Absolute Configuration and Alkene Isomerism Notes

The Nature of Mirror Images (Enantiomers) * In the study of stereochemistry, a molecule and its mirror image are expected to exhibit opposite absolute configurations at their chiral centers. * If a specific center is determined to be $S$ (sinister), its corresponding mirror image must be assigned as $R$ (rectus). This definitive relationship allows for the verification of stereochemical assignments.
Step-by-Step Assignment of $R$ and $S$ Configurations * The process begins by assigning primality (priority) to the substituents attached to a chiral center, typically ranked 1, 2, 3, and 4 based on atomic number. * Assignment of $S$ : When the sequence of priorities 1, 2, and 3 proceeds in a counter-clockwise direction, the configuration is definitively designated as $S$ . * Assignment of $R$ : When the sequence of priorities 1, 2, and 3 proceeds in a clockwise direction, the configuration is designated as $R$ .
Spatial Orientation and Priority Positioning * The orientation in which a molecule is drawn is critical for accurate assignment. * Hydrogen in Front: A specific caveat occurs when the lowest priority group, such as Hydrogen ( $H$ ), is positioned at the front of the molecule (oriented toward the viewer). In such instances, the perceived direction of the 1-2-3 sequence must be interpreted carefully, as it represents the reverse of the standard perspective (where the lowest priority group is in the back).

Definitions of Cis and Trans * Trans-2-Butene ( $\text{trans-2-butene}$ ): This configuration indicates that the primary substituents (in this case, methyl groups) are located on opposite sides of the carbon-carbon double bond axis. * Cis-2-Butene ( $\text{cis-2-butene}$ ): This configuration indicates that the substituents are located on the same side of the double bond.
Substitution Patterns on Vinylic Carbons * The carbons that form the double bond are referred to as the vinylic carbons (noted phonetically in the transcript as "vanillic"). * The cis/trans nomenclature is traditionally utilized when there is a single substituent on each of the two vinylic carbons.

The Constraint of Cis/Trans * Once an alkene possesses three or more substituents (e.g., three distinct groups attached to the double bond), the cis/trans system is no longer applicable or sufficient for clear identification. * In these more complex cases, the $E/Z$ system is employed to provide a definitive description of the spatial arrangement.
German Etymology and the $Z$ Configuration * The designation $Z$ is derived from the German word Zusammen, which translates to "together." * Mnemonic for the $Z$ Isomer: A common mnemonic used to remember this configuration is that the high-priority substituents are on the "zame side" (same side) of the double bond.