R and S Nomenclature for Stereoisomers.pptx

R and S Nomenclature for Stereoisomers

1. R and S Nomenclature for Stereoisomers

2. Stereochemistry is the study of the relative arrangement of atoms or groups in a molecule in three dimensional space. Stereo-isomers are molecules, which have the same chemical formula and bond connectivity but different relative arrangement in three-dimensional space. WHAT IS STEREOCHEMISTRY

3. CLASSIFICATION

4. Enantiomers: Nonsuperimposable mirror images, different molecules with different properties. STEREOISOMERS

5. Cis-1,2-dichlorocyclohexane is achiral because the molecule has an internal plane of symmetry. Both structures above can be superimposed. Trans-1,2-dichlorocyclohexane does not have a plane of symmetry so the images are nonsuperimposable and the molecule will have two enantiomers. Cis and Trans Cyclic Compounds

6. Different molecules (enantiomers) must have different names. Usually only one enantiomer will be biologically active. Configuration around the chiral carbon is specified with (R) and (S). (R) and (S) Nomenclature

7. Cahn–Ingold–Prelog Rules Assign a priority number to each group attached to the chiral carbon. Priority is assigned according to atomic number. The highest atomic number assigned is the highest priority #1. In case of ties, look at the next atoms along the chain. Double and triple bonds are treated like bonds to duplicate atoms. (R) and (S) Nomenclature

8. Once priorities have been assigned, the lowest priority group (#4) should be moved to the back if necessary. Atomic number : F > N > C > H Assign Priorities

9. Draw an arrow from Group 1 to Group 2 to Group 3 and back to Group 1. Ignore Group 4. Clockwise = (R) and Counterclockwise = (S) Assign Priorities Counterclockwise (S)

10. When rotating to put the lowest priority group in the back, keep one group in place and rotate the other three. Example Clockwise ( R )

11. Same boiling point, melting point, and density. Same refractive index. Rotate the plane of polarized light in the same magnitude, but in opposite directions. Different interaction with other chiral molecules: Active site of enzymes is selective for a specific enantiomer. Taste buds and scent receptors are also chiral. Enantiomers may have different smells. Properties of Enantiomers

12. Enantiomers rotate the plane of polarized light in opposite directions, but same number of degrees. Optical Activity

13. Polarimeter Clockwise Dextrorotatory (+) Counterclockwise Levorotatory (-) Not related to (R) and (S)

14. Observed rotation depends on the length of the cell and concentration, as well as the strength of optical activity, temperature, and wavelength of light. Where  (observed) is the rotation observed in the polarimeter, c is concentration in g/mL and l is length of sample cell in decimeters. Specific Rotation [  ] =  (observed) c  l

15. Equal quantities of d - and l - enantiomers. Notation: ( d,l ) or (  ) No optical activity. The mixture may have different boiling point (b. p.) and melting point (m. p.) from the enantiomers!. Racemic Mixtures

16. Equal quantities of d - and l - enantiomers. Notation: ( d,l ) or (  ) No optical activity. The mixture may have different boiling point (b. p.) and melting point (m. p.) from the enantiomers!. Racemic Mixtures

17. Bromine is the priority and the hydrogen is number four. Carbon “a” is connected to one oxygen and two hydrogens. Carbon “b” is connected to one oxygen and one hydrogen. However, because of the double bond, carbon “b” is treated as if it is connected to two oxygens. The same rule is applied for any other double or triple bond. So, when you see a double bond count it as two single bonds when you see a triple bond cut it as three single bonds. Double and triple bonds in the R and S configurations

18. The arrow goes clockwise, however, the absolute configuration is S, because the hydrogen is pointing towards us. Double and triple bonds in the R and S configurations

19. A meso compound is a stereoisomer with two or more chiral centres but no optical activity due to an internal plane of symmetry. Mesomers are compounds with zero net rotation of plane polarised light. In other words, mesomers are organic molecules that have two chiral carbons that are identical, resulting in zero net rotation. Meso compounds feature several chiral centres and are achiral substances. Despite having stereocenters, it is overlaid on its mirror counterpart and is optically inactive. Meso compounds

20. Meso compounds feature chiral centres (sp3 hybridised tetrahedral atoms bound to four distinct groups), yet they are achiral generally due to symmetry. As mentioned, a meso compound should include at least two chiral s p 3 hybridised atoms with the exception of the nitrogen atom of a tertiary amine and at least one internal plane that splits the molecule into two mirror copies. Meso compounds