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How does the length of the bridge of the resulting bicycle change the rate of the Diels Alder reaction (examples below)? I can't seem to think of how one would be more electron donating than the other to result in a more electron rich diene.

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    $\begingroup$ What is your source for the reaction rates? $\endgroup$ Jan 26, 2022 at 10:18
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    $\begingroup$ What is your experimental evidence for this, are there kinetic data for experiments running (like in the [suggestive] illustration added) where either cyclopentadiene, or 1,3-cyclohexadiene are used as dienophile for the same diene at otherwise same conditions? Are these differences significant (e.g., beyond $\pm 3 \sigma$)? Perhaps there is some difference in reactivity for a DA among small ring dienophiles (less than or equal to six atoms in the cyclic dienophile) by ring tension, but without hard data, this is a speculation. $\endgroup$
    – Buttonwood
    Jan 26, 2022 at 10:27
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    $\begingroup$ First off, do we even have rate constants? I see none in the question. $\endgroup$ Jan 26, 2022 at 19:43
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    $\begingroup$ The diene portion of the 6-ring is less coplanar than the diene portion of the 5-ring. $\endgroup$
    – ron
    Jan 27, 2022 at 21:39
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    $\begingroup$ I'd fully expect the 5-ring to react faster than the 6-ring, because it has less conformational flexibility (cf @ron's comment and Oscar's answer). Essentially, the 5-ring is more constrained to be in a conformation that is suitable for the DA reaction. But I'd probably still like to see hard data. $\endgroup$ Jan 27, 2022 at 23:56

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Assuming the claimed difference is reaction rates is real (see the comments), we may want to consider the distance between the ends of the conjugated diene chain. You need these ends to overlap with the pi orbitals of the other alkene, and making the diene end-to-end distance closer would improve the overlap thus facilitating the reaction. Thus the pentagonal ring, by creating a closer distance than the hexagonal one, is expected to lead to a faster reaction rate.

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