Stereochemistry: Chirality, Stereoisomers, and Enantiomer Separation

Chapter 3. Stereochemistry

1. Introduction to Stereochemistry

Stereochemistry is the study of the three-dimensional structure of molecules and how this structure influences their chemical and physical properties. Understanding stereochemistry is crucial in biology, biochemistry, and chemistry because the spatial arrangement of atoms affects molecular interactions, metabolism, and biological activity.

• Biology: The sense of smell can distinguish between stereoisomers (e.g., the different smells of lemon and orange are due to different stereoisomers).

• Biochemistry: Enzymes and metabolic pathways often act on only one stereoisomer (e.g., natural vs. unnatural alanine).

• Chemistry: Only one isomer of a drug may be biologically active (e.g., only the S-isomer of ibuprofen is active as an anti-inflammatory drug).

2. Chirality

Chirality refers to the property of a molecule that makes it non-superimposable on its mirror image. Objects like hands are chiral because the left and right hands are mirror images but not identical.

• Chiral molecules have non-superimposable mirror images.

• Enantiomers are pairs of chiral molecules that are mirror images of each other.

• An object is chiral if its mirror image is different from the original object.

3. Achiral Objects

Achiral objects or molecules are those whose mirror images can be superimposed. They possess a plane of symmetry.

• Example: A simple chair or a molecule with a plane of symmetry is achiral.

4. Chiral Carbon (Stereocenter)

A chiral carbon (also called an asymmetric carbon or stereocenter) is a carbon atom bonded to four different groups. Most organic molecules owe their chirality to the presence of such stereocenters.

• Any molecule that is chiral must have an enantiomer.

• Example: 2-bromobutane has a chiral center and exists as two enantiomers.

5. Chiral Carbon: Isomerism and Nomenclature

• Enantiomers have different spatial arrangements of the four groups attached to the chiral carbon.

• The two possible spatial arrangements are called configurations.

• Each asymmetric carbon atom is assigned a letter, (R) or (S), based on its 3-D configuration using the Cahn–Ingold–Prelog convention.

6. Cahn–Ingold–Prelog Convention

This system is used to assign the configuration (R or S) to chiral centers.

1. Assign a relative "priority" to each group bonded to the asymmetric carbon based on atomic number (higher atomic number = higher priority).

2. Orient the molecule so that the lowest priority group is pointing away from you.

3. Draw an arrow from the highest to the lowest priority group (excluding the lowest, which is in the back).

4. If the arrow goes clockwise, the configuration is R; if counterclockwise, it is S.

Priority order example: I > Br > Cl > S > F > O > N > C > H

7. Assignment of R and S Configurations

• Follow the priority rules for the four groups attached to the chiral center.

• Clockwise sequence = R; Counterclockwise = S.

8. Examples of Chiral Carbons

• Assign priorities based on atomic number (e.g., F > N > C > H).

• Rotate the molecule to place the lowest priority group in the back, then determine R or S configuration.

9. More Examples: Chiral Carbons in Molecules

• 1,3-dibromobutane: Has two chiral centers, each can be R or S.

• S-carvone: Responsible for the smell of caraway; R-carvone for spearmint.

10. Additional Examples

• Practice assigning R/S configurations to molecules with different substituents (e.g., I, Br, Cl, F, NH2).

11. Fischer Projections

Fischer projections are a way to represent 3-D molecules in 2-D, facilitating the comparison of stereoisomers and clarifying chemical differences.

• Chiral carbon is at the intersection of horizontal and vertical lines.

• Horizontal lines represent bonds coming out of the plane (toward the viewer).

• Vertical lines represent bonds going behind the plane (away from the viewer).

• Highest oxidized carbon is at the top.

• Rotation of 180° in the plane does not change the molecule; rotation of 90° is not allowed.

12. Assigning R/S in Fischer Projections

• If the lowest priority group is on a horizontal line (coming forward), assignment rules are reversed.

• Clockwise 1-2-3 is S; Counterclockwise 1-2-3 is R (when lowest priority is forward).

13. Rotation Rules for Fischer Projections

• Rotation of 180° is allowed and does not change the configuration.

• Rotation of 90° changes the orientation and is not allowed.

14. Multiple Chiral Centers

• The same rules for R/S assignment apply to each chiral center in a molecule with more than one chiral carbon.

15. Enantiomers and Diastereoisomers

• Enantiomers: Stereoisomers that are non-superimposable mirror images (all chiral centers have opposite configurations).

• Diastereoisomers: Stereoisomers that are not mirror images (some, but not all, chiral centers have opposite configurations).

• Fischer projections help distinguish between enantiomers and diastereoisomers.

16. Meso Compounds and Isomer Counting

• Meso compounds have internal mirror planes and are achiral despite having chiral centers.

• Maximum number of stereoisomers is $2^n$, where $n$ is the number of chiral centers.

17. Importance of Chirality

• In nature, stereoisomers can have drastically different biological effects (e.g., one isomer may be sweet, the other bitter).

• Drugs: Only one enantiomer may be active (e.g., S-ibuprofen is anti-inflammatory, R-ibuprofen is not active).

• Most drugs are sold as racemic mixtures (50:50 of R and S), but only one may be therapeutically useful.

18. Biological Discrimination and Optical Activity

• Enantiomers interact differently with biological systems (e.g., enzymes, receptors).

• Optical activity: Enantiomers rotate plane-polarized light in opposite directions.

• Specific rotation ($[\alpha]$) is measured using a polarimeter:

$[\alpha] = \frac{\alpha_{\text{observed}}}{c \cdot l}$ where $c$ is concentration (g/mL), $l$ is path length (dm), and $\alpha_{\text{observed}}$ is the observed rotation.

19. Polarimeter

• A polarimeter measures the optical rotation of chiral compounds.

• It consists of a light source, monochromator, polarizing filter, sample cell, and analyzing detector.

20. Separation of Enantiomers

Enantiomers have identical physical properties except for their interaction with plane-polarized light and chiral environments. Separation (resolution) is achieved by converting enantiomers into diastereomers, which have different physical properties and can be separated.

1. Create diastereomers by reacting the racemic mixture with a chiral reagent.

2. Separate the diastereomers by physical methods (e.g., crystallization, chromatography).

3. Regenerate the pure enantiomers from the separated diastereomers.

Example: Separation of a racemic mixture of 2-butanol or ibuprofen.

21. Summary

• Chirality: chiral carbon

• Stereoisomers: R or S

• Fischer projection

• Enantiomers

• Diastereoisomers

• Meso compounds

• Separation of enantiomers

22. Practice Problems

1. Draw the structure of (S)-1-bromo-1-chloropropane.

2. Mark the relationships (enantiomers, diastereomers, identical) between given structures.

3. Designate the R/S configuration for any chiral centers in the provided molecules.

4. Would a 50/50 mixture of two compounds be optically active? Briefly explain your answer.

Additional info:

• For more advanced study, consider the impact of chirality on drug design, asymmetric synthesis, and the use of chiral catalysts.