1. 14.5 Chiral Molecules
An object whose mirror image is identical to the original and can be superimposed on it is achiral. Objects such as hands that have nonsuperimposable mirror images are chiral.
Learning Goal Identify chiral and achiral carbon atoms in an organic molecule.

2. Stereoisomers
Molecules are structural isomers when they have the same molecular formula but different bonding arrangements. In stereoisomers, the atoms are bonded in the same sequence but differ in the way they are arranged in space.

3. Chirality
Left and right hands are chiral because they have mirror images that cannot be superimposed on each other.
When the mirror images cannot be completely matched, they are nonsuperimposable.
Core Chemistry Skill Identifying Chiral Molecules

4. Chiral or Achiral

5. Chiral Carbon Atoms
Carbon atoms are chiral if they have four different atoms or groups.
When stereoisomers cannot be superimposed, they are called enantiomers.

6. Achiral Carbon Atoms
If a molecule with two or more identical atoms bonded to the same atom is rotated, the atoms can be superimposed and the mirror images represent the same structure.

7. Study Check
For each of the following molecules, indicate whether the carbon in red is chiral or achiral. A. B.

8. Solution
For each of the following molecules, indicate whether the carbon in red is chiral or achiral. A. achiral B. chiral

9. Fischer Projections A Fischer projection
is a two-dimensional representation of a molecule.
places the most oxidized group at the top.
uses vertical lines for bonds that go back.
uses horizontal lines for bonds that go forward.
The intersections of vertical and horizontal lines represent a carbon atom that is usually chiral.
Core Chemistry Skill Identifying d- and l- Fischer Projections

10. Drawing Fischer Projections
For glyceraldehyde, the only chiral carbon is the middle carbon. In the Fischer projection, the carbonyl group, which is the most highly oxidized group, is drawn at the top.
In a Fischer
projection, the chiral carbon atom is at the center, with horizontal lines for bonds that project forward and vertical lines for bonds that point away.

11. Drawing Fischer Projections
Fischer projections can also be drawn for compounds that have two or more chiral carbons:
In the mirror images of erythrose, both of the carbon atoms at the intersections are chiral.
The mirror image for L-erythrose is drawn by reversing the positions of all the — H and — OH groups on the horizontal lines of D-erythrose

12. Drawing Fischer Projections
The designation as a D or an L stereoisomer of a Fischer projection is determined by the position of the — OH group attached to the chiral carbon farthest from the carbonyl group.

13. Chemistry Link to Health: Enantiomers in Biological Systems
Molecules in nature also have mirror images, and often one stereoisomer has a different biological effect than the other one.
One enantiomer of carvone, produced by the spearmint plant, smells and tastes like spearmint, whereas its mirror image, produced by the caraway plant, has the odor and taste of caraway in rye bread.
Our senses of smell and taste are responsive to the chirality of molecules.

14. Chemistry Link to Health: Enantiomers in Biological Systems

15. Chemistry Link to Health: Enantiomers in Biological Systems
Molecules in nature also have mirror images, and often one stereoisomer has a different biological effect than the other one.
One enantiomer of carvone, produced by the spearmint plant, smells and tastes like spearmint, whereas its mirror image, produced by the caraway plant, has the odor and taste of caraway in rye bread.
Our senses of smell and taste are responsive to the chirality of molecules.

16. Chemistry Link to Health: Enantiomers in Biological Systems
A substance used to treat Parkinson’s disease is L-dopa, which is converted to dopamine in the brain, where it raises the serotonin level.
The D-dopa enantiomer is not effective for the treatment of Parkinson’s disease.

17. Study Check
Identify each as a chiral or an achiral compound. A. C. B.

18. Solution Identify each as a chiral or an achiral compound. A. C. B.
chiral chiral achiral

19. Study Check
Indicate whether each of the following pairs of Fischer projections can or cannot be superimposed: A. and B. and

20. Solution
Indicate whether each of the following pairs of Fischer projections can or cannot be superimposed: A. and B. and
cannot be superimposed
can be superimposed

21. Study Check
Identify the following Fischer Projection as a d or an l enantiomer:

22. Solution
Identify the following Fischer Projection as a d or an l enantiomer:
The –OH group is
on the right,
it is a d- enantiomer.

23. Study Check
Draw the Fischer projection for the following dash-wedge structure:

24. Solution
Draw the Fischer projection for the following dash-wedge structure:

25. Concept Map