14 5 Chiral Molecules An object whose mirror

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. General, Organic, and Biological Chemistry: Structures of Life, 5/e Karen C. Timberlake © 2016 Pearson Education, Inc.

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. General, Organic, and Biological Chemistry: Structures of Life, 5/e Karen C. Timberlake © 2016 Pearson Education, Inc.

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 General, Organic, and Biological Chemistry: Structures of Life, 5/e Karen C. Timberlake © 2016 Pearson Education, Inc.

Chiral or Achiral General, Organic, and Biological Chemistry: Structures of Life, 5/e Karen C.

Chiral Carbon Atoms • Carbon atoms are chiral if they have four different atoms or groups. • When stereoisomers cannot be superimposed, they are called enantiomers. General, Organic, and Biological Chemistry: Structures of Life, 5/e Karen C. Timberlake © 2016 Pearson Education, Inc.

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. General, Organic, and Biological Chemistry: Structures of Life, 5/e Karen C. Timberlake © 2016 Pearson Education, Inc.

Study Check For each of the following molecules, indicate whether the carbon in red is chiral or achiral. A. B. General, Organic, and Biological Chemistry: Structures of Life, 5/e Karen C. Timberlake © 2016 Pearson Education, Inc.

Solution For each of the following molecules, indicate whether the carbon in red is chiral or achiral. A. achiral B. chiral General, Organic, and Biological Chemistry: Structures of Life, 5/e Karen C. Timberlake © 2016 Pearson Education, Inc.

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 General, Organic, and Biological Chemistry: Structures of Life, 5/e Karen C. Timberlake © 2016 Pearson Education, Inc.

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. General, Organic, and Biological Chemistry: Structures of Life, 5/e Karen C. Timberlake © 2016 Pearson Education, Inc.

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 General, Organic, and Biological Chemistry: Structures of Life, 5/e Karen C. Timberlake © 2016 Pearson Education, Inc.

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. General, Organic, and Biological Chemistry: Structures of Life, 5/e Karen C. Timberlake © 2016 Pearson Education, Inc.

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. General, Organic, and Biological Chemistry: Structures of Life, 5/e Karen C. Timberlake © 2016 Pearson Education, Inc.

Chemistry Link to Health: Enantiomers in Biological Systems General, Organic, and Biological Chemistry: Structures

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. General, Organic, and Biological Chemistry: Structures of Life, 5/e Karen C. Timberlake © 2016 Pearson Education, Inc.

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. General, Organic, and Biological Chemistry: Structures of Life, 5/e Karen C. Timberlake © 2016 Pearson Education, Inc.

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

Solution Identify each as a chiral or an achiral compound. A. C. chiral B. chiral General, Organic, and Biological Chemistry: Structures of Life, 5/e Karen C. Timberlake achiral © 2016 Pearson Education, Inc.

Study Check Indicate whether each of the following pairs of Fischer projections can or cannot be superimposed: A. and B. and General, Organic, and Biological Chemistry: Structures of Life, 5/e Karen C. Timberlake © 2016 Pearson Education, Inc.

Solution Indicate whether each of the following pairs of Fischer projections can or cannot be superimposed: A. B. and cannot be superimposed and can be superimposed General, Organic, and Biological Chemistry: Structures of Life, 5/e Karen C. Timberlake © 2016 Pearson Education, Inc.

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. General, Organic, and Biological Chemistry: Structures of Life, 5/e Karen C. Timberlake © 2016 Pearson Education, Inc.