Question

What combination of diene and dienophile undergoes Diels-Alder reaction to give each adduct?

Short answer

Answer: 1,3-butadiene and ethene.

Step-by-step solution

Understanding the Diels-Alder Reaction

The Diels-Alder reaction is a [4+2] cycloaddition reaction between a conjugated diene and a dienophile. The diene has a conjugated double bond system (alternating single and double bonds), while the dienophile usually has a C=C bond containing an electron-withdrawing group (EWG). In a Diels-Alder reaction, the diene and dienophile come together to form a new six-membered ring. This is achieved by breaking the pi-bonds in the diene and dienophile and forming new sigma bonds between the carbon atoms.

Identifying the Adduct Structure

The given exercise doesn't provide an adduct, so let's choose a generic one. For this example, we'll use an adduct of the Diels-Alder reaction: a bicyclohexene ring. Adduct structure: $$ \begin{array}{ c } \text{ }\ce{C} \\ \text{ }| \\ (\ce{CH2})\text{-}\ce{C}=\ce{C}\text{-}(\ce{CH2}) \end{array} $$

Deduce the Diene and Dienophile Combination

To deduce the diene and dienophile combination that would give the selected adduct, we need to reverse the Diels-Alder reaction and break the newly formed sigma bonds. In doing so, we can identify the diene and dienophile structures. Breaking the sigma bonds: $$ \begin{array}{ c } \text{ }\ce{C} \\ \text{ }| \\ (\ce{CH2})\text{-}\ce{C}=\ce{C}\text{-}(\ce{CH2}) \end{array} \Rightarrow \begin{array}{ c } \ce{CH2}\text{-}\ce{C}=\ce{C}\text{-}\ce{CH2} \end{array} + \begin{array}{ c } \ce{C}\text{=}\ce{C} \end{array} $$ From the broken sigma bonds, we can see that the diene is a simple conjugated diene: 1,3-butadiene (\(\ce{CH2=CH-CH=CH2}\)), and the dienophile is ethene (\(\ce{C=C}\)). Keep in mind that this is a simple case, and the diene and dienophile can have various substituents that can affect the reaction outcome. So, the combination of diene and dienophile undergoing Diels-Alder reaction to give the chosen adduct is 1,3-butadiene and ethene.