Choices drop down: 1) endo/exo 2) endo/exo 3)lower/higher 4) endo/exo 5)lower/hi

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Choices drop down: 1) endo/exo 2) endo/exo 3)lower/higher 4)endo/exo 5)lower/higher 6)endo/exo 7)endo/exo 8)endo/exo 9)endo/exo When furan and maleimide undergo a Diels-Alder reaction at 25°C, the major product is the endo adduct G. When the reaction is carried out at 90°C, however, the major product is the exo isomer H. The endo adduct isomerizes to the exo adduct when it is heated to 90°C. Propose an explanation that will account for these results. 25 NH Endo adduct 90°C Furan Maleimide 90 °C NH Exo adduct When a Diels-Alder reaction takes place, products from both the endo and exo transition states can be formed, but the product typically predominates because the transition state is usually of energy. The reason that the approach is generally favored has to do with orbital overlap that the transition state energy in the orientation, though for steric reasons the product is thermally the more stable one. For this reason, the product is formed faster (it is the kinetic product). Only by heating the isomer is transformed into the more stable somer

Explanation / Answer

Nice questtion.

The major product is endo because there are favorable interactions between the newly forming pi bond and the electron withdrawing groups of the dienophile (maleimide).

It turns out that the rate of formation of the "expected" endo product is actually faster than the rate of formation of the exo product. However, the Diels–Alder is a reversible reaction. In this case, the exo product is thermodynamically favoured over the endo product by about 1.9 kcal/mol1.9 kcal/mol. So, even though the predicted endo product is the kinetically favored product (and formed first), thermodynamics eventually takes over and the more stable exo adduct. Hence when you heat the reaction the endo product becomes exo product.

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