Question

Explain why maleic anhydride reacts rapidly with 1,3-butadiene but does not react at all with ethene under thermal conditions.

Short answer

The reaction of maleic anhydride with 1,3-butadiene involves suprafacial ring closure; thus, maleic anhydride reacts rapidly with 1,3-butadiene, whereas the reaction of maleic anhydride with ethene involves antarafacial ring closure.

Step-by-step solution

Suprafacial and Antarafacial bond formation

In a cycloaddition reaction, the new pie bonds in the product are formed by the donation of electron density from one reactant to the other reactant.

There are two modes of orbital overlap for the simultaneous formation of two pie bonds, suprafacial and antarafacial, which are as follows:

• In suprafacial bond formation, both pie bonds form on the same side of the pie system.
• In antarafacial bond formation, the two s bonds form on opposite sides of the pie system

cycloaddition of Maleic Anhydride

Cycloaddition reaction of maleic anhydride with 1,3 butadiene involves the three pie bonds ( two from1,3 butadiene and one from maleic anhydride), and reaction occurs under a thermal condition, so such reaction involvessuprafacial ring closure; thus,maleic anhydride reacts rapidly with 1,3-butadiene.

Cycloaddition reaction of maleic anhydride with ethene involves the two pie bonds (one from ethene and one from maleic anhydride), and reaction occurs under a thermal condition, so such reaction involvesantarafacial ring closure, which cannot occur with the formation of the four-membered ring; thus,maleic anhydride does not react at all with ethene.