Science Poetry

Mere Images

“These chem terms are far from generic:
First noted with acids, tartaric;   
The handed behaviors 
That explain endeavors: 
R/S forms, enantiomeric.”

The 28 April 2022 Twitter limerick described some context and vocabulary related to enantiomers: compounds that are identical in connectivity but in terms of their three-dimensional structures are non-superimposable mirror images. 

“These chem terms are far from generic: /
First noted with acids, tartaric…”

The first few lines note that this poem will introduce some precise vocabulary, “far from generic,” for stereochemical properties of chemical species.  

The properties described here were observed by Louis Pasteur in 1847 with crystal samples derived from tartaric acid.  Using a magnifying glass, a pair of tweezers, and (presumably) no small amount of patience, he separated the pertinent crystals into two piles based on their optical properties: one set of crystals rotated plane-polarized light in a clockwise direction, while the other rotated plane-polarized light in a counter-clockwise direction.  

This difference in optical activity is the only difference in physical properties for enantiomers: in other physical properties (like melting or boiling points), they are identical.  

“The handed behaviors /
That explain endeavors: / 
R/S forms, enantiomeric.”

Stereochemistry involves the three-dimensional (3-D) arrangement of atoms in a molecule, rather than the molecule’s composition or connectivity.  Enantiomers, specifically, demonstrate “handed behaviors”; they are non-superimposable mirror images of one another, just as hands are.  This 3-D-specific information can be delineated in a variety of ways; a common shorthand is called R or S notation.  

As stated above, enantiomers have identical physical properties aside from their optical activity; however, their chemical reactivities differ in chiral environments.  In those cases, the R enantiomer of a molecule would react differently than the S enantiomer: “the handed behaviors… explain endeavors,” or chemical reactivities.  

The depictions of these “mirror-image” compounds can seem simple and often can look identical at first glance (resulting in the post’s title!).  However, once discerned, the “handed” differences in their three-dimensional compositions would have significant implications for how such molecules behave.