“A need for alkene can be fill-ed /
Through synthetic effort most skill-ed: /
Reaction approaching, /
On ketone encroaching, /
Employing the chem of the ylide.”
The 24 April 2024 limerick summarized the Wittig reaction, a well-known process in organic chemistry. The structure of the poem allowed for pronunciation hints about one of the novel vocabulary terms involved.
“A need for alkene can be fill-ed /
Through synthetic effort most skill-ed…”
The Wittig reaction is named for German chemist Georg Wittig (1897-1987), who won the Nobel Prize in Chemistry in 1979 for developing important reagents in organic synthesis. The Wittig reaction is illustrated here. It is famous as a synthetic pathway to alkenes: compounds with carbon-to-carbon double bonds (C=C).
“Reaction approaching, /
On ketone encroaching, /
Employing the chem of the ylide.”
The Wittig reaction occurs between a compound containing a carbonyl group (an aldehyde or a ketone) and a compound called a triphenylphosphine ylide.
The ylide has both a positive and negative charge within one structure; it “encroaches” on the ketone (or aldehyde) due to this unusual reactivity. The reaction ultimately yields an alkene and a side product of triphenylphosphine oxide.
“Ylide” seems one of organic chemistry’s more confounding instances of jargon, at first glance; it is pronounced in such a way as to rhyme with “fill-ed” and “skill-ed,” as the poem ideally suggests.
***
A brief postscript to this particular essay:
Speaking of “ylides,” I was intrigued as a student with the etymology of the unusual word, but I never tracked it down. When this poem came to mind last year, I was pleased to have a new inspiration… after only fifteen-or-so years of having the question running in the background!
I’ve learned in my time preparing and teaching courses that the Nobel Prize lectures, compiled online, are unique chances to read narrative accounts of research from scientists, rather than the technical accounts found in journal articles.
I suspected that Wittig’s lecture might include his inspiration for the novel term, and I was glad to confirm this. In his Nobel lecture, Wittig writes: “We gave the name N-ylides to this new class of substances since the bonding of the carbon to the neighboring nitrogen is homopolar (yl) and ionic (ide) at the same time.” The phosphorus ylides from the Wittig reaction demonstrate similar chemistry, with aspects of both molecular and ionic bonding patterns.
(Wittig’s lecture title highlights this unusual chemical species and others in a creative way: “From Diyls to Ylides to My Idyll.”)
Chemphasis 