
1. Molecules have more than one stereogenic (chiral) center:
2. are stereoisomers that are not mirror images of each other.
3. Diastereomers must have opposite configurations at some (one or more) stereogenic centers, but the same configurations at other stereogenic centers
DIASTEREOMERS

15. Enantiomers & Diastereomers
For a molecule with 1 chiral center, 21 = 2 stereoisomers are possible
For a molecule with 2 chiral centers, a maximum of 22 = 4 stereoisomers are possible
For a molecule with n chiral centers, a maximum of 2n stereoisomers are possible

16. Meso Compounds
A meso compound is a stereoisomer that is superimposable on its mirror image due to an internal plane of symmetry (is an imaginary plane that bisects a molecule in such a way that the two halves of the molecule are mirror images of each other).
A meso compound is an optically inactive compound even through it possesses more than one chiral centre.
(2R,3S)-
(2R,3R)-
(2S,3S)-

20. The biological importance of chirality
1) All but one of the 20 amino acids that make up proteins are chiral and left-handed .
2) The molecules of natural sugars are almost all classified as right-handed (R configuration).
3) the active site of enzyme is chiral, and only one enantiomer of a chiral reactant fits it
Thus Enzymes are capable of distinguishing between stereoisomers
The biological importance of chirality