Which of the following compounds is the most reactive dienophile in a Diels-Alder reaction with 1,3-butadiene, $CH_2=CH-CH=CH_2$ ?

Question

$CH_2=CH-CH=O$
$(CH_3)_2C=CH_2$
$CH_2=CH-OCH_3$
$CH_3-CH=CH-CH_3$
$CH_2=CH_2$

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Answer 1 · 2016-10-14T20:58:01

The most reactive dienophile is propenal.

The Diels-Alder reaction

The Diels-Alder reaction is a [2+4]-cycloaddition reaction between a 1,3-diene and an alkene (a dienophile) to form a cyclohexene.

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The transition state involves the overlap of the frontier orbitals — usually the HOMO of the diene and the LUMO of the dienophile.

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Nature of the Dienophile

Adding an electron-withdrawing group (EWG) to the alkene lowers the energy of the LUMO.

DA Orbitals
(From Wikipedia)

This decreases the energy gap between the HOMO and the LUMO and causes a dramatic increase in the rate of cycloaddition.

Typical EWGs are carbonyl ( $"C=O"$ ) and cyano ( $"C≡N"$ ) groups.

The groups attached to the dienophile in this question are $"CHO"$ (EWG), $"CH"_3$ (weakly donating), and $"OCH"_3$ (strongly donating).

The most reactive dienophile is the aldehyde — propenal.