Science Poetry

Rotational Profiles

“Gauche interactions are 
Torsional infractions;
Likewise, groups eclipsing, so see option third…
Butane in profile
Rotates from erstwhile 
Higher-strain conformers: anti’s preferred.”

The 22 July 2019 Twitter poem examines a concept from organic chemistry, the energetic costs and benefits available to a molecule as it rotates through its conformations: specifically, the poem discusses the ways that the molecule butane can arrange itself in three-dimensional space.  This is a highly visual topic, so I’m intrigued to see what I can communicate in the 280 words below (with many links!).    

Gauche interactions are /
Torsional infractions; /
Likewise, groups eclipsing, so see option third…
The molecule butane (C4H10) consists of four carbon atoms in a line, covalently bonded.  Carbon atoms form four bonds, so butane’s terminal carbon atoms (first and last in line) each form three additional bonds to hydrogen atoms, while the middle two carbon atoms each form two additional bonds to hydrogen atoms.    

Rotation around butane’s central carbon-carbon bond leads to a variety of conformers.  Different conformers’ atoms interact with one another differently (their electron clouds repel, incurring energetic costs) through three-dimensional space.  Chemists have vocabulary to describe these torsional interactions: so named since interactions arise from the molecule’s torsion (twisting).    

The most intuitively named is the eclipsed conformer; if the methyl groups (the terminal carbon atoms, bonded to three hydrogen atoms apiece) are eclipsing, these groups line up with one another like the hands of a clock at noon. This is most easily seen through a chemistry model called a Newman projection.  Eclipsing incurs the highest possible energetic cost, or “torsional infraction,” in this molecule.  

Other energetic penalties arise in the gauche conformer, where the methyl groups are in something akin to a “2 p.m.” orientation. 

Butane in profile /
Rotates from erstwhile / 
Higher-strain conformers: anti’s preferred.
As portrayed in a rotational profile of butane, the anti conformer (an approximation of a clock’s “6 p.m.” orientation) keeps the methyl groups as far away from one another as possible and is the most energetically beneficial (“preferred”) conformation.  The anti conformer avoids torsional strain, although butane can still rotate into other conformations (“erstwhile higher-strain conformers”) as well.