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So I was doing an exercise from the book and i saw this question: Norbornadiene + hexachloro-1,3-cyclopentadiene gives aldrin and I had to determine the structure of aldrin. The reagents of course undergoes a Diels-Alder reaction. In the end i came to conclusion that there are 2 options, either 2 hexachloro-1,3-cyclopentadienes get connected to norbornadiene or just one, I checked the answers and it was stated exclusively that only molecule gets connected, and the other option wasn't even mentioned.

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So i think there aren't any things making connecting second hexachloro-1,3-cyclopentadiene to aldrin impossible, the double bond is still relatively electron abundant, the chlorines are far away and not withdrawing the electrons from double bond, there isnt any steric strain and stuff like that, so why doesn't norbornadiene react with 2 hexachloro-1,3-cyclopentadienes but with only 1?

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  • $\begingroup$ Have you tried building a model? Based on intuition, I'd have trouble believing that you can squeeze a second reaction onto the norbonyl skeleton, but I could be wrong... $\endgroup$
    – Zhe
    Commented Mar 23 at 21:02
  • $\begingroup$ @Zhe I haven't as I don't have such resources, but when i saw the 3d structure of aldrin (both done by me and those looked up on internet) it looked like one more reaction could happen, but I might be wrong $\endgroup$
    – Roxtron
    Commented Mar 24 at 1:13

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It is true that norbornadienie and hexachlorocyclopentadiene can give two products: aldrin (1) and a product of another Diels-Alder cycloaddition between 1 and hexachlorocyclopentadiene (2). There are scientific articles showing syntheses of both of these compounds. And published procedures for synthesis of 1 and 2 look as if there is no significant reduction in reactivity in Diels-Alder reaction between 1 and hexachlorocyclopentadiene when compared to norbornadiene. In short, article 1 describes synthesis of compound 1 at conditions at which norbornadiene to hexachlorocycopentadienie are in 3 : 1 ratio, i. e. there is significant excess of norbornadiene - probably to prevent formation of 2. When it comes to synthesis of 2 the norbornadiene to hexachlorocycopentadienie ratio is 1 : 2.4, i. e. with a slight excess of hexachlorocycopentadiene. Both reactions are heated under reflux for similar time and both give 65% yield of respective products in the end.

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I don't think that exercise from your book was meant to show that only one product is possible. Seems to me that the intention was just to present a problem regarding Diels-Alder reaction between not so trivial substrates - of which one is bicyclic and overall electronics are somewhat reversed as diene substrate is electron depleted by chlorine substituents.

References

  1. T. A. Robbins, V. T. Vien, J. W. Givens III, D. A. Lightner J. Am. Chem. Soc. 1992, 114, 10799-10810. https://doi.org/10.1021/ja00053a016
  2. N. R. Lokan, M. N. Paddon-Row, M. Koeberg, J. W. Verhoeven J. Am. Chem. Soc. 2000, 122, 5075-5081. (Procedure for synthesis of 2 can be found in supporting information which is accessible free of charge) https://pubs.acs.org/doi/10.1021/ja991895m
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