Comprehensive Study Notes on Molecular Models II: Enantiomers and Diastereomers

Institutional and Administrative Context of the Molecular Models Laboratory

The laboratory exercise titled "Modelos Moleculares 2" (Molecular Models 2) was conducted within the Department of Natural Sciences at the University of Puerto Rico at Aguadilla (UPR Aguadilla), specifically under the Chemistry Area. The report pertains to course Quin 3462, section L11, supervised by Professor Carlos Nieves Marrero. The student investigators responsible for the clinical observations and structural modeling were Arashel Romero Pérez and Natalie Valentín Hernández. The documented work timeframe spans from April 6, 2026, to June 8, 2026.

Fundamental Principles of Chirality and Superposition

A primary focus of the study was the practical application of the superposition test to determine molecular chirality. Chirality is defined by the inability of a molecule to be superimposed on its mirror image. In one specific laboratory observation, it was noted that a pair of models could not be superimposed because, upon any attempt at alignment, two of the substituents consistently remained in opposite positions. This physical property confirms that the molecule is chiral. In contrast, the students observed another set of models where both physical representations occupied the exact same spatial configuration, leading to the conclusion that both models represent the same exact molecule, which is classified as an achiral molecule.

Comparative Definition and Examples of Enantiomers and Diastereomers

The laboratory material establishes a clear distinction between two types of stereoisomers: enantiomers and diastereomers. Enantiomers are defined as stereoisomers that are non-superimposable mirror images of one another. To illustrate this, the report provides the specific example of $(R)-2-bromobutane$ and $(S)-2-bromobutane$. These two molecules share the same connectivity but differ in their three-dimensional orientation such that they reflect each other perfectly across a plane but cannot be overlaid.

Diastereomers, conversely, are stereoisomers that are not mirror images of each other. The transcript notes that when attempting to superpose diastereomeric models, the groups do not coincide, and they fail to exhibit a mirror-image relationship. A specific example provided is the pair consisting of $(2R,3R)-2,3-dibromobutane$ and $(2R,3S)-2,3-dibromobutane$. This distinction is critical in organic chemistry, as diastereomers possess different physical properties (such as melting points and solubilities), whereas enantiomers share identical physical properties except for their interaction with plane-polarized light.

Optical Activity and the Implications of Symmetry

The presence of an internal plane of symmetry within a chemical structure significantly alters its physical properties. According to the laboratory findings, if a compound possesses a plane of symmetry, it is usually achiral. Such compounds do not deviate or rotate plane-polarized light, a state described as being optically inactive. This lack of optical rotation is a diagnostic physical property used to identify meso compounds or other achiral structures despite the presence of stereocenters.

Cahn-Ingold-Prelog (CIP) Priority System and Configuration Assignment

In assigning the absolute configuration (R or S) of stereocenters, the Cahn-Ingold-Prelog priority system must be applied. The laboratory documentation establishes a specific ascending order of priority for a set of organic functional groups. The groups, ranked from lowest priority (1) to highest priority (5), are as follows: $-CH_3 < -HC=CH_2 < -CHCl_2 < -NH_2 < -OH$. This ranking is determined by the atomic number of the atoms directly attached to the chiral center and the subsequent atoms in the chain in cases of ties.

Specific exercises were conducted on Structure G, Structure H, and Structure I. Upon performing configuration assignments, it was determined that for the stereoisomer illustrated in Figure 9.8, Carbon 1 holds an $S$ configuration while Carbon 2 holds an $R$ configuration. Furthermore, after analyzing the various spatial rotations and projections of Structures G, H, and I, it was concluded that all three structures (G, H, and I) represent the same identical enantiomer possessing the $S$ configuration.

Fischer Projections and Molecular Representation

The laboratory manual details the construction of Fischer projections for molecules in the $S$ configuration. Two specific compounds were used as primary examples for this structural representation:

For 2-bromopentane in the $S$ configuration, the projection is organized as follows: The methyl group ($-CH_3$) is positioned at the top vertical position. On the horizontal axis, the Hydrogen atom ($H$) is placed on the left side, and the Bromine atom ($Br$) is placed on the right side. The propyl chain ($-CH_2CH_2CH_3$) occupies the bottom vertical position.

For 3-hydroxy-1-butene in the $S$ configuration, the vinyl group ($-CH=CH_2$) is at the top. The horizontal axis contains the hydroxyl group ($OH$) on the left and the Hydrogen atom ($H$) on the right. The methyl group ($-CH_3$) is located at the bottom of the structure. These projections provide a standardized two-dimensional method to convey the three-dimensional stereochemistry of chiral molecules.