Enantiomeric excess (ee).pptx

Enantiomeric excess (ee)

1. Specific Rotation

2. Chiral and Achiral Molecules Although everything has a mirror image, mirror images may or may not be superimposable .

3. Not superimposed ( enantiomers ) ( Chiral ) superimposed (identical) ( Achiral )

4. Enantiomers Enantiomers are chiral molecules that are mirror images of one another. Furthermore, the molecules are  non- superimposable  on one another. This means that the molecules cannot be placed on top of one another and give the same molecule. Enantiomers have identical chemical and physical properties ( M.p & B.P) except for their ability to rotate plane-polarized light  (+/−) by equal amounts but in opposite directions, this property of enantiomer is called (Optical Activity).

5. A chiral compound with plane polarized light

6. Chiral compound with plane polarized light

7. (+) charge meaning: ( right ) (clockwise) ( dextro - rotatory ) (-) charge meaning: ( left) (counter-clockwise) ( Levo-rotatory )

8. Enantiomeric state (inside polarimeter tube) Pure enantiomer Mixtures of  enantiomers Enantiomeric Excess & Optical purity

9. 1. Pure Enantiomers It is solution containing a single enantiomer and the other enantiomer is entirely absent. It may also called: Optically pure (or) enantiomerically pure. Example: ( R )-2-bromobutane is an  optically active  compound with a specific rotation of -23.1 o . 100 % R 100 % S

10. Quiz + 1.99 0.03 1 + 66.3

11. 2. Mixtures of  enantiomers It is solution containing equal amounts of both enantiomers . It is called a  racemic mixture.  Racemic mixtures are not chiral as they are not optically in active . This is a result of rotating the plane of the light by the two enantiomers to the same extent but opposite directions. The net rotation is simply canceled out.                                           

12. 3. Enantiomeric Excess &Optical purity It is solution containing un -equal amounts of both enantiomers . It is mixture contains for example: 80% ( R )- 2-bromobutane & 20 % ( S )-2-bromobutane. The sample is neither optically pure, nor is it a racemic mixture.

13. Enantiomeric Excess and Optical purity There is more of one enantiomer than the other, or, in other words one enantiomer (in this case the ( R )-2-bromobutane) is said to be in excess. In order to describe and quantify the sample, the term enantiomeric excess   ( ee )  is used. Enantiomeric excess tells us  how much more  of one enantiomer is present in the mixture.

14. Enantiomeric Excess and Optical purity In this example, the  ee   is determined by the difference of percentages of the two enantiomers :

15. Enantiomeric Excess can be measured by Of total un-equal solution

16. Quiz - 45 - 53 % 85 ( excess (S The reminder = 15 % where 7.5 % for (R) 7.5 % for (S) So, The Total where 7.5 % for (R) 92.5 % for (S)