“Diene and a dienophile
In movement, concerted, beguile.
Results in compaction:
Route cyclohexenic in style.”
The penultimate mechanism-themed poem of NaPoWriMo 2022 was posted on 7 April 2022 and celebrated the Diels-Alder reaction, a well-known process in which two molecules combine to yield a single product.
Just as chemists are often keenly interested in reaction pathways via which molecules can alter their stereochemistry (the 3-D arrangement of atoms), they also celebrate processes by which new carbon-carbon bonds can be formed, and the latter occurs here. The Diels-Alder reaction also forms a particularly stable molecular shape, a six-membered ring. Both aspects are particularly useful in the key objectives of organic synthesis: making new molecules.
Thus, this particular reaction is another early one from the curriculum of Organic Chemistry. As with the past three posts, this post is intended to help summarize some of the most pertinent material for students learning the reaction. Likewise, as with the other organic mechanisms cited in this series, it is probably useful to include a diagram.
This drawing uses the convention of the skeletal structure for simplicity’s sake: that is, only the “carbon skeletons” of the molecules are shown. Each vertex or terminus represents a carbon atom bonded to a number of hydrogen atoms appropriate to achieve its desired number of four bonds total (so it can obey the octet rule). For instance, the “dienophile” above, ethene in this case, is depicted as a short set of parallel lines. In a chemist’s reading, this translates instantly to H2C=CH2. Each of the two carbon atoms must be bonded to two hydrogen atoms, along with participating in the double bond. Moreover, electron movement in the reactants (left-hand side of the reaction arrow) is depicted via red curved arrows. The new bonds formed by these electron movements are shown in red in the product (right-hand side of the reaction arrow).
Covalent bonds in organic molecules are represented with lines; a single line represents a single bond (also known as a sigma bond), and a double line represents a double bond, consisting of one sigma bond and one pi bond. As the diagram shows, the two reactants that participate in a Diels-Alder reaction are classified as a diene (a molecule with two double bonds) and a dienophile (a molecule that wants to react with a diene). Here, the simplest diene is 1,3-butadiene, and the simplest dienophile is ethene; they yield the simplest Diels-Alder product of cyclohexene.
As with the last few weeks, we have much build-up and context here, given all of the shorthand and jargon inherent in a chemical mechanism. Ideally, this background will help the next 280 words or so make more sense.
“Diene and a dienophile /
In movement, concerted, beguile…”
The Diels-Alder reaction involves the interaction of a diene and a dienophile. The mechanism is generally postulated to occur all at once (“movement concerted”), as shown with three red arrows of electron movement in the single reaction step. The electrons in each of the pi bonds here (a pi bond is often thought of as the “second” bond in a double bond) participate in what is called a cycloaddition. This means that two new bonds form between the diene and dienophile to yield a six-membered ring, as the single product takes on a hexagonal shape. Additionally, the electrons from a third pi bond shift their location.
“Diels-Alder reaction /
Results in compaction…”
The reaction is named for Otto Diels and Kurt Alder, who published their findings in 1928 and received the Nobel Prize in Chemistry in 1950. The reaction results in the “compaction” of the molecular geometry of interest: it is smaller, since two reactant molecules have reacted to form a single product molecule.
“Route cyclohexenic in style.”
The reaction shown above, with the simplest possible dienophile and diene, yields a molecule named cyclohexene. The name gives us several clues about its structure: “cyclo” (the molecule is cyclic; it is a ring); “hex” (six carbon atoms are involved); and “ene” (the structure includes a double bond).
This closing line was my favorite from the mechanism poems, as I appreciated the wordplay possible with “scenic route” and “cyclohexenic route.” I also find the “scenic route” title fitting for this post, given the extensive background, since that phrase is often a euphemistic shorthand used to explain that something will take much more time!