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The Diels-Alder reaction, like many reactions in organic chemistry, is named after the chemists who discovered it—in this case Otto Paul Hermann Diels (1876–1954) and Kurt Alder (1902–1958). Kurt Alder was actually Diels’ student at the University of Kiel, and Alder was awarded a PhD in 1926. Alder and his advisor Diels jointly received the Nobel Prize in Chemistry in 1950.

What’s a cycloaddition reaction?

In the most general terms, a cycloaddition reaction is one that forms a ring from multiple π-bonds. To illustrate some of the concepts and ways that these reactions are classified, let’s look at the cycloaddition reaction shown below. This reaction, known as the Diels-Alder reaction, is between a conjugated diene (double alkene) and a another alkene that likes to react with dienes (hence dienophile, or diene lover). It is classified as a [4 + 2] cycloaddition, referring to the number of atoms directly involved in bond-forming events.

(There’s a second system for classifying cycloaddition reactions that uses the number of π-electrons involved in ring formation, but let’s just leave that aside.)

What’s a pericyclic reaction?

A cycloaddition reaction is actually a type of pericyclic reaction, but the term “pericyclic” includes other types of reactions. The textbook definition of pericyclic is a reaction whose transition state has a cyclic structure (i.e., the electrons are flowing in a closed loop). In addition to cycloaddition reactions (which exchange two π-bonds for two σ-bonds, or vice versa), pericyclic reactions include sigmatropic reactions, electrocyclic reactions, and cheletropic reactions (and a few others which we’ll ignore).

What’s a sigmatropic reaction?

While a cycloaddition reaction uses two π-bonds to make two new σ-bonds, a sigmatropic reaction takes one σ-bond and makes…well…one σ-bond. One of the most well-known sigmatropic rearrangements is known as the Cope rearrangement. Using the system we talked about earlier, this reaction is classified as a [3,3]-sigmatropic reaction (the methyl substituent is not directly involved, so we don’t count it).

What’s an electrocyclic reaction?

Okay, so a cycloaddition reaction was 2π2σ, and we just saw that sigmatropic reactions are 1σ1σ, so what about a pericyclic reaction that is 1π1σ? That’s an electrocyclic reaction. Like most pericylic reactions, electrocyclic reactions can either make or break a σ-bond. If a σ-bond is made in the process, it’s an electrocyclic ring-closing reaction. If the σ-bond is consumed in the process, the process is named an electrocyclic ring-opening reaction, but it’s the same process.

One noteworthy example of an electrocyclic ring-closing reaction is the Nazarov cyclization, which converts divinyl ketones into cyclopentenes, usually in the presence of an acid.

What’s a cheletropic reaction?

Finally, there’s a second type of reaction that involves two π-bonds being transformed into two σ-bonds. The difference here is that two bonds are formed (or broken) at a single atom, while in cycloaddition reactions, only one bond is made at each reactive atom. An example involving SO₂ is shown below (surprisingly this reaction isn’t named after anyone!). In this reaction both σ-bonds are made (or broken) at the sulfur atom. Compare this with the Diels-Alder reaction mentioned above, where the two new σ-bonds are connected to the two different ends of the dieneophile.

What is a tautomerization reaction?

A tautomerization reaction interconverts two constitutional isomers of an organic molecule, referred to as tautomers, which differ only by the positions of hydrogen atoms. The scheme below shows how the reaction typically takes place at the ketone functional group of an organic molecule. The ketone form of the molecule is commonly referred to as the “keto” form of the molecule, while the alcohol form is commonly referred to as the “enol” form of the molecule.