Organic Molecules: Chirality and Stereochemistry Notes

Learning Objectives:

• Understand chirality and its importance
• Understand enantiomers
• Identify R and S isomers
• Understand diastereomers

Chirality:

• A compound not superimposable on its mirror image is chiral.
• Most chiral compounds contain an asymmetric carbon atom (stereocentre) with four different groups attached.
• Enantiomers possess identical chemical and physical properties but interact differently with chiral environments (e.g., biological systems).

Example of Chirality:

• Enantiomers of 2-butanol:
• Structure: [ CH3-CH(OH)-CH2-CH_3 ]
• Mirror images are not superimposable.

Importance of Enantiomers in Nature:

• Nature can distinguish between enantiomers (e.g., pharmaceutical effects).
• Thalidomide case: one enantiomer caused birth defects.
• Ibuprofen: inactive form converted to the active form in the body.

Optical Rotation:

• Enantiomers have equal but opposite specific rotations.
• A racemate (1:1 mixture of enantiomers) has no net optical activity.

Absolute Configuration of Asymmetric Carbon:

• Assign priorities to substituents based on atomic number.
• Use a bonding visual from the lowest priority group to assign R (clockwise) or S (anticlockwise) configuration.

Diastereomers:

• Stereoisomers that are not enantiomers; occur with compounds having more than one chiral center.
• Do not have similar physical properties.
• Example: 4 stereoisomers can be derived from 2 chiral centers (2^n where n is number of stereocenters).

Questions to Consider:

• Boiling point of enantiomers is identical (e.g., (+)-2-butanol and (-)-2-butanol).
• Identify chiral carbons in given structures (e.g., lactic acid).