1. The 3-D Shape of Molecules
CHAPTER 15
The 3-D Shape of Molecules
General, Organic, & Biological Chemistry
Janice Gorzynski Smith

2. CHAPTER 15: The 3-D Shape of Molecules
Learning Objectives:
Understand the difference between Chiral and Achiral molecules
Identify chiral centers
Draw the enantiomers & diastereomers of a chiral molecule (using dashes & wedges to show geometry)
Draw fischer projections of chiral molecules
Explain the relevance of chiral centers in Biochemistry
Smith. General Organic & Biological Chemistry 2nd Ed.

3. ISOMERS Stereoisomers Constitutional Isomers Spatial Isomers
Same chemical formula
Stereoisomers
Spatial Isomers
Constitutional
Isomers
Structural Isomers
Different connectivity between atoms
Different spatial arrangement of atoms but with the same connectivity
Enantiomers
Diastereomers
Mirror images of a chiral molecule that are not superimposable
All stereoisomers that are not mirror images of one another
Use modeling kit!
Cis/Trans
Isomers
Conformational
Isomers
Rotation around C-C single bonds R R R R

4. Left hands and right hands are mirror images of each other.
Stereo-
isomers
Chiral & Achiral
Left hands and right hands are mirror images of
each other.
A molecule that is not superimposable on its mirror
image is chiral.
Smith. General Organic & Biological Chemistry 2nd Ed.

5. Two socks from a pair are mirror images that are superimposable.
Stereo-
isomers
Chiral & Achiral
Two socks from a pair are mirror images that are
superimposable.
A molecule that is superimposable on its mirror
image is achiral.
Smith. General Organic & Biological Chemistry 2nd Ed.

6. To test whether a molecule is chiral or achiral:
Stereo-
isomers
Chiral & Achiral
To test whether a molecule is chiral or achiral:
Draw the molecule in 3 dimensions.
Draw its mirror image.
Try to align all bonds and atoms.
To superimpose a molecule and its mirror image
you can perform any rotation but you cannot
break bonds.
Smith. General Organic & Biological Chemistry 2nd Ed.

7. Stereo- isomers Chiral & Achiral For CH2BrCl: Rotate the molecule to
align bonds:
ACHIRAL
Smith. General Organic & Biological Chemistry 2nd Ed.

8. Stereo- isomers Chiral & Achiral For CHBrClF: CHIRAL
Smith. General Organic & Biological Chemistry 2nd Ed.

9. Identify the chiral centers in…
Stereo-
isomers
Chiral & Achiral
Identify the chiral centers in…
cholesterol
glucose
cysteine

10. Chiral Molecules Enantiomers
Many drugs are chiral, and often they must interact with a chiral receptor to be effective.
Thalidomide was an anti-nausea drug sold to pregnant in the 60’s as a mixture with its enantiomer. It was found that the enantiomer interacted with the body differently, and caused catastrophic birth defects.
Smith. General Organic & Biological Chemistry 2nd Ed.

11. Ibuprofen is an active anti-inflammatory agent
Chiral Molecules
Enantiomers
Ibuprofen is an active anti-inflammatory agent
whose enantiomer is inactive, and it is sold as a
racemic mixture.
Smith. General Organic & Biological Chemistry 2nd Ed.

12. Two different enantiomers can interact differently
Chiral Molecules
Enantiomers
Two different enantiomers can interact differently
with smell receptors in the brain and have
different perceived odors.
Smith. General Organic & Biological Chemistry 2nd Ed.

13. Stereo-
isomers
Multiple Chiral Centers
Draw the Enantiomer of the amino acid Glutamate

14. Multiple Chiral Centers
Chiral Molecules
Multiple Chiral Centers
Smith. General Organic & Biological Chemistry 2nd Ed.

15. Multiple Chiral Centers
Stereo-
isomers
Multiple Chiral Centers
chemwiki.ucdavis.edu

16. A Fischer Projection takes a 3D tetrahedral shape, and re-draws it:
Chiral Molecules
Fischer Projections
A Fischer Projection takes a 3D tetrahedral shape,
and re-draws it:
Smith. General Organic & Biological Chemistry 2nd Ed.

17. We can draw the cross and Fischer projection
Chiral Molecules
Fischer Projections
We can draw the cross and Fischer projection
for both enantiomers of 2-butanol:
Smith. General Organic & Biological Chemistry 2nd Ed.

18. Fischer Projections with multiple chiral centers:
Chiral Molecules
Fischer Projections
Fischer Projections with multiple chiral centers:
A&C; A&D; B&C; B&D are all Diastereomers
Smith. General Organic & Biological Chemistry 2nd Ed.

19. Chiral Molecules
Fischer Projections
Draw a Fischer Projections of the carbohydrate erythrose’s stereoisomers (all 4).

20. They differ in how they interact with plane-polarized light.
Chiral Molecules
Optical Properties
Two enantiomers have identical physical properties (melting pt., boiling pt., solubility, etc.)
They differ in how they interact with plane-polarized light.
Smith. General Organic & Biological Chemistry 2nd Ed.

21. Chiral Molecules Optical Properties
An achiral compound does not change the direction of plane-polarized light and is optically inactive.
A chiral compound rotates the plane of polarized light through an angle α and is optically active.
Smith. General Organic & Biological Chemistry 2nd Ed.

22. Chiral Molecules Optical Properties
Assign Priority to the substituents bonded to the chiral carbon 1-highest, 4-lowest, to determine R or S rotation
1.  The higher the atomic number of the immediate substituent atom, the higher the priority.
For example, H–  <  C–  <  N–  <  O–  <  Cl–.
2.  If two substituents have the same immediate substituent atom, evaluate atoms progressively further away from the chiral center until a difference is found.
For example, CH3–  <  C2H5–  <  ClCH2–  <  BrCH2–  <  CH3O–.
3.  If double or triple bonded groups are encountered as substituents, they are treated as an equivalent set of single-bonded atoms.
For example, C2H5–  <  CH2=CH–  <  HC≡C–

23. Optical Properties: R & S
Chiral Molecules
Optical Properties: R & S
Enantiomers have opposite optical properties: ie, R,R vs S, S rotation of light
One of the chiral centers of diastereomers will rotate light in the same way: ie, R,R vs R,S
chemwiki.ucdavis.edu

24. Chiral Molecules
Optical Properties: L & D
L is “left handed”
(equivalent to “S”)
D is “right handed”
(equivalent to “R”)