# 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?

## $C {H}_{2} = C H - C H = O$ ${\left(C {H}_{3}\right)}_{2} C = C {H}_{2}$ $C {H}_{2} = C H - O C {H}_{3}$ $C {H}_{3} - C H = C H - C {H}_{3}$ $C {H}_{2} = C {H}_{2}$

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.

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

Nature of the Dienophile

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

(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 ($\text{C=O}$) and cyano ($\text{C≡N}$) groups.

The groups attached to the dienophile in this question are $\text{CHO}$ (EWG), ${\text{CH}}_{3}$ (weakly donating), and ${\text{OCH}}_{3}$ (strongly donating).

The most reactive dienophile is the aldehyde — propenal.