Refer to the molecular orbital diagrams of allyl cation (Figure 11.2), ethylene, and 1,3-b...

Refer to the molecular orbital diagrams of allyl cation (Figure 11.2), ethylene, and 1,3-butadiene (Figure 11.11) to decide which of the following cycloaddition reactions are allowed and which are forbidden according to the Woodward-Hoffmann rules.

(a)

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(b)

Figure 11.2 The π molecular orbitals of allyl cation, radical, and anion. Allyl cation has two π electrons, allyl radical has three, and allyl anion has four.

Figure 11.11 The LUMO of ethylene and the HOMO of 1,3-butadiene have the proper symmetry to allow cycloaddition. The two molecules approach each other in parallel planes, and electrons flow from the HOMO of 1,3-butadiene to the LUMO of ethylene. σ Bonds form when orbitals of the same symmetry (blue-toblue and red-to-red) overlap.