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Making a ring of six carbon atoms is one of the most common jobs in organic synthesis, and one of the cleanest ways to do it was found by a professor and his student in Kiel in the late 1920s. Otto Diels and Kurt Alder showed that two molecules can join end to end to form a ring in one step, often without even heating. This post covers who they were, what they found, and how to count the bonds in their reaction.
Who were Diels and Alder?
The Nobel Prize biography of Otto Diels records that he was born in Hamburg on 23 January 1876, studied chemistry in Berlin under Emil Fischer, and graduated in 1899. In 1916 he moved to the University of Kiel as professor and director of its chemistry institute, where he stayed until 1945. Early in his career he discovered carbon suboxide, and he introduced selenium as a reagent for removing hydrogen from ring compounds. He died on 7 March 1954.
Kurt Alder's biography says he was born in Königshütte, Upper Silesia, on 10 July 1902, studied in Berlin and then Kiel, and took his doctorate in 1926 with Diels as his supervisor. From 1936 he led a department at I. G. Farben in Leverkusen working on synthetic rubber, and in 1940 he became professor at Cologne. He died on 20 June 1958.
The 1950 Nobel Prize in Chemistry was shared equally between them "for their discovery and development of the diene synthesis".
What did they discover in 1928?
Their first paper, Synthesen in der hydroaromatischen Reihe ("Syntheses in the hydroaromatic series"), appeared in Liebigs Annalen der Chemie in 1928.
In his Nobel lecture, Alder explains the starting point. Chemists had seen cyclopentadiene and isoprene add to quinone, but nobody could prove the structure of the products, so several explanations competed. Diels and Alder set out to prove structures directly, varying one partner at a time. Moving to butadiene as the simplest diene, and from quinone to maleic acid and its anhydride, was, he said, the step that made the general pattern clear.
The presentation speech gave a plain description. The diene has a chain of four carbon atoms joined by two double bonds with a single bond in the middle. The partner, the dienophile, has a carbon-carbon double bond. The two join to form a ring of six atoms, and of the three double bonds only one remains. The speech notes that the reaction often runs "quite spontaneously, without external heating", and that it was used to build the carbon framework of camphor and to make raw materials for plastics.
Worked example: counting the Diels-Alder reaction
Take buta-1,3-diene with maleic anhydride.
Formulas: butadiene C₄H₆, maleic anhydride C₄H₂O₃. Product C₈H₈O₃. Every atom of both reactants ends up in the product, so nothing is wasted.
Molar masses (C 12.01, H 1.008, O 16.00): butadiene 54.09 g/mol; maleic anhydride 98.06 g/mol; product 152.15 g/mol, which equals the sum.
Bonds: three π bonds break (two in the diene, one in the dienophile). Two new C–C σ bonds form at the ends of the diene, and one new π bond forms between what were carbons 2 and 3 of the diene.
Quantities: 5.0 g of butadiene is 5.0 ÷ 54.09 = 0.0924 mol. It needs 0.0924 × 98.06 ≈ 9.06 g of maleic anhydride and can give at most 0.0924 × 152.15 ≈ 14.06 g of product.
Common mistakes:
Leaving the new double bond where the old ones were. It moves to the middle of the former diene.
Counting a five-membered ring. Number the atoms 1 to 6 around the ring to check.
Forgetting that the diene's two ends must point the same way for the ring to close.
Why does the reaction matter?
Diels and Alder showed that new carbon-carbon bonds can be made cleanly, in one step, with no reagents left over. The speech noted its role in studying natural products and in industry, and the Nobel facts page says it became a link in making plastics and synthetic rubber. For an earlier chemist thinking about how electrons move in butadiene, see our post on Robert Robinson and curly arrows. If addition to a single double bond is where you are now, start with our guide to alkene addition and Markovnikov's rule.
What can a student take from Diels and Alder's way of working?
Change one thing at a time. Their method was to vary one partner and prove each structure.
Prove, do not guess. Competing explanations ended only when structures were proved directly.
Look for the general rule. One reaction became a method once they saw the pattern.
If drawing the ring and moving the double bond still confuses you, the Fermi Concept Library covers every concept in physics, chemistry, biology, mathematics and computing, so you can revise conjugated dienes one idea at a time.
Try it now: draw the product of butadiene with ethene, number its six ring carbons, and mark the one remaining double bond.
