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#?

#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#

1 Answer
Oct 14, 2016

The most reactive dienophile is propenal.

Explanation:

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.