Dear Diels & Alder,

Dear Diels & Alder,

Oh, Diels-Alder reaction, it was love at first sight. I saw your cute mechanism and knew it was meant to be. Ever since that day you have been my favorite organic chemistry reaction. The more I learned about you, the more fun you were to me. Here is why…



It is possible that many of you read that paragraph and it brought back some horrible memories of arrows, lines, hours of horrendous studying, and confusion. However, organic chemistry to me was an exciting course consisting of various chemical puzzles (see end of post for your chance to solve). Believe it or not, I really did fall in love and it is one reason why I spent the last six years since that glorious course becoming a physical organic chemist.

I thought it would be fun to do some organic chemistry basics. It is an undergraduate course required for many science majors and professions including medicine, biology, and engineering. I know, I know many people dread, hate, or fail the course. However, here is a fun introduction and Diels-Alder puzzles at the end for you problem solvers.

Organic chemistry examines carbon-based chemistry. Carbon, generally, makes four covalent bonds. Covalent bonds are when two atoms share a pair of electrons. Carbon, one of the sluttiest chemical elements, usually wants to share its valence electrons. Valence electrons are defined as electrons available to participate in chemical bonding.

The first barrier to understanding the complex subject is to comprehend the ‘line notation’ used to draw the compounds. Below I show the four-carbon compound, butane. Each of the vertices is a carbon that is fully saturated with hydrogen atoms. This is how the line notation translates to a fully drawn structure.

Multiple chemical bonds can also be drawn using line notation. Counting the carbon-carbon bonds allows you determine the number of hydrogens required to fully saturate the hydrocarbon.

With the basics aside, let’s take a closer look at the Diels-Alder reaction. If you have taken an organic chemistry course, then you are probably familiar with this reaction or have forgotten it amidst the many named reactions you were forced to remember. This reaction is a cycloaddition. A cycloaddition is where two components, in this case a diene and a dienophile, add together to form a cyclic compound. A diene has two carbon double bonds, in the case of the Diels-Alder reaction it has to be a conjugated diene (alternating double bonds). As the name might suggest, a dienophile ‘loves’ to react with a diene. However, the requirement of a dienophile is that it has one carbon-carbon double bond.

Electron movement is depicted by arrows in organic mechanisms. Electrons move from the arrow end to where the arrow is pointed.

The diene and dienophile undergo a reaction to form a six-membered ring. The Diels-Alder reaction simultaneously moves six electrons to form cyclohexene. Diels and Alder won the Nobel prize for this elegant reaction in 1950. Even today it is used in many synthetic reactions and in drug design, as it is one of the best ways to form the cyclohexene compound.

Puzzle Time! I am going to give the folks still interested two puzzles to work out on their own. The second will be more difficult than the first. (They are intended for an organic chemistry novice or someone who likes to think in 3D.)

Puzzle 1: Here are the reactants and product of a Diels-Alder reaction. Predict where the arrows should be drawn.

Puzzle 2: Here are the reactants of a Diels-Alder reaction. Predict the arrows and product. Use Puzzle 1 as a hint to predict the product.

The answers are easily found by googling. If requested in the comments, I will post the answers to the puzzles next week.


Jacqueline, a true Floridian, wandered up to the tundra of Athens, Georgia to receive her PhD in computational quantum chemistry. Returning to her roots, she is currently working as a postdoctoral researcher in Tampa in the field of computational biochemistry investigating the wonders of penicillin-like drugs. When she is not slaving over the computer, her varied interests include international travel, Brazilian jiu jitsu, kickboxing, fancy food, (American) football, and Belgian quadrupels. She is also the founder of, a football blog with an exclusive female writing staff. Check out her sports ramblings there or follow her on Twitter @jhargis9.

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  1. “Pages and pages of data; over 63,000 in all. And on the perimeter of each–”

    “Line symbols, registration marks but they don’t line up.”

    “Oh but they do–if you think like a Vegan.”

    1. Our conversation is where I came up with half of the idea. Hopefully the chemistry is clear enough (sometimes it is hard to go back to the beginning). Let me know if there are any questions.

  2. I just finished up my first organic chemistry course – and my first chem course at all in something like 8 years. I took it as a component of my PhD minor, which since I’m doing Bioinformatics I thought would be relevant.

    Oh my god I love organic chemistry and I want to do it ALL THE TIME.

  3. I, too, ADORED the organic chemistry classes I took a couple of years ago. They were challenging (O Chem 1 was the first class I received anything less than a straight A), but the puzzle aspect was wonderful. In the second semester I made myself a set of cards and played a sort of “O Chem Solitaire” to study, randomly making different chemicals and then trying to figure out how I could get the second chemical from the first using the reactions we learned in class.

  4. Ah, happy memories! Let’s see, going back 40 years here: (and let’s see if I can describe it in plan view)

    Q1 line up the ethylene between carbons 1 and 4 of the ring ;

    place the arrows as per your example except:

    the top arrow would point down into the page and the bottom arrow up out of the page

  5. Q2 I guess ditto with the ethylene bridging carbons 1 and 3 of the ?cyclopentadiene

    (I swear I have not cheated here, this is just off the top of my head)

    Some neurones jingling here -is the product a turpene??

  6. As a student I was quite good at this stuff but never used it IRL, instead went into Clinical Biochemistry and am living out my days as a Protein and Enzyme electrophoresis “expert”.

    I shit you not, that’s what they say, but seriously I think “What, me?!” – the real experts all left in disgust during the purges of the mid nineties. Thanks to the influence of Thatcher and Reagan.

  7. This is a wonderful reaction. I took a course last semester in computational chemistry where we calculated the properties of the ‘Puzzle 2’ reaction in Gaussian. We looked at the HOMO LUMO interactions; IR spectra; activation barriers and reaction enthalpies.

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