Chirality and Stereoisomers

Chirality and Chiral Molecules

• Objects may or may not be superimposable on their mirror images.

• Definition of chirality is crucial for understanding chiral molecules.

Definition of Terms

• Enantiomers: A specific set of chiral molecules.

• Stereoisomer: An umbrella term for isomers that differ in the orientation of atoms.

• Structural Isomers: Isomers that differ in connectivity (not stereoisomers).

Isomer Formation

• Structural Isomer Formation:

  • Achieved by disconnecting a group from one carbon atom and reconnecting it to another carbon atom.

• Stereoisomer Formation:

  • Formed by changing the orientation of groups around the same atom.

  • Example: Identical molecules become different through orientation changes around a carbon atom.

Key Concepts in Chirality

• Chiral Molecule Criteria:

  • A molecule must have four different groups bonded to the same carbon atom to be chiral.

• Mirror Images:

  • Two molecules can be mirror images, but if their configuration allows them to be superimposed, they are identical, not chiral.

Enantiomers Specifics

• Each enantiomer must have:

  • Four different groups attached to one carbon.

  • Must not be superimposable on each other.

• Real-world examples discussed indicate the significance of chirality in molecular structures.

Types of Stereoisomers

Geometric Isomers vs. Enantiomers

• Geometric Isomers: Involves double bonds, allowing orientation changes leading to stereoisomers.

• Enantiomers (as previously defined): Must contain a chiral carbon with four different groups.

Distinguishing Stereoisomers

• Cis and Trans Isomers: Used for certain stereoisomers that contain a chiral carbon.

• D and L Designation:

  • Used for stereoisomers to indicate configurations that reflect the rotation of polarized light.

  • D Isomers: Produce positive (clockwise) rotation.

  • L Isomers: Produce negative (counterclockwise) rotation.

Polarized Light

• Polarized light is light oscillating in one plane, as seen through polarized sunglasses.

• Polarized light reflection creates glare, which sunglasses filter.

Monosaccharides

• Definition of Monosaccharides: The simplest sugar structures that cannot be hydrolyzed into simpler sugars.

• Structural Features:

  • Polyhydroxy (multiple hydroxyl groups) and contains a carbonyl group.

Types of Monosaccharides

1. Aldose:

   • Contains an aldehyde functional group at the end of the carbon chain.

2. Ketose:

   • Contains a ketone functional group, not necessarily at the end of the carbon chain.

Drawing Conventions

Fischer Projection

• A method to depict monosaccharide structures where the chiral carbons are represented by intersecting lines.

• Important to understand three-dimensional aspects:

  • Vertical Lines: Indicate bonds going into the plane of the board.

  • Horizontal Lines: Bonds coming out of the plane of the board.

Identifying Chiral Carbons

• Not all carbons are chiral; for a carbon to be chiral, it must be attached to four different groups.

• Examples of achiral and chiral carbons in various molecules were illustrated.

Distinguishing Between Enantiomers and Diastereomers

• Enantiomers: Must have all corresponding groups identical across chiral carbons.

• Diastereomers: Results when at least one but not all groups are different on the chiral carbons.

• The number of chiral centers determines the potential for stereoisomers.

Sugars with Multiple Chiral Carbons

• Sugars generally contain multiple chiral centers.

• The comparison for enantiomers is made using pairs of chiral centers.

D and L Sugar Identification

• The designation of D and L sugars is determined by the position of the hydroxyl group (-OH) on the last chiral carbon of the chain.

  • Right side ---> D Sugar

  • Left side ---> L Sugar

Summary Points

• D and L sugars are enantiomers and will have the same molecular name but different configurations due to their stereochemistry.

• The names help clarify distinct biological functions and reactivity in biochemical processes.