Refer to the molecular orbital diagrams of allyl cation (Figure 10.14) and those    presen...

Refer to the molecular orbital diagrams of allyl cation (Figure 10.14) and those    presented earlier in this chapter for ethylene and 1,3-butadiene (Figures 10.11 and 10.12) to decide which of the following cycloaddition reactions are allowed and which are forbidden according to the Woodward-Hoffmann rules.