Stereochemistry: Isomers, Chirality, Enantiomers, and Diastereomers

Chapter 5: Stereochemistry

Overview

This chapter introduces the fundamental concepts of stereochemistry in organic chemistry, focusing on the classification of isomers, the nature of chirality, enantiomers, stereocenters, and diastereomers, as well as the nomenclature and properties associated with these species.

Types of Isomers

Definitions and Classification

• Isomers: Compounds with the same molecular formula but different arrangements of atoms.

• Constitutional isomers: Isomers that differ in the connectivity of their atoms.

• Stereoisomers: Isomers that have the same connectivity but differ in the spatial arrangement of their atoms.

• Configuration: The particular three-dimensional arrangement of atoms in a molecule.

Classification Flowchart:

Question	Type
Do the atoms have identical connectivity?	If NO: Constitutional isomers If YES: Stereoisomers
Are the stereoisomers non-superimposable mirror images?	If YES: Enantiomers If NO: Diastereomers

Chirality

Definition and Examples

• Chiral object: An object (or molecule) that is non-superimposable upon its mirror image. Example: human hands.

• Achiral object: An object that is superimposable upon its mirror image. Example: a pair of identical socks.

• The term chiral is derived from the Greek word cheir, meaning hand.

Example: The molecule CH2BrCl is chiral, as its mirror image cannot be superimposed by rotation.

Enantiomers & Stereocenters

Key Concepts

• Enantiomers: A pair of stereoisomers that are non-superimposable mirror images of each other.

• Stereogenic center (stereocenter): A tetrahedral (sp3) atom with four different substituents attached. In organic compounds, this is typically a carbon atom.

• Molecules with a single stereocenter are always chiral.

• Molecules with more than one stereocenter may or may not be chiral, depending on symmetry.

Example: CHBrClF has a stereocenter at the carbon atom, making it chiral.

A Few Caveats

Exceptions and Special Cases

• Molecules with multiple stereocenters may be achiral if they possess an internal mirror plane.

• A stereocenter does not have to be a carbon atom. Neutral amines with three different groups can be chiral, but may undergo rapid pyramidal inversion, making them achiral in practice.

• Some molecules are chiral even without stereocenters (e.g., certain allenes and biphenyls).

Example: 7-stereocenter molecule with a plane of symmetry is achiral.

Varying the Amount of Each Enantiomer

Enantiopure, Enantioenriched, and Racemic Mixtures

• Enantiopure sample: Contains only one enantiomer (e.g., biologically derived substances).

• Enantioenriched sample: Contains more of one enantiomer than the other.

• Racemic mixture: Contains equal amounts (50:50) of both enantiomers, denoted by the (±) symbol.

Type	Ratio
Enantiopure	0:100 D:L or 100:0 D:L
Enantioenriched	e.g., 70:30 D:L or 5:95 D:L
Racemic mixture	50:50 D:L

Example: (±)-ibuprofen is sold as a racemate, but only the S-enantiomer is bioactive in vivo.

Identifying Pairs of Enantiomers

Drawing and Recognizing Enantiomers

• A molecule with one stereogenic center is a chiral compound and exists as a pair of enantiomers.

• To draw the mirror image, invert a pair of substituents (e.g., switch H and Me).

Example: L-alanine and D-alanine are enantiomers; methionine and fluoxetine (Prozac) also exist as pairs of enantiomers.

Stereocenters in Cyclic Compounds

Assessing Chirality in Rings

• Cyclic organic molecules can be analyzed as flat polygons to assess stereochemistry.

• If substituents around the ring are identical on each side, the molecule has a plane of symmetry and is achiral.

• If substituents are different, a stereocenter may be present and the molecule can be chiral.

Example: Methylcyclopentane is achiral due to symmetry; methylcyclohexene is chiral due to different groups at C2/C3.

Application: Thalidomide was sold as a racemate, but only one enantiomer was effective as an anti-nausea drug, while the other caused birth defects (teratogenic).

*Additional info: These notes cover the introductory and foundational aspects of stereochemistry, including definitions, classification, and real-world pharmaceutical relevance. Further details on nomenclature, physical and chemical properties, and advanced stereochemical analysis are typically included in subsequent sections of the chapter.*