Which of the following compounds cannot react as a diene in

Which of the following compounds cannot react as a diene in a Diels-Alder reaction? IH2C H3C A) B) H-C CHs C) H2C D) D A) A B)B(C)

Solution

(C)

One quirk of the Diels-Alder reaction is that the diene is required to be in the s-cis conformation in order for the Diels-Alder reaction to work. The s-cis conformation has both of the double bonds pointing on the same side of the carbon-carbon single bond that connects them. In solution, the carbon-carbon single bond in the diene that connects the two alkenes is constantly rotating, so at equilibrium there is usually some mixture of dienes in the s-trans conformation and some in the s-cis conformation. The ones that are at that moment in the s-trans conformation do not react, while the ones in the s-cis conformation can go on to react.