“Clockwise or counter?
Enantiomeric:
We note designation as R or S, seen.
Note how substituents
Yield structure ‘handed’:
A configurational naming routine.”
After National Library Week drew to a close, the 14 April 2024 Twitter poem shifted back to a more directly scientific theme, with a focus on a specific concept used to understand the reactivity of organic molecules. Other posts on this site have engaged with the theme more broadly; this one examined a specific naming convention.
“Clockwise or counter? /
Enantiomeric: /
We note designation as R or S, seen…”
Part of learning organic chemistry involves understanding molecules in three-dimensional space. Stereochemistry is the general discussion of this understanding.
Within the broad topic of stereochemistry, enantiomeric molecules (enantiomers) are those that are non-superimposable mirror images of one another. They contain the same atoms connected in the same order, but the implications of their different three-dimensional arrangements can be immense.
The primary way students describe enantiomers is via the designation R or S, which is a shorthand for how attached atoms or groups are arranged as one sees them on a chiral center, when the least significant attached group is pointing away from the viewer.
If the other three groups are arranged in a clockwise configuration in terms of their “priority”, the chiral center is designated R (from the Latin term recto, for “right”). If the other three groups are arranged in a counterclockwise configuration, based on these rules, the chiral center is designated S (from the Latin term sinister, for “left”).
Other pairs of vocab terms are employed with enantiomers, as well, such as dextrorotatory and levorotatory. (Part of what is challenging in learning this material can be the lack of overlap across the distinguishing categories, which rely on different classification criteria.)
“Note how substituents /
Yield structure ‘handed’: /
A configurational naming routine.”
Adjusting to three-dimensional viewing can be a challenge. It typically helps to remind students of the parallels between paired enantiomeric structures and right and left hands. Just as a pair of hands would be a set of non-superimposable mirror images, so are two enantiomers.
Classifying molecules as R or S is a “configurational naming routine.”
Chemphasis 