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Stereoisomerism Chapter 26

Stereoisomerism Chapter 26

Hein * Best * Pattison * Arena

Colleen KelleyChemistry DepartmentPima Community College

© John Wiley and Sons, Inc.

Version 1.0

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Chapter Outline26.1 Review of Isomerism

26.2 Plane-Polarized Light

26.3 Optical Activity

26.4 Projection Formulas

26.5 Enantiomers

26.6 Racemic mixtures

26.7 Diastereomers and Meso Compounds

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Review of IsomerismReview of Isomerism

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• Isomerism is the phenomenon of two or more compounds having the same number and kind of atoms.

• In structural isomerism, the difference between isomers is due to different structural arrangements of the atoms that form the molecules.– e.g. butane and isobutane

• In stereoisomerism, the isomers have the same structural formula, but differ in spatial arrangement of atoms.

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• Compounds that have the same structural formulas but differ in their spatial arrangement are called stereoisomers.

• There are two types of stereoisomers:1. Cis-trans or geometric isomers2. Optical isomers

– Have the ability to rotate plane-polarized light.

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Plane-Polarized LightPlane-Polarized Light

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• Plane-polarized light is light that is vibrating only in one plane. – Ordinary (unpolarized) light consists of

electromagnetic waves vibrating in all directions (planes) perpendicular to the direction in which it is traveling.

– When ordinary light passes through a polarizer, it emerges vibrating in only one plane and is called plane-polarized light.

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Figure 26.1 (a) Diagram of ordinary light vibrating in all possible directions (planes), and (b) diagram of plane-polarized light vibrating in a single plane. The beam of light is coming toward the viewer.

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Figure 26.2 Two Polaroid filters (top) with axes at right angles. In (a), light passes through both filters and emerges polarized. In (b) the polarized light that emerges from one filter is blocked and does not pass through the second filter, which is at right angles to the first. With no light emerging, the filters appear black.

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Figure 26.3 Schematic diagram of a polarimeter.

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Specific Rotation• The specific rotation, [], of a compound

is the number of degrees that polarized light would be rotated by passing through 1 decimeter of a solution of the substance at a concentration of 1 g/mL.

[] = Observed rotation in degrees(length of sample tube in dm)(sample concentration g/mL)

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Optical ActivityOptical Activity

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Optical Activity• Many naturally occurring substances are

able to rotate the plane of polarized light.– optically active

• When plane-polarized light passes through an optically active substance, the plane of polarized light is rotated.– right (clockwise) dextrarotatory

– left (counterclockwise) levorotatory

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Asymmetry• The tetrahedral arrangement of single

bonds around a carbon atom makes asymmetry (lack of symmetry) possible in organic molecules.

• When four different atoms or functional groups are bonded to a carbon atom, the molecule formed is asymmetric, and the carbon atom is called an asymmetric carbon atom.

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Figure 26.4 Three-dimensional representation of an asymmetric carbon atom with four different groups bonded to it. The carbon atom is a sphere. Bonds to A and B project from the sphere toward the observer. Bonds to C and D project from the sphere away from the observer.

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Chiral• A molecule that is not superimposable on

its mirror image is said to be chiral.– e.g. your left and right hands

• An asymmetric carbon atom is also called a chiral carbon atom or chiral center.

• A molecule cannot be chiral if it has a plane of symmetry.

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Projection FormulasProjection FormulasProjection FormulasProjection Formulas

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Projection Formulas• Molecules of a compound that contain one

chiral carbon atom occur in two optically active isomeric forms.

• This is because the four different groups bonded to the chiral carbon atom can be oriented in space in two different configurations.

• It is important to understand how to represent such isomers on paper.

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Figure 26.7 Methods of representing three-dimensional formulas of a compound that contains one chiral carbon atom. All three structures represent the same molecule (lactic acid). Formulas II and III are called projection formulas.

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EnantiomersEnantiomers

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Enantiomers

• Chiral molecules that are nonsuperimposable mirror images of each other are stereoisomers and are called enantiomers.

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Figure 26.8 Mirror images of lactic acid. Each isomer is the mirror reflection of the other. (-)-Lactic acid rotates plane-polarized light to the left, and (+)-lactic acid rotates plane-polarized light to the right. They are enantiomers of one another.

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• Many, but not all, molecules that contain a chiral carbon are chiral.

• To decide whether a molecule is chiral and has an enantiomer, make models of the molecule and of its mirror image, and see if they are superimposable.– If they are not superimposable, then the

molecule is chiral and has an enantiomer.

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Enantiomers

• Same chemical properties

• Same physical properties (except for optical rotation)

• They rotate plane-polarized light the same number of degrees, but in opposite directions.

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Enantiomers• Differ in their biochemical properties.

– (+)-Glucose (“blood sugar”) is used for metabolic energy whereas (-)-glucose is not.

– (+)-Lactic acid is produced by reactions occurring in muscle tissue, and (-)-lactic acid is produced by the lactic acid bacteria in the souring of milk.

– thalidomide

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Key Factors of Enantiomers and Optical Isomerism

1. A carbon atom that has four different groups bonded to it is called an asymmetric or chiral carbon atom.

2. A compound with one chiral carbon atom can exist in two isomeric forms called enantiomers.

3. Enantiomers are nonsuperimposable mirror-image isomers.

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Key Factors of Enantiomers and Optical Isomerism

4. Enantiomers are optically active; that is, they rotate plane-polarized light.

5. One isomer of an enantiomeric pair rotates polarized light to the left (counterclockwise). The other isomer rotates polarized light to the right (clockwise). The degree of rotation is the same but in opposite directions.

6. Rotation of polarized light to the right is indicated by a (+) and to the left by a (-).

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Racemic MixturesRacemic Mixtures

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Racemic Mixtures• A mixture containing equal amounts of a

pair of enantiomers is known as a racemic mixture.

• Such a mixture is optically inactive and shows no rotation of polarized light when tested in a polarimeter.– Each enantiomer rotates the plane of

polarized light by the same amount, but in opposite directions. Thus, the rotation by each isomer is canceled.

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Figure 26.9 Some examples of common chiral drugs. Many pharmaceuticals are synthesized as racemic mixtures.

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Diastereomers and Diastereomers and Meso CompoundsMeso Compounds

Diastereomers and Diastereomers and Meso CompoundsMeso Compounds

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The number of stereoisomers increases as the number of chiral carbon atoms increases.

2n = maximum number of stereoisomers for a given chiral compound

n = number of chiral carbon atoms in a molecule

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Diastereomers

• When there are two or more chiral carbon atoms in a molecule, diastereomers can exist.

• Stereoisomers that are not enantiomers (not mirror images of each other) are called diastereomers.

H

CH3

Br

ClH

CH3

H

CH3

Br

HCl

CH3

diastereomers

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Meso Compounds• Stereoisomers that contain chiral carbon atoms

and are superimposable on their own mirror images are called meso compounds, or meso structures.

• All meso compounds are optically inactive.

H

COOH

OH

OHH

COOH

plane of symmetry

meso-tartaric acid

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