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why can't the diene react with itself like thisenter image description here

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    $\begingroup$ it does it just has a really low yield $\endgroup$ Feb 29 at 8:39
  • $\begingroup$ ... because it's a bad dienophile $\endgroup$
    – Karl
    Mar 1 at 21:46

1 Answer 1

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It can and does given the right diene. The classic case is cyclopentadiene:

At room temperature, this cyclic diene dimerizes over the course of hours to give dicyclopentadiene via a Diels–Alder reaction. This dimer can be restored by heating to give the monomer.

Note the thermoreversibility of the reaction in this case, which opens up a relatively easy route to aromatic cyclopentadienide-ion rings.

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  • $\begingroup$ Freshly distilled cyclopentadiene cracked from the dimer is neutral, and its dimer/product of the Diels-Alder reaction is uncharged, too. Neither cyclopentadiene, nor dicyclopentadiene feature a planar cyclic array of $(2n + 2)$ $\pi$ electrons to qualify as aromatic. The large gain of stability if cyclopentadiene is deprotonated because the resulting anion/base then equally is an aromatic aromatic structure is separate, independent of cyclopentadiene's ability to dimerize quickly at room temperature. $\endgroup$
    – Buttonwood
    Mar 2 at 18:03

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