1. Chirality
Chirality - the Handedness of Molecules

2. Isomers
we concentrate on enantiomers and diastereomers

3. Enantiomers Enantiomers: nonsuperposable mirror images
as an example of a molecule that exists as a pair of enantiomers, consider 2-butanol

4. Enantiomers
one way to see that the mirror image of 2-butanol is not superposable on the original is to rotate the mirror image

5. Enantiomers
now try to fit one molecule on top of the other so that all groups and bonds match exactly
the original and mirror image are not superposable
they are different molecules
nonsuperposable mirror images are enantiomers

6. Enantiomers
Objects that are not superposable on their mirror images are chiral (from the Greek: cheir, hand)
they show handedness
The most common cause of enantiomerism in organic molecules is the presence of a carbon with four different groups bonded to it
a carbon with four different groups bonded to it is called a stereocenter

7. Enantiomers
If an object and its mirror image are superposable, they are identical and there is no possibility of enantiomerism
such an object is achiral (without chirality)
An achiral molecule, consider 2-propanol
notice that it has no stereocenter

8. Enantiomers
to see the relationship between the original and its mirror image, rotate the mirror image by 120°
after this rotation, we see that all atoms and bonds of the mirror image fit exactly on the original
the original and its mirror image are the same

9. Enantiomers To summarize
an object that is nonsuperposable on its mirror image is chiral (it shows handedness)
the most common cause of chirality among organic molecules is the presence of a carbon with four different groups bonded to it
we call a carbon with four different groups bonded to it a stereocenter
an object that is superposable on its mirror image is achiral (without chirality)
nonsuperposable mirror images are called enantiomers
enantiomers, like gloves, always come in pairs

10. Drawing Enantiomers
Following are four different representations for one of the enantiomers of 2-butanol
both (1) and (2) show all four groups bonded to the stereocenter and show the tetrahedral geometry
(3) is a more abbreviated line-angle formula; although we show the H here, we do not normally show them in line-angle formulas
(4) is the most abbreviated representation; you must remember that there is an H present on the stereocenter

11. Drawing Mirror Images on the left is one enantiomer of 2-butanol
on the right are two representations for its mirror image (in this case, its enantiomer)

12. The R,S System To assign an R or S configuration
assign a priority from 1 (highest) to 4 (lowest) to each group on the stereocenter;
orient the stereocenter so that the group of lowest priority is facing away from you
read the three groups projecting toward you in order from (1) to (3)
if reading the groups is clockwise, the configuration is R (Latin, rectus, straight, correct)
if reading the groups is counterclockwise, the configuration is S (Latin: sinister, left)

13. The R,S System
The first step in assigning an R or S configuration to a stereocenter is to arrange the groups on the stereocenter in order of priority
priority is based on atomic number
the higher the atomic number, the higher the priority

15. The R,S System
Example: assign priorities to the groups in each set

16. The R,S System
Example: assign priorities to the groups in each set

17. The R,S System
example: assign an R or S configuration to each stereocenter

18. The R,S System
example: assign an R or S configuration to each stereocenter

20. The R,S System
Because enantiomers are different compounds, each must have a different name
here are the enantiomers of the over-the-counter drug ibuprofen
the R,S system is a way to distinguish between enantiomers without having to draw them and point to one or the other

21. The R,S System
returning to our original three-dimensional drawings of the enantiomers of ibuprofen

22. Chirality in Biomolecules
a molecule and its enantiomer or one of its diastereomers elicit different physiological responses
as we have seen, (S)-ibuprofen is active as a pain and fever reliever, while its R enantiomer is inactive
the S enantiomer of naproxen is the active pain reliever, but its R enantiomer is a liver toxin!

23. Stereocenters
A molecule with n stereocenters has a maximum number of 2n stereoisomers
a molecule with one stereocenter, 21 = 2 stereoisomers (enantiomers) are possible
for a molecule with two stereocenters, a maximum of 22 = 4 stereoisomers (two pair of enantiomers)
for a molecule with three stereocenters, a maximum of 23 = 8 stereoisomers (four pairs of enantiomers) is possible
and so forth

24. Fischer Projection Formulas
Fischer Projection: show configuration of chiral molecules in two-dimensional representation
Vertical bonds are directed away
Horizontal bonds are directed toward you

25. Enantiomers & Diastereomers
2,3,4-Trihydroxybutanal
two stereocenters; 22 = 4 stereoisomers exist

26. Two Stereocenters 2,3,4-trihydroxybutanal
diastereomers: stereoisomers that are not mirror images
(a) and (c), for example, are diastereomers

27. Meso Compounds
Meso compound: an achiral compound possessing two or more stereocenters
tartaric acid
two stereocenters; 2n = 4, but only three stereoisomers exist

28. Stereoisomers
example: mark all stereocenters in each molecule and tell how many stereoisomers are possible for each

29. Stereoisomers
example: mark all stereocenters in each molecule and tell how many stereoisomers are possible for each
solution:

30. Three Or More Stereocenters
how many stereocenters are present in the molecule on the left?
how many stereoisomers are possible?
one of the possible stereoisomers is menthol
assign an R or S configuration to each stereocenter in menthol

31. Three Or More Stereocenters* R * *
23 = 8 possible stereoisomers

32. Stereoisomers
The 2n rule applies equally well to molecules with three or more stereocenters