Stereochemistry: Classification, Representation, and Chemical Reactions

Stereochemistry

Introduction to Stereochemistry

Stereochemistry is the branch of chemistry concerned with the study of isomerism in three-dimensional space. Isomers are compounds that have the same molecular formula but differ in the arrangement of atoms or groups in space.

Classification of Isomers

Types of Isomerism

• Constitutional (Structural) Isomerism: Isomers differ in the connectivity of atoms.

• Stereoisomerism: Isomers have the same connectivity but differ in spatial arrangement.

• Conformational Isomerism: Isomers differ by rotation around single bonds.

A. Constitutional Isomers

• Chain Isomers: Differ in the arrangement of the carbon skeleton. Example: n-butane vs. isobutane.

• Positional Isomers: Same functional group, different position. Example: 1-butanol vs. 2-butanol.

• Functional Group Isomers: Same molecular formula, different functional groups. Example: C2H6O: ethanol (alcohol) vs. dimethyl ether (ether).

B. Stereoisomerism

• Optical Isomerism: Isomers differ in their effect on plane-polarized light due to the presence of chiral centers.

• Geometrical Isomerism: Isomers differ due to restricted rotation, commonly seen in alkenes and cycloalkanes (cis/trans isomerism).

C. Conformational Isomerism

Conformational isomers (conformers) arise from rotation around single bonds. These structures are interconvertible and differ in energy. The most stable conformer is the one with the lowest energy.

Graphical Representation of Stereochemistry

Sawhorse Projection

Shows the side view of two adjacent atoms. Bonds are depicted as lines projecting in front of and behind the plane of the paper.

Newman Projection

Visualizes the molecule by looking straight down the bond between two atoms. The front atom is shown as a dot, the back atom as a circle.

Fischer Projection

Used for chiral molecules, especially carbohydrates and amino acids. The vertical lines represent bonds going away from the viewer, and horizontal lines represent bonds coming towards the viewer.

• Do not rotate the formula by 90° or 270° (causes inversion of configuration).

• Allowed to rotate by 180°.

• Allowed to switch three groups around the chiral center in one direction while fixing the fourth.

Optical Isomerism

Chirality and Optical Activity

• A compound is optically active if it rotates plane-polarized light.

• Requirements: One or more asymmetric (chiral) atoms, or a molecule lacking a plane of symmetry.

• Enantiomers: Non-superimposable mirror images; one rotates light to the right (dextrorotatory, +), the other to the left (levorotatory, -).

• Diastereomers: Stereoisomers that are not mirror images, often with more than one chiral center.

• Meso Compounds: Have multiple chiral centers but are optically inactive due to an internal plane of symmetry.

Racemic Mixture

A racemic mixture contains equal amounts of both enantiomers and is optically inactive due to external compensation.

Basic Terminology in Stereochemistry

• Enantiomers: Mirror-image isomers, non-superimposable.

• Diastereomers: Non-mirror-image stereoisomers.

• Meso Compounds: Achiral despite having chiral centers.

Rules for Fischer Projection Formula

• All carbons are written vertically with C1 at the top.

• Horizontal bonds project above the plane; vertical bonds project below.

• Do not rotate by 90° or 270°; allowed to rotate by 180°.

• Allowed to switch three groups around the chiral center in one direction while fixing the fourth.

Compounds with Multiple Asymmetric Carbon Atoms

Number of Optical Isomers

• For n chiral centers: Number of optical isomers = $2^n$

• Number of pairs of enantiomers = $2^{n-1}$

Example: 2,3,4-trihydroxybutanal has 3 chiral centers, so 8 optical isomers.

Nomenclature of Configuration

D and L System

Used in carbohydrate and amino acid chemistry. D and L isomers are based on the configuration of glyceraldehyde.

R and S System (Cahn-Ingold-Prelog Rules)

• Assign priorities to groups attached to the chiral center based on atomic number.

• Orient the molecule so the lowest priority group is away from you.

• If the sequence 1-2-3 is clockwise, configuration is R; if counterclockwise, it is S.

Resolution of Racemic Modification

Resolution is the process of separating enantiomers from a racemic mixture. This is important in nature and pharmaceuticals, as enantiomers can have different biological activities.

Geometrical Isomerism

Cis-Trans Isomerism

• Occurs in alkenes and cycloalkanes due to restricted rotation around double bonds.

• Cis isomer: Similar groups on the same side.

• Trans isomer: Similar groups on opposite sides.

E/Z Nomenclature

• Used when four groups attached to the double bond are different.

• Z (zusammen): Higher priority groups on the same side.

• E (entgegen): Higher priority groups on opposite sides.

Geometrical Isomerism Around C=N

Occurs in oximes and ketoximes due to restricted rotation around the C=N bond.

Stereochemistry of Chemical Reactions

Addition Reactions to C=C (Olefins)

• Syn-addition: Both groups add to the same side of the double bond. Examples: Catalytic hydrogenation (H2/Ni), oxidation with KMnO4 or OsO4.

• Anti-addition: Groups add to opposite sides of the double bond. Examples: Addition of halogens (Br2), hydroxyation by peracids.

Summary Table: Types of Isomerism

Type	Definition	Example
Constitutional Isomerism	Different connectivity of atoms	n-butane vs. isobutane
Stereoisomerism	Same connectivity, different spatial arrangement	Cis/trans-2-butene
Conformational Isomerism	Rotation around single bonds	Staggered vs. eclipsed ethane
Optical Isomerism	Non-superimposable mirror images	2-butanol enantiomers
Geometrical Isomerism	Restricted rotation (cis/trans)	Cis/trans-2-butene

Key Equations

• Number of optical isomers: $2^n$

• Number of pairs of enantiomers: $2^{n-1}$

Additional info: These notes cover the essential aspects of stereochemistry, including isomer classification, graphical representation, rules for projections, and the stereochemistry of chemical reactions, suitable for college-level Organic Chemistry.