Which of the following dienophiles reacts fastest with buta-1,3-diene in a normal-electron-demand Diels-Alder reaction?

Apropenal, $CH X_{2} = CH - CHO$
Bethene, $CH X_{2} = CH X_{2}$
Cpropene, $CH X_{2} = CH - CH X_{3}$
D2-methylpropene, $(CH X_{3}) X_{2} C = CH X_{2}$

Answer: A

Normal-demand Diels-Alder reactions are accelerated by electron-withdrawing groups on the dienophile, which lower its LUMO energy. The formyl group of propenal does so, while methyl groups are electron donating and slow the reaction.

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Which of the following dienophiles reacts fastest with buta-1,3-diene in a normal-electron-demand Diels-Alder reaction?

Answer: A

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