Science Poetry

Mainly in the Plane

“Classifying aromatic?  
Benzene ring, one emblematic
Structure seen; its systematic 
Look yields answer automatic.   
Tougher cases?  Check prolongéd:
‘4n plus two’ pi electrons; 
Conjugated; planar; cyclic.
(Faulty rhymes but fair mnemonic.)”

The 20 April 2022 poem was a checklist-style poem intended to highlight some of the many questions for discerning whether or not an organic molecule is aromatic: an adjective that, in a chemistry context, designates increased stability, compared to what would be predicted from its structure. (Chemists use the terms aromatic, antiaromatic, and non-aromatic as useful classifications of molecular structure.)  

Historically, it was seen that many compounds that had distinct aromas were some of the first to be classified as chemically aromatic; however, now, the two meanings are distinct.    

“Classifying aromatic? / 
Benzene ring, one emblematic /
Structure seen; its systematic /
Look yields answer automatic…”

The first lines ask whether the molecule of interest contains a benzene ring or not, given that benzene is the iconic example of an aromatic molecule.  A molecule could have both aromatic and non-aromatic (aliphatic) regions, but the benzene ring would always be aromatic.  

“Tougher cases?  Check prolongéd: /
‘4n plus two’ pi electrons; / 
Conjugated; planar; cyclic. /
(Faulty rhymes but fair mnemonic.)”

The latter lines include more complex discussions (i.e., a “prolonged check”).  Is the molecule conjugated (alternating single and double bonds, as drawn in an electron dot structure)?  Planar (flat)?  Cyclic (ring-shaped)? If the answer to all three is yes, how many pi electrons (those populating the delocalized “pi bond” system) does the molecule have?   Learning to count these electrons and apply Hückel’s rule is a traditional organic chemistry endeavor. 

If a planar, cyclic, conjugated molecule contains “4n + 2” pi electrons, where n is any number including zero (so that the formula yields two, six, ten, fourteen, etc.), then the molecule would be aromatic.  If the answer to the electron count is “4n” (four, eight, twelve, sixteen, etc.), then the molecule would be antiaromatic: decreased stability, compared to expectation from structure alone.  Benzene (six pi electrons) is aromatic; cyclobutadiene (four pi electrons) is antiaromatic.

This poem uses imperfect rhymes, and it rivals the mechanism-themed verses for density of jargon, but it was fun to try to create here a useful mnemonic.  The title borrows (and misspells) a lyric from Lerner and Loewe’s “The Rain in Spain,” noting a molecule characterized as aromatic overall would be “mainly in the plane,” among other criteria.