1. Lecture Nine : Stereochemistry
Biological Chemistry
First Year Organic Chemistry
Lecture Nine : Stereochemistry
Convener : Dr. Fawaz Aldabbagh

2. Stereoisomers differ only in the arrangement of their atoms in space
Isomers are different compounds that have the same molecular formula
Constitutional isomers are isomers that differ because their atoms are connected in a different order
Stereoisomers differ only in the arrangement of their atoms in space
Geometric Isomers

3. Enantiomers are stereoisomers whose molecules are nonsuperimposable mirror images of one another
Objects that are superimposable on their mirror images are said to be achiral
Involves a tetrahedral sp3 atom
Interchanging any two groups at a chiral centre (stereocentre) that bears four different groups converts one enantiomer into another

4. Screwdriver is achiral Socks are achiral Golf club is chiral
One structure can be superimposed on another
If any of the attached groups onto the tetrahedral atom are the same, the centre is achiral.
The ultimate way to test for molecular chirality is to construct models of the molecule and its mirror image and then determine whether they are superimposable
Screwdriver is achiral
Socks are achiral
Golf club is chiral
Gloves are chiral

5. Properties of Enantiomers
Enantiomers have identical melting points and boiling points
Enantiomers have identical solubilities in solvents
Enantiomers have identical spectra and refractive index
Enantiomers interact, and react with achiral molecules in the same manner
Enantiomers interact and react with other chiral molecules at different rates
Enantiomers rotate plane-polarised light by equal amounts but in opposite directions
Chiral molecules are optically active
Polarimeter is a devise used to measure the effect of plane-polarised light on an optically active compound

6. Nomenclature of Enantiomers (R-S) System
Enantiomers have opposite configuration at the chiral or stereocentre
Priority
1. Highest atomic number first; 2. If the first atom attached to the chiral centre has the same mass, then next set of atoms in the unassigned groups are examined; 3. We now rotate the formula (or model) so that the group with lowest priority is away from us; 4. Unsaturated substituents have greater priority than saturated substituents
The Absolute Configuration

8. No Correlation between the direction of rotation of plane polarised light and the absolute configuration of a molecule
Clockwise Rotation (+) – dextrorotatory
Anti-Clockwise Rotation (-) – levorotatory
An equimolar mixture of two enantiomers is called a Racemic Mixture
It is Optically Inactive

10. Draw the R- and S- forms of thaildomide
Biochemical effects arise when molecules interact with biomolecules such as enzymes or receptors. These are constructed of chiral building blocks called amino acids.
Therefore, the biomolecule only accepts molecules of a certain configuration or shape.
Thus, only one enantiomer will fit into the receptor or enzyme active site
S – induces a teratogenic response
R – induces a hypnotic or seductive response
Draw the R- and S- forms of thaildomide

11. Nobel Prize 2001 SN2 Professor William Knowles Professor Ryoji Noyori
Professor K. Barry Sharpless
                               
                               
                               
                               
For synthesis of optically active compounds – asymmetric synthesis
SN2