Organic Chemistry and Stereochemistry

Introduction to Organic Chemistry

Organic chemistry focuses on the structure, properties, and reactions of carbon-containing compounds. Carbon is a fundamental element in biochemistry, making up about 62% of the dry weight of the human body, which highlights its importance in life processes.

• Stereochemistry: Refers to the spatial 3D arrangement of atoms in molecules.

• Stereoisomers: Compounds with the same atomic composition but different spatial 3D arrangements.

Example: Malic acid and fumaric acid are stereoisomers, differing in the arrangement of their carboxyl groups.

Configurations & Conformations

Stereochemistry considers both a molecule's configuration and possible conformations.

• Configuration: A fixed 3D arrangement (only changed by breaking/reforming bonds).

• Conformation: Potentially flexible 3D arrangements due to rotation around single bonds.

• Chirality center: A carbon in a molecule bonded to four distinct chemical groups. A chiral center can have one of two possible configurations: R or S.

• Enantiomers: Stereoisomers that are non-superimposable mirror images of each other.

Example: Thalidomide exists as two enantiomers, one of which is therapeutic and the other teratogenic.

Chiral Configurations

• Enantiomers have opposite chiral configurations at all chiral centers.

• Chiral molecules can be labeled as R (rectus) or S (sinister) based on the Cahn-Ingold-Prelog priority rules.

Ethane Conformations

• Staggered conformation: Lower energy, more stable.

• Eclipsed conformation: Higher energy, less stable.

Resonance

Resonance Structures

Resonance is the delocalization of electrons within a molecule, which can have a stabilizing effect. Resonance structures are not actual transient states of the molecule; the 3D structure is the best representation.

• Resonance increases molecular stability by distributing electron density.

Example: The acetate ion has two resonance structures, with the negative charge delocalized over two oxygen atoms.

Practice Problems

Alkene Configuration

• Determine the configuration (E/Z) of given alkenes based on the priority of substituents.

Chirality Centers

• Identify chirality centers in a molecule.

• Determine the configuration (R/S) of each chirality center using the Cahn-Ingold-Prelog rules.

Fischer Projections

Introduction to Fischer Projections

Fischer Projections are a molecular drawing style used commonly to portray chiral compounds, especially carbohydrates and amino acids in biochemistry.

• Horizontal bonds are depicted as wedges (coming out of the page).

• Vertical bonds are depicted as dashes (going into the page).

Assigning R/S Configuration in Fischer Projections

• Assign priorities to the four groups attached to the chiral center.

• Orient the lowest priority group vertically (usually on top or bottom).

• Determine the direction (clockwise for R, counterclockwise for S).

Example: Assign the R/S configuration to each chirality center in a Fischer projection of an amino acid or carbohydrate.

Key Terms and Definitions

• Organic Chemistry: The study of carbon-containing compounds.

• Stereochemistry: The study of the spatial arrangement of atoms in molecules.

• Chirality Center: A carbon atom bonded to four different groups.

• Enantiomers: Non-superimposable mirror image molecules.

• Resonance: Delocalization of electrons in a molecule.

• Fischer Projection: A 2D representation of a 3D molecule, commonly used for sugars and amino acids.

Important Equations and Rules

• Cahn-Ingold-Prelog Priority Rules: Used to assign R/S configuration to chiral centers.

Table: Comparison of Stereoisomers

Additional info: Stereochemistry is foundational for understanding biomolecular function, drug design, and metabolic pathways in biochemistry. Fischer projections are especially important for carbohydrate and amino acid chemistry, which are central topics in biochemistry.