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.

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.

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

Chiral or Achiral

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

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.

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

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

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

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.

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

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.

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.

Chemistry Link to Health: Enantiomers in Biological Systems

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.

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.

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

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

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

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

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

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.

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

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

Concept Map